Muscles

Following on from bones and joints, I'm going to talk about muscles, since they also play a vital role in allowing us to move around!

There are three main types of muscles, but I'm only really going to talk about one in this post. The type of muscle that I'm going to talk about is called skeletal muscles- the muscles that allow movement at the joints. Aside from these, there are also smooth/involuntary muscles, which are the muscles in the internal organs and cardiac muscle, which is the heart muscle.

All kinds of muscles, however, do have a few things in common. Firstly, they can only contract on their own. Contraction allows skeletal muscles to move bones, cardiac muscles to pump the heart, and smooth muscles to carry out functions specific to the organ in question. Although muscles cannot stretch on their own, they can be stretched. This ability to be stretched is extensibility. Elasticity, on the other hand, is the ability of muscles to return to their original length after stretching. Contractability, extensibility and elasticity are all vital for allowing muscles to create movement.

Now let's look at the stuff that's specific to skeletal muscles!

Skeletal muscles, as the name implies, are attached to the bones of the skeleton via tendons, which is a fibrous, inelastic connective tissue. Muscles are positioned in order to bridge the joints- the two ends of each muscle will generally be attached to different bones. The end attached to the stationary bone is called the origin, while the end attached to the moving bone is called the insertion. (The bit in the middle is called the belly.) Contraction of a muscle will therefore pull the two bones closer together. Since muscles can't stretch themselves out, moving the bones apart again generally requires a muscle on the other side of the limb to contract. Hence, muscles are generally grouped in pairs known as antagonists.

During movement, the muscle that causes the desired action is called the agonist or prime mover. The other muscle of the pair, on the other hand, is known as the antagonist (yup, there appears to be two definitions of that word here). There are sometimes other muscles involved called synergists or fixators, which help to steady joints, prevent unwanted movement in other areas, and allow the agonist to function more effectively.

Another way of looking at the movement of the bones and muscles is by looking at how the bones act as levers. A lever is a structure that moves around a fixed point called a fulcrum when force is applied. In the human body, the bones are the levers, the joints are the fulcrums, and the muscles provide the force.

Now for some more technical stuff...

The Structure of Skeletal Muscles

Skeletal muscles are made up of bundles of muscle fibres. Muscle fibres are surrounded by a sheath of connective tissue, which allows adjacent bundles to slide over each other during contraction. The sheaths of each bundle join each other, tapering towards the end of the muscle to form tendons. The amount of connective tissue increases with age, and is thought to contribute to the decrease in muscular strength that comes as we get older.

The muscle fibres themselves are elongated cylindrical muscle cells with many nuclei. Each cell is surrounded by a thin, transparent plasma membrane called the sarcolemma, which contains a kind of cell fluid called sarcoplasm (this is basically the cytoplasm of muscle cells- more on cytoplasm and other parts of the cell in a future post). The cells are 10-100 micrometres in diameter, and their length varies from a few millimetres to a few centimetres. The sarcoplasm contains hundreds to several thousands of thread-like myofibrils.

Myofibrils can be divided into units called sarcomeres, which contain many smaller myofilaments. Myofilaments come in two varieties and are the units actually responsible for the muscles being able to contract. Thick myofilaments are mainly comprised of a protein called myosin, while thin myofilaments are mainly comprised of a protein called actin. According to the sliding filament model (remember, a model is just a simplified representation of a scientific concept), these myofilaments can slide past each other to shorten the sarcomeres and allow the muscles to contract. They are then pulled past each other when the muscle relaxes.

Not all muscle fibres have to contract and relax at the same time: in fact, at any given time, some fibres will be relaxed while others will be contracted. Muscle tone is the maintenance of partial contraction of muscles. This is not due to the same fibres remaining contracted all the time, but rather by the muscle fibres "taking turns" at contracting so that contraction can be maintained for a long period of time.

Muscles of the Upper Limbs

In my post about the skeleton, I mentioned briefly that the pectoral girdle (shoulder bones), while allowing a wide range of movement, do not provide very strong support. To make up for this, the upper limb bones have a lot of muscular attachments to the axial skeleton (i.e. the ribs and spine). Here's a quick overview of only some of the muscles in our upper limbs:

• Trapezius- attaches scapula to axial skeleton. Allows us to move our shoulders around in various ways (e.g. shrugging).
• There are nine muscles that cross the shoulder joint to attach to the humerus. Seven of these are from the scapula. The shallow joint is held in place by ligaments, while the many muscles allow a wide range of movements.
• Biceps and triceps- you probably already know what these do. The biceps allows us to bend our arms at the elbow; the triceps allows us to straighten our arms out afterwards.
• The brachialis, a muscle that lies beneath the biceps, allows us to flex our forearms.
• Our forearms have many muscles in a number of layers: those on the lower layers allow us to move our fingers, while those on the upper layers move our wrists and palms. In order to reach the fingers and palms, the tendons of the muscles extend over the wrist.

Muscles of the Lower Limbs

While the muscles of the upper limbs are more about extending the range of movement, the muscles of the lower limbs are more about stability and strength. Although the pelvic girdle doesn't need any muscular attachments to the axial skeleton, strength is still required in order to help those leg muscles constantly resist the pull of gravity. Here's an overview of the muscles in the lower limbs:

• Three large gluteal muscles extend from the pelvis to the femur of each leg, each of which has a "neck" that bends inwards so as to provide optimal leverage for the muscles that pull on it. The gluteal muscles serve to extend and rotate the thighs. The largest gluteal muscle is called the gluteus maximus.
• There are two main muscle groups of the thigh: the hamstrings, which bend the knee and extend the thigh backwards, and the quadriceps, which can straighten the lower leg. All quadriceps muscles have a common tendon which crosses the knee joint to join with the tibia.
• The thigh also contains adductors, which move the thigh towards the centre line of the body (as mentioned in my post on joints, "adduction" is movement towards the centre line of the body). It's antagonistic (i.e. produces the opposite effect) with two of the gluteal muscles (which, surprise surprise, move the thigh away from the centre line of the body).
• The calf muscle is made up of two muscles, the more prominent being the gastrocnemius (which I have absolutely no idea how to pronounce without sounding stupid). It looks kinda like two muscles that merge into one, which then becomes the calcanean tendon, which you might know by its more common name- the Achilles tendon.
• The soleus is another muscle which is closely associated with the gastrocnemius, as the tendon of the soleus joins that of the gastrocnemius.
• The anterior tibialis is the front of the lower leg. It allows us to point our toes upwards (I think), taking weight onto the heels.
• The arch of the foot is made up of tendons from calf muscles. One of these muscles is called the posterior tibialis.
• There are many short muscles in the foot which support the arches, make up the fleshy part of the sole, and contribute to the suppleness and flexibility of the foot.

The next part of my book has a whole load of propaganda interesting facts on why we should exercise and stuff. I'm just going to cover the terms related to muscles:

• Muscular strength- the force that a muscle group can exert against a resistance
• Muscular endurance- the ability of a muscle to contract repeatedly or sustain a contraction for an extended period of time
• Flexibility- range of motion about a joint
• Atrophy- the decrease in size of a muscle (normally happens in muscles that aren't used or are only used for very weak contractions)

And now for the old obligatory "what can go wrong" part!

• Paralysis- occurs when the spinal cord is damaged. Any limbs below the break become paralysed (lose sensation and voluntary muscular movement). Paraplegia is paralysis of both of the lower limbs while quadriplegia is paralysis of all four limbs.
• Strain- normally occurs when a muscle or tendon is overstretched. Symptoms include a sudden pain and loss of power in the limb.
• Spasms- short, sudden, involuntary contractions
• Cramps- sustained involuntary contractions that lack even partial relaxation
• Convulsions- violent, involuntary contractions. May be caused by muscle fibres receiving impulses from nerves, which in turn might have been stimulated by fever, poisons and so on.
• Fibrillation- uncoordinated contractions of individual muscle fibres. This prevents the muscle from contracting smoothly
• Tics- involuntary, spasmodic twitching of muscles
• Muscular dystrophy- inherited diseases in which individual muscle cells degenerate. Leads to a progressive reduction in the size of the skeletal muscle, an increase in connective tissue and possibly the replacement of muscle fibres with fatty tissue. There are two forms: the Duchenne form and the fascioscapulohumeral form.

My Human Bio posts have been rather bland and picture-less as of late... but don't worry, this post isn't going to be one of them, as you shall soon see!

Back in Skills Week in Year 10, we had the option to go to a university and participate in one of three projects under the guidance of a researcher. The project my group did was looking at the effectiveness of different dietary interventions for Duchenne muscular dystrophy. What follows are some pictures that my group (or half of my group, to be precise, since we split into halves and took turns crunching numbers and taking pictures) took of muscle cells from both healthy mice and mice affected by Duchenne muscular dystrophy.

This first picture is titled "pikachu bulbasaur( absolutley [sic] nothing is in this image dont bother opening it)." (Yup, we were really mature back in Year 10.) It's a picture of normal muscle tissue. Those purplish dots are the nuclei of cells IIRC. (I think the tissues were stained with something beforehand to highlight the nuclei, but I can't remember.)

This second picture is also of healthy muscle tissue. It's titled "H***** [name blanked out to protect her privacy, not that I think you guys are going to stalk her or anything] is awesome (According to her...[Connecting Tissue in Normal Muscle])" so I'm assuming that that diagonal strip thing is some connective tissue. Or maybe it's not, and the other white bits are connective tissue (to be honest that makes more sense to me at the moment now that I know that bundles of muscle fibres are surrounded by connective tissue, but I could be wrong).

This next picture, titled "k***** is awesome(MDX fat and necrosis )" shows, as the title implies, fat and necrosis (cell death) in the tissue of an mdx mouse (i.e. a mouse affected by muscular dystrophy). The white bubble things are fat, while the clusters of purple nuclei indicate necrosis. (Presumably the breakdown of cells means that the nuclei originally inside them just end up floating around with nowhere to go.)

This picture is titled "awesome threesome (black stuff and fat and necrosis). The black stuff isn't important, by the way- we think it's just something that fell onto the slide.

Yet more necrosis from a picture titled "g******* is awesome (MDX NECROSIS)"

This last picture is titled "L**** is awesome no im not ( mdx purple splotches." This picture also displays necrosis- I'm wondering whether or not those big purple splotches indicate cells that are in the process of dying.

And that's it from me! Good night everyone!

Joints

Just a quickie (okay, a quickie following my definition of "quickie") on joints!

There are three main types of joints that can be classified according to range of movement (functional classification) or their structure (structural classification):

• Fixed or fibrous joints: No movement occurs between the bones because they're held in place by fibrous connective tissue. Very difficult to damage this type of joint. Can be found between bones of the skull.
• Slightly movable or cartilaginous joints: Bones held together by cartilage. Slight movement can occur, but not a lot. Examples: between the two pelvic bones (symphysis pubis), between vertebrae, between ribs and sternum
• Freely movable or synovial joints: Most of the joints can move in many directions. Even though their movement is somewhat restricted according to the shape of the joint, they're still classified as freely movable joints.

As you can probably guess, we're going to be focusing on the 3rd type (probably the most interesting type, since there's movement involved!).

There are many types of freely movable joints:

• Ball-and-socket joints: One bone has a spherical head, while the other has a kind of cup-like cavity for the spherical head to fit into. Allows movement in all directions. Examples: shoulder joint, hip joint
• Hinge joints: One bone has a convex surface which slots into the concave surface of the other. It might sound a bit like a ball-and-socket joint, but the surfaces are less rounded (I think) which results in the joint only allowing movement in one direction but not in others. Examples include the elbow and the knee.
• Pivot joints: One bone has a circular, pointed or conical end, which rotates upon the axis of another bone. A prime example of this is the joint between the first vertebra (on which the head is balanced) and the second vertebra.
• Gliding joints: Movement can occur in any direction, hindered only surrounding ligaments or bones. Examples: between carpal bones, between tarsal bones, between the sternum and clavicle, between the scapula and clavicle
• Saddle joints: The two bones of the joint are saddle-shaped (i.e. concave in one direction, convex in the other). This allows both side-to-side and back-and-forth movements. Pretty much only seen in the joint between the thumb and the palm of the hand.

Now for a bit of technical stuff on the structure of synovial (freely movable) joints!

• The whole joint is surrounded by a capsule, which consists of two layers. The outer layer is called the fibrous capsule and consists of dense, fibrous connective tissue, which is attached to the periosteum of the bones (periosteum = the hard white bit. See my post on the skeleton for more info). It's strong yet flexible, allowing movement but resisting dislocation.
• The inner layer of the capsule is called the synovial membrane (hence the name "synovial joint"). It consists of loose connective tissue with blood capillaries. The synovial membrane lines the entire joint cavity aside from the articular cartilages (cartilage that "caps" the ends of the bones) and the articular discs (I'll get to them later).
• The synovial membrane secretes synovial fluid, which lubricates the joint, provides nourishment for the cells of the articular cartilage, and carries phagocytic cells that remove microorganisms and debris resulting from wear-and-tear. Only a small amount of fluid is usually present, but if the joint is injured or inflamed, more fluid may be produced, resulting in swelling and discomfort.
• Articular cartilage is located on the ends of the bones, providing a smooth surface for movement.
• Articular discs occur in some joints and divide the synovial cavity into two, allowing synovial fluid to be directed to areas of greatest friction. In the knee, the articular discs are called menisci (singular meniscus), and consist of fibrocartilage which extends inwards from the articular capsule.
• Bursae are little sacs of synovial fluid and, like articular discs, they are only present in some joints. Bursae are positioned in such a way to prevent friction in certain parts of the joint.
• Accessory ligaments hold the bones together in many joints.

Several forces keep the articulating bones in contact with each other: the shape of the bones, the strength of the joint ligaments and the tension provided by the muscles around the joint.

Now for some more technical terms on the types of movement that occurs at joints:

• Flexion or bending: The angle between bones is decreased (e.g. bending the knee).
• Extension or straightening: Opposite of flexion. Angle between bones is increased (e.g. straightening out the leg after flexion.)
• Abduction: (no, I do NOT mean "kidnapping.") Movement away from the midline of the body (e.g. moving your arm up)
• Adduction: Opposite of abduction. Movement towards the midline of the body.
• Rotation: Bone rotates around its long axis. The humerus, for example, can rotate around quite a bit.

Now for the obligatory "what can go wrong" part...

• Arthritis: Includes many types of inflammation of the joints.
  Rheumatoid arthritis is a severe form involving inflammation of the joint, swelling, pain and loss of function. Firstly, the synovial membrane is inflamed, and then abnormal tissue known as pannus is produced, which grows over the surface of the articular cartilage. It can destroy the cartilage, or even erode the bone. Eventually it becomes invaded by fibrous tissue. In severe cases this tissue ossifies (changes into bone), making the joint entirely immovable.
  Osteoarthritis is much more common, but much less damaging. It involves deterioration of the articular cartilage, causing bony spurs to develop from the ends of the bone forming the joint. Due to these spurs, the space in the joint is decreased, which in turn restricts the movement of the joint.
• Dislocation: A bone is displaced, and ligaments, tendons or capsules are torn. Symptoms include temporary paralysis of the joint, pain, swelling and occasionally shock.
• Sprains: A ligament is torn from the bones. Blood vessels, muscles, tendons, ligaments and nerves may also be damaged. Symptoms include swelling, pain and discolouration due to ruptured blood vessels.
• Slipped disk: Part of the invertebral disc (fibrocartilage between vertebrae) is squeezed to one side, displacing the disc. Depending on how the disc has "slipped," it can put pressure on spinal nerves of the spinal cord, causing severe pain and numbness. This can then result in nerve damage, which then causes weakness and possible degeneration to the tissue of the muscles supplied by the damaged nerves.
• Tendinitis: Inflammation of the tendon sheaths surrounding certain joints. Symptoms include swelling and pain upon movement of the affected joint. RSI (repetitive strain injury) is a form of tendinitis that might affect the wrists of computer keyboard operators or clarinet players.
• Whiplash: The cervical vertebrae allow quite a lot of flexibility which is great most of the time, but in a sharp impact this flexibility can cause the head to fling back and forth. This can then cause ligaments to tear and internal bleeding to occur. Nerves may also be injured. There are many symptoms of whiplash, including headaches, dizziness, nausea, pain and weakness. If the injury is such that the axis is driven into the brain stem, death may occur.

Maybe I should leave it there so that the post can end on an ominous note... nah. I'm too kind for that. Good night and sweet dreams! (Unless you're reading this during class, in which case... GET BACK TO YOUR WORK!)

The Skeleton

I was just going to call this post "bones," as the chapter in the book that I'm reading is also called "bones," but I didn't want to give too many opportunities for the more sick-minded to make jokes about it. Then again, maybe I'm the one who's sick-minded for thinking about this enough to censor it out.

Anyway.

First I'm going to talk about the structure of the bones that make up the skeletal system and talk a bit about what the skeletal system does as a whole before throwing the names of different bones at you.

What's in a Bone?

Bones are made up of several different materials, including connective tissues, cartilage and marrow.

The long shaft of a typical long bone is called the diaphysis. It's made up of a hollow cylinder of compact bone (which is a type of connective tissue) surrounding the yellow bone marrow cavity, which is a fat storage site.

The ends of bones are called the ephiphyses (singular: epiphysis). They have compact bone on the outside, but on the inside they have spongy or cancellous bone, which is more porous than compact bone and thus contains many large spaces. These spaces are filled with marrow, which may or may not be red bone marrow, depending on the bone. Red bone marrow is where many blood cells are produced (see my post titled "Blood" for more info about the different types of blood cells).

The outer surface of the bone is covered by a dense, white, fibrous covering known as the periosteum. The epiphyses are also capped by thin layers of articular cartilage (I'm assuming it's called that because it helps the bones move in the joints or something).

Although you might not think of bones as being alive, bones do carry living cells which carry out various processes, such as growth and repair. These cells are located in a bony matrix (which is known as lamella- plural, lamellae) which is full of non-cellular material, including inorganic salts. These inorganic salts are part of what gives bone its strength.

The lamellae are arranged differently depending on the type of bone. Spongy bone consists of trabeculae, an irregular arrangement of thin, bony plates. The spaces in the trabeculae contain bone cells (osteocytes), nerves and blood vessels. In compact bone, however, there are many structures known as Haversian systems, which are arranged parallel to the long axis of the bone, providing maximum strength. Each Haversian system consists of a Haversian canal, which contains at least one capillary, and possibly nerves and lymph capillaries as well. The canal is surrounded by concentric layers of lamellae. Between the lamellae are small spaces called lacunae (singular: lacuna) which hold the bone cells. Small canals known as canaliculi run between the lacunae, allowing materials and so forth to be passed from cell to cell.

Functions of the Skeleton

The skeleton, aside from providing support and stopping us from going all floppy, also has a variety of different functions:

1. Movement: the muscles can attach to the bones, allowing bones that articulate to move relative to each other. (Articulation is basically the positioning of the bones that allows them to move- I'm assuming this basically means joints and so on.)
2. Protection of vital organs
3. Storage areas for mineral salts and fat. Mineral salts stored here (such as calcium, phosphorus, sodium and potassium) can then be distributed to other regions of the body via the circulatory system.
4. Blood cell production (in bones containing red bone marrow).

Now that's all out of the way, time to throw the names of bones at you! YAAAAAAAAAAYYYYYYYYYYYYYYYY!! not. (Would probably be more fun for everyone involved if I just threw the actual bones at y'all. Never mind.)

There are 206 bones of the skeleton, but I'm not going to give you the names of all of them for two reasons: 1) I'm merciful like that and 2) I can't be bothered looking up the names of all of them. I'm just going to talk about the major bones in the two main parts of the skeleton-

Oh yeah, I should probably provide a quick note on that first. The axial skeleton is the name given to the bones around the central "axis" of the body- that is, the skull, the backbone and the ribs. The appendicular skeleton consists of the bones that make up the limbs as well as the shoulders and hips.

Here we go...!

The Axial Skeleton's Main Bones

• Skull: The cranium of the skull is made up of a number of bones all fused together. In fact pretty much all of the bones of the skull are fused together without moveable joints- the only exception is the mandible, which is the bone that makes up the lower jaw. (For curious people out there- the bone that forms the upper jaw, as well as part of the mouth, eye sockets and nasal cavities, is called the maxilla.)
• Vertebral column: The vertebral column consists of many smaller bones called vertebrae. The top 7 are called the cervical vertebrae, followed by 12 thoracic vertebrae (which are attached to the ribs), 5 lumbar vertebrae, and then the sacrum (which normally consists of 5 vertebrae) and coccyx. There are openings between vertebrae to allow spinal nerves to pass through to various parts of the body.
• Thorax (chest): The thorax is formed by the sternum (breastbone), as well as ribs, costal cartilages and the aforementioned thoracic vertebrae. There are 12 pairs of ribs, which are joined at the back with the corresponding thoracic vertebrae. Some ribs are called "true ribs" because they are directly attached to the sternum by some costal cartilage; some are "false ribs" because they are not directly attached to the sternum by cartilage (instead, the cartilage meets up with another bit of cartilage which then attaches to the ribs); while others are "floating ribs" (don't attach to the sternum at all).

The Appendicular Skeleton's Main Bones

There are a lot here, so I'm going to section this bit up.

Pectoral Girdle (Shoulder Girdle)

The pectoral girdle only has two bones on each side: the scapula (shoulder blade) and the clavicle (collar bone). The clavicle attaches to the sternum and holds the shoulder away from the rib cage, giving somewhat weak support but allowing a range of movement.

The Upper Limb (Arms)

The upper arm bone is called the humerus. The lower arm has two bones- the ulna and the radius. The ulna forms the point of the elbow and joins the wrist on the small-finger side, while the radius joins the wrist on the thumb side and forms the wrist joint. The wrist has 8 carpals arranged in two rows of four, the palm has metacarpals while the finger bones are known as phalanges. There are three phalanges per finger except for the thumb, which only has two.

The Pelvic Girdle (Hip Girdle or Pelvis)

The two main bones of the pelvic girdle are called the pelvic or hip bones. They are joined at the front by a cartilaginous joint called the symphysis pubis and are joined at the rear by the sacrum. Each pelvic bone also has a socket known as the acetabulum, which forms part of the hip joint.

The Lower Limb (Legs)

The upper leg bone is called the femur. The knee joint is protected by a triangular bone called the patella, or kneecap. The lower leg is made up of the tibia and fibula. The tibia is larger, allowing it to bear a greater proportion of weight. The fibula, on the other hand, is quite slender and articulates with the tibia at the hip joint and with one of the ankle bones at its lower end. The ankle consists of seven tarsals. One of these is the talus, which is the only ankle bone to articulate with the fibula and tibia. Another is the calcaneus, which is the heel bone, and is the largest of the ankle bones. The foot contains metatarsals which then lead into the phalanges which make up the toes. Just like in the fingers, there are three phalanges for each toe except for the big toe. (Though given how small and inflexible my little toe is, I seriously doubt there are three bones in there. Um.)

Now, in order to wrap up this post about the skeleton, I'm going to keep up the tradition with the "things that can go wrong with this part of the body" section. Yay!

Stuff That Can Go Wrong

• Bone fractures- occur for a variety of reasons and normally require months to heal properly. Bone-forming cells called osteoblasts help the bones to heal. They are stimulated when the bone is used for support and movement, and thus prolonged immobilisation of a bone may be detrimental to healing (according to this book, anyway. If you break a bone, don't listen to me, listen to your doctor).
• Osteoporosis- A gradual reduction in the rate of bone formation while the rate of bone absorption remains normal. This results in the bones becoming porous, fragile and relatively easily broken. Happens for a variety of reasons, including a decrease in sex hormones, calcium deficiency, vitamin D deficiency, inactivity, and so on.
• Spina bifida- A birth defect where the spine fails to join together at the back, resulting in a gap that membranes and parts of the spinal cord may push through.
• Rickets- A condition in which the bones are soft. Can be caused by lack of vitamin D (which is required to synthesise a protein that transports calcium into the extracellular fluid).

Okay that's it from me, unless I feel like writing a post about joints later tonight (highly unlikely at this stage though). TTFN!

The Respiratory System

I've spoken before about the circulatory system, which carries nutrients and oxygen to the cells that need it, as well as the digestive system, which carries nutrients to the blood in the first place. Now I'm going to talk about the respiratory system, which carries oxygen to the blood, and removes carbon dioxide from the body.

The respiratory system, like every other system, consists of several organs (which is, incidentally, pretty much the very definition of a "system" IIRC). The air you breathe in goes through the nose or mouth, then goes through the pharynx and larynx, into the trachea and then into the bronchi which enter the lungs. From there, the bronchi branch out into smaller bronchi, which branch out into bronchioles, and then into small air sacs called alveoli, where the membrane walls are thin enough for gases to diffuse through into the blood.

tl;dr: basically the air just goes through a whole bunch of tubes until they reach a point where gases can diffuse into and out of the blood.

Long version:

The Nose

Yes, I know that sometimes people breathe through their mouths, especially during exercise (because it's pretty difficult to comfortably breathe in lots of air through your nose), but today we're going to talk about the nose as it's the starting point, okay? The reason I'm doing this is because the nose- or rather the nasal cavity, which is the internal portion of the nose inside the skull- has more relevant stuff to talk about.

This aforementioned nasal cavity has a central partition in the middle, which divides the cavity into a left and right chamber. Each chamber has three "shelves" known as conchae. Conchae divide up the passages and increase the overall surface area.

So why is surface area actually important? After all, what is it that the nasal cavity does in the first place, and why are its functions important enough to require a larger surface area for the nasal cavity to work at optimal capacity? Here are some of the functions of the nasal cavity:

1. Filtering the air: The nose contains coarse hairs which filter out large dust particles. Additionally, the nasal cavity contains a mucous membrane, which traps dust particles that make it through the first set of hairs.
2. Warming the air: Capillaries in the nasal cavity contain warm blood, warming the air as well.
3. Moistens the air: The mucous membrane takes care of this part.
4. Smell: The upper part of the nasal cavity contains olfactory receptors, which are nerve endings that are sensitive enough to distinguish different smells.
5. Pushing down mucus etc.: The mucous membrane in the lower part of the nasal cavity has hair-like projections called cilia, which push mucus and any trapped dust towards the throat. They do this by rhythmically beating back and forth.

The next part of the respiratory system is...

The Pharynx

I briefly mentioned the pharynx in my post about the digestive system, and I'm going to make another quick mention here. And by quick mention I literally mean that all I have to say about it this time is that it's a tube that's roughly 13 centimetres long, extending from the nasal cavity downwards. Oh and there's also a tube called the Eustachian tube that leads to the middle ear. Just a random fact that popped up in my book. I have no idea why it's relevant in this chapter, but maybe you might find a linkage somewhere.

Okay now that's done. Onto...

The Larynx

The larynx is also called "the voice box." It connects the pharynx with the trachea, and thus it is yet another organ that the air has to pass through to get to the lungs. It's made up of a few different pieces of cartilage (including the "Adam's apple," the large one at the front of the neck), with some mucous membranes stretched between them. These membranes are called the vocal folds, and the edges, otherwise known as the vocal cords, have elastic ligaments that can vibrate. The opening between the vocal cords is called the glottis. The muscles that move the cartilages can move the vocal folds, which then changes the size of the glottis.

When the muscles are relaxed, air simply passes through; when the muscles are contracted, the vocal cords vibrate, which in turn makes the air in the pharynx, nose and mouth vibrate, producing sound. More air produces a louder sound, while the pitch is controlled by the tension on the vocal cords. Thus the larynx is able to fulfil one of its important functions- allowing us to talk! (It also allows us to shut up after talking, but that bit's hard. I'm not good at that yet. Which is why I ramble a lot on this blog- hey wait, writing doesn't require my vocal cords! Scrap that, then.)

The larynx has yet another important function for such a humble small organ. When swallowing, it moves upwards, and meets a flap of cartilage called the epiglottis, which projects from the rear wall of the larynx. The epiglottis closes the glottis, which stops food from passing through the larynx and into the trachea and the lungs. And speaking of the trachea...

The Trachea

The trachea is about 12cm long and about 2.5cm in diameter. It contains C-shaped bands of cartilage, which allow the trachea to be flexible without being at risk of collapsing. Just like the other parts of the respiratory system covered so far, the inside of the trachea also has a mucous membrane. The membrane in the trachea contain cilia, like the lower part of the nasal cavity. The cilia in the trachea beat mucus and other solid stuff upwards, towards the pharynx, where they can be swallowed. Sounds gross, but preferable to entering the lungs.

At the bottom of the trachea, the tube divides into two smaller ones, otherwise known as...

The Bronchi

The bronchi (singular bronchus) bring air in and out of the lungs as well as through the lungs. The first set of bronchi, the pair that first enter the lungs, are known as primary bronchi. They then divide into several secondary bronchi, which then divide into tertiary bronchi, and so on. The bronchi have cartilage like the trachea, and a ciliated mucous membrane like pretty much every other part we've seen so far. Soon the bronchi divide out into...

Bronchioles

Bronchioles are basically even smaller bronchi, but with some big differences: they have no cartilage (only walls of smooth muscle), and they have no cilia. The smallest bronchioles then end in...

Alveoli

Alveoli (singular alveolus) are tiny air sacs that occur in clusters throughout most of the area of the lung. Just like the walls of villi and capillaries, alveoli walls only have one layer of cells, allowing gases to diffuse through easily. Also like villi and capillaries, alveoli come in large numbers in order to maximise surface area, thereby making them more efficient at their job, which in this case is to exchange gases between the inside of the alveoli and the numerous blood capillaries surrounding the alveoli. The inside of the alveoli have thin layers of moisture, which is prevented from evaporating completely due to the lungs' placement deep inside the body. This moisture is essential for dissolving gases, which is essential for allowing them to diffuse into the blood.

The blood in the capillaries surrounding the alveoli is the blood that's come through the pulmonary arteries after going through the rest of the body and the right side of the heart (see my post on the circulatory system). Hence, the blood has a low concentration of oxygen, much lower than that in the alveoli, allowing oxygen to dissolve into the moisture on the inside of the alveoli and diffuse through into the blood.

Once inside the blood, only around 3% of oxygen dissolves in the plasma, as it's not very soluble in water. The other 97% is combined with haemoglobin to form a compound called oxyhaemoglobin. Oxygenated blood is red since oxyhaemoglobin is bright red. As the concentration of oxygen in the blood is reduced, particularly around cells that use up oxygen, oxyhaemoglobin breaks down to release haemoglobin and oxygen. Haemoglobin is dark red, so deoxygenated blood is also dark red.

Aside from allowing oxygen to be absorbed by the blood, the alveoli also absorb carbon dioxide, which gets exhaled later. The concentration of carbon dioxide in the deoxygenated blood that arrives at the lungs has a relatively high concentration of carbon dioxide, while the alveoli have a relatively low concentration. This also provides great conditions for diffusion.

There are several ways in which carbon dioxide is transported to the alveoli for the all-important diffusion stage. Around 7-8% is dissolved in the plasma, just like the 3% of oxygen mentioned above. This carbon dioxide simply diffuses into the alveoli. Another 22% of carbon dioxide combines with the globin part (i.e. the protein part) of the haemoglobin molecule, which forms a compound called carbaminohaemoglobin, which later breaks down, allowing the carbon dioxide released to diffuse into the alveoli. The remaining 70% or so of carbon dioxide reacts with the water to produce carbonic acid, which then breaks down to produce hydrogen and bicarbonate (HCO3-) ions (there's a bit about this reaction on one of my posts about reactions and equations). These ions are carried in the plasma. Later on, the ions recombine to form carbonic acid and then, with the aid of enzymes, decompose into water and carbon dioxide, the latter of which is able to diffuse into the alveoli.

Now that I've rambled on for a bit about the mechanics of breathing, let's look at the lungs in general, as well as some other muscles that aid in breathing.

The Lungs

The lungs are located in the thoracic cavity, which is basically the area bounded by the ribs and diaphragm. Aside from the lungs, the thoracic cavity also contains the heart, aorta (the main artery), venae cavae (the two main veins leading into the heart), pulmonary veins and arteries, oesophagus, thymus gland and part of the trachea and bronchi, inside a space called the mediastinum, which is located between the two lungs.

The lungs have a two-layered membrane called the pleural membrane or pleura. Between the two, there is a narrow space called the pleural cavity, which is full of pleural fluid, which provides some lubrication, allowing the two layers to slide against each other. The fluid also holds the lungs in place. The outer layer of the pleural membrane adheres to the inner wall of the chest cavity (i.e. the ribs. Or at least I'm pretty sure it's the ribs). Meanwhile, the inner layer covers the inner surface of the lungs.

The Diaphragm

The diaphragm is the muscle separating the thoracic and abdominal cavities. It can contract and relax to change the volume of the thoracic cavity.

Intercostal Muscles

Intercostal muscles are the muscles between the ribs. They come in two varieties: external and internal. The fibres of the internal intercostal muscles are at right angles to those of the external intercostal muscles. The external intercostal muscles can contract to move the ribcage upwards and outwards in order to increase the volume of the thoracic cavity; the internal intercostal muscles contract to pull the ribs closer together and decrease the volume of the thoracic cavity.

Now that we've familiarised ourselves with the tools, let's put it all together to get a better picture of ventilation, or breathing:

Inspiration

The word "inspiration" here doesn't mean "the thing that gives you a good idea for some creative work," but "inhalation," or "taking in air." Inspiration works by increasing the volume of the thoracic cavity, thus making the air pressure inside the thoracic cavity lower than the pressure outside (my post on the Kinetic Theory explains why volume and pressure are inversely proportional). Air then rushes into the lungs from outside in order to make the pressure equal (pretty much the same principle as that of diffusion).

There are several processes in place that make the thoracic cavity bigger. The diaphragm and external intercostal muscles contract, flattening the diaphragm and moving the ribs upwards and outwards. The outer layer of pleural membrane adheres to the inner wall of the thoracic cavity, so as the thoracic cavity expands, the lungs expand too. During normal breathing, the diaphragm does most of the work; during heavier breathing, the rib cage becomes more important.

Expiration

"Expiration," which refers to "exhalation" here and not expiry dates or whatever, occurs as a result of the thoracic cavity's volume being reduced, which increases the pressure in the lungs, thus forcing the air outside in order to make the pressure equal again. The process is basically the opposite of inhalation: the diaphragm and external intercostal muscles relax, making the diaphragm bulge into the cavity and moving the rib cage downwards. More forceful expiration also requires the contraction of the intercostal muscles to lower the rib cage more actively.

Now for some more random bits and pieces about the respiratory system before I get off and get back to having a life doing other nerdy stuff because I have no life:

Respiratory Volumes

Respiratory volumes are basically different measures of lung capacity. They can be measured with different instruments, such as spirometers and vitalographs. Here's some terms for you:

• Residual volume- the volume of air remaining after maximum expiration (since you can't completely empty the lungs). This is normally around 1 200 mL for men, and 1 000 mL for women.
• Tidal volume- the volume of air that moves in and out with each regular breath. This is about 500mL for both men and women. About 150mL of tidal air doesn't reach the alveoli, but stays in dead space instead- the interior of the other organs of the respiratory system that are not involved in the actual exchange of gases.
• Vital capacity- The maximum amount of air that can be exhaled after inhaling as much air as possible. Roughly 4 800 mL for men and 3 400 mL for women.

Disorders of the Respiratory System

I'm just going to list and add in a few key points because I really should get off the computer at some point...

• Asthma- An allergic response which results in the muscles surrounding the bronchioles going into spasm (sudden involuntary contractions). As the bronchioles have no cartilage to keep them open, this can cause difficulty in breathing. Sometimes, the irritation of the membranes lining the air passages results in excessive mucus being secreted, which then also restricts air movement.
• Emphysema- Caused by long-term exposure to irritating particles. Air in general contains irritating particles, but certain groups, such as smokers and people who live in highly polluted cities, are more at risk. The particles damage the alveoli, which lose their elasticity, are replaced with fibrous tissue, and may break down. The loss of elasticity means that the lungs are constantly inflated, which then means that exhalation requires voluntary effort.
• Lung cancer- Involves the development of a tumour, just like other cancers. Risk factors include exposure to certain irritants like asbestos and tobacco. Normally begins in the walls of the bronchi- excessive production of mucus is caused by irritation of the mucous membrane lining. Trapped mucus causes alveoli to rupture, resulting in emphysema. Eventually a cancerous growth develops in a bronchus and may spread to other parts of the body.
• Laryngitis- Swelling of the mucous membrane covering the vocal cords, making it difficult for them to vibrate.
• Bronchitis- An irritation that causes an increase in mucus production in the bronchi and bronchioles, which can result in an accumulation of mucus which can be cleared by coughing.
• Pneumonia- An infection caused by some bacteria, viruses or fungi, most notably the pneumococcus bacterium. The inflammation causes fluid to accumulate in the alveoli.
• Carbon monoxide poisoning- Carbon monoxide (CO) can combine with haemoglobin 250 times more readily than oxygen can. When CO combines with haemoglobin, the oxygen-carrying capacity of the blood is reduced.
• Altitude sickness (or mountain sickness)- Higher altitudes contain lower pressure air (i.e. fewer gas molecules in the same amount of space as compared to lower altitudes). People who aren't used to the lower pressures may feel sick as their bodies are unable to absorb enough oxygen. The body can, however, adapt by firstly increasing the rate of breathing while more red blood cells (and therefore more haemoglobin) are produced to increase the oxygen-carrying capacity of the blood. People who have lived at high altitudes for a very long time may also have more alveoli and more blood vessels than those at lower altitudes, and their haemoglobin can combine with oxygen at the lower concentrations experienced at high altitudes.

Phew! I am done. I am so done. Good night!

(Mind you, I guess I didn't really have to do all this in the first place. Heck, I'm not even taking courses on human bio. Not at the moment, anyway.)

More About Blood and the Heart

I don't really have much more to say about blood, apart from a couple more points about blood clotting and blood groups. I don't really have a lot more to say about the heart, either, except talk about cardiovascular disease.

Blood Clotting

In my previous post about blood, I mentioned that platelets are responsible for blood clotting but I didn't go into too much detail. I'm still not going to go into too much detail, but I will go into more detail than I did last time ;)

For most small tears to blood vessels, not too much has to be done. First of all, the muscles in the walls of the blood vessels constrict to reduce blood loss, and then platelets stick to the rough surface caused by damage (normally the internal walls of blood vessels are smooth, but not when damaged). As more and more platelets are attracted to the injury, the platelets begin to stick to each other and form a plug which reduces blood loss further. Next the platelets release substances that prolong the constriction of the damaged vessels until they heal themselves.

If the damage is much worse, then coagulation (a.k.a. blood clotting) is necessary. Many reactions take place involving a large number of chemical substances in the plasma called clotting factors. These result in the formation of threads of insoluble protein, which trap the cells in the blood, forming a clot. The threads hold the clot in position, before a slower process called clot retraction takes place, in which the threads contract, becoming denser and stronger and pulling the edges of the blood vessels together. While this happens, a fluid called serum is squeezed out. After this, the clot dries and forms a scab which protects the wound from infections.

There are several factors that stop blood from forming clots in undamaged vessels. First of all, as mentioned before, undamaged blood vessels have smooth walls that platelets can't stick to. Secondly, the plasma also contains anticlotting factors.

Blood Groups

I'm sure you've all heard of the ABO and Rhesus blood group systems, right? Well, here I'm going to explain how they work.

Basically your red blood cells (erythrocytes) may or may not contain specific antigens which determine your blood group. Antigens are substances that can stimulate the formation of antibodies, and the antigens on your red blood cells can consist of either antigen A only, antigen B only, both antigens A and B, or neither antigen. This corresponds to blood groups A, B, AB and O, respectively.

Meanwhile, your plasma contains the antibodies that react against the antigens that you don't have- for example, if you have group A blood (i.e. your red blood cells only have antigen A), you will have anti-B antibodies. Similarly, group B blood has anti-A antibodies. Type AB blood doesn't have any antibodies, while type O blood has both anti-A and anti-B antibodies.

You can't receive blood that contains antigens that your antibodies are going to attack, otherwise the donated blood will clump and disintegrate. For example, if someone has group B blood, with anti-A antibodies in the plasma, receiving A or AB blood will just result in a lot of cells clumping together before disintegrating. Type AB is probably the best off in this regard- their blood has no anti-A or anti-B antibodies, so they can receive blood from any other blood group! Type O, on the other hand, can only receive type O blood (since type O has both anti-A and anti-B antibodies), but thankfully type O is the most common type of blood (followed by type A, then type B, then type AB). Generally attempts are made to give the patient blood of the same type, but if that fails then, unless the patient is O negative (more on the Rhesus +/- system in a bit), another compatible type is given.

One thing that I don't really understand is why it isn't a problem the other way. As in, if someone has type AB blood and they receive type O, why don't the anti-A and anti-B antibodies in the donated blood react with the A and B antigens in the patient's blood? Could someone please answer this for me?

Now we're onto the Rhesus system! As well as the antigens listed above, there are also Rh antigens which might also appear on our red blood cells! If you have the antigens, you're Rh positive; if not, you're Rh negative. If you're Rh negative, your body will most likely produce anti-Rh antibodies when confronted with Rh positive blood. This is fine the first time, but subsequent exposures will cause your newly-created antibodies to spring into action.

This can also be a bit of a problem during pregnancy, if an Rh-negative woman gives birth to multiple Rh-positive children. The first child will cause the mother's body to produce anti-Rh antibodies; a second Rh-positive child might have its blood cells attacked by these antibodies. This can be prevented or managed through several medical interventions, including Rh immune-globulin injections. (See http://kidshealth.org/parent/pregnancy_center/your_pregnancy/rh.html for more info.)

Transfusions

Determining the patient's blood type is necessary for transfusions. As stated above, attempts are made to get the best match possible, or at the very least a compatible blood type. Another kind of transfusion is an autologous transfusion where the patient's own blood is used, which carries fewer risks, but obviously has a lot of drawbacks (for example, they are pretty much only used for non-emergency surgeries that are planned in advance, so they know when to collect your blood).

Aside from whole blood transfusions, there are several other kinds of transfusions, including red cell concentrates (the red cells are separated out, but the concentrate may also include platelets and leucocytes), plasma, platelet concentrates, cryoprecipitate (it's what you get if you freeze the plasma and let the other bits thaw out. Contains substances necessary for blood clotting) and immunoglobulins (contain antibodies against particular diseases).

Cardiovascular Diseases

Cardiovascular diseases, or heart diseases, come in many different varieties. Many of them are related to arteriosclerosis, which is a "hardening of the arteries." One type of arteriosclerosis, atherosclerosis (which looks too similar to "arteriosclerosis" IMO, which might be why I keep mixing the two up), is caused by the deposition of fatty substances onto the artery walls which creates fatty deposits known as plaques, which then cause fibrous tissue to develop, which results in the deposition of salts, which results in hard, calcified areas. These hard, calcified areas then cause obstructions and result in the artery walls losing their elasticity. 

One treatment for atherosclerosis is balloon angioplasty. First the patient is given drugs that help to dissolve clots, and then a tube with a tiny balloon at the tip is inserted. When the balloon is inflated, the artery expands, loosening the plaque from the wall.

Bypass surgery can be used to treat more severe cases where the blockage is leaving the patient at risk of a heart attack. Veins are taken and placed on the outside of the heart to allow the circulatory system to bypass those arteries affected by atherosclerosis.

Arteriosclerosis can lead to several other heart cardiovascular diseases:

• Coronary heart disease is a result of atherosclerosis in the coronary arteries (arteries that supply blood to the heart- see my post on the circulatory system). The obstructing plaques restrict the supply of blood to the heart muscle, so there might not be enough oxygen supply to the heart, particularly during exercise.
• Angina is pain in the chest that normally occurs during physical activity. It is normally called by atherosclerosis of the coronary arteries. Normally, the pain subsides during rest and there is no damage to the heart muscle, so people with angina can still lead relatively normal lives.
• A heart attack, or myocardial infraction, is a result of a complete stop in blood flow to a part of the heart muscle. A severe obstruction in the coronary arteries is usually the cause- sometimes this is due to plaque, or it might be due to a blood clot (clots can form on the rough surface caused by plaque). As oxygen supply is halted, the muscle in that area dies, resulting in sudden and severe chest pain. If only a small area is affected, it can heal- firstly, scar tissue forms, and then arterial branches develop to supply blood. Heart attacks, can, however, have very adverse effects: if it is severe enough to disturb the normal beat of the heart, cardiac arrest, where the heart pumps little to no blood, may occur. This can result in death within minutes.
  If someone is displaying the signs of a heart attack, you should get them to hospital immediately. Symptoms include chest pain, pain radiating to the left arm, shoulder, neck or jaw, sweating, breathlessness, faintness and palpitations.
• A stroke occurs when atherosclerosis affects the cerebral arteries (arteries supplying the brain with blood). This causes part of the brain tissue to die. Aside from blocked arteries, another cause of a stroke is cerebral haemorrhage, where bleeding occurs at a weak point in one of the arteries. Strokes range from mild to severe, some allowing people to continue leading normal lives, others resulting in death.
• A transient ischaemic attack (TIA) is like a stroke, but shorter, with less severe symptoms. They do not cause any permanent damage, but people who have a TIA should seek treatment as a TIA shares the same causes as other cardiovascular diseases (i.e. if you have a TIA, you're at more risk of stroke etc.)
• Peripheral vascular diseases are cardiovascular diseases affecting the limbs. One type of peripheral vascular disease is arteriosclerosis in the limb arteries, slowing blood supply to a limb. Other types of peripheral vascular disease include phlebitis (inflammation of a vein) which may cause blood clots, or varicose veins (enlarged and lengthened veins which cannot carry blood back to the heart efficiently, potentially resulting in the accumulation of blood in lower limbs).

Aside from these diseases, there are many other kinds of heart diseases. One other is congestive heart failure, or simply "heart failure," which is where the heart is too weak to pump sufficient amounts of blood. This could be due to excessive alcohol consumption, a heart attack, or an infection.

Now that we know what types of heart disease there are, let's look at factors that influence your risk at getting a heart disease so that you can avoid some of them! (Granted, there are some that you can't avoid, for example those related to gender and age, but lifestyle changes can go a long way.)

• Age: Incidence of atherosclerosis increases with age.
• Gender: Men are more likely to suffer from cardiovascular diseases at a younger age than women are (women rarely suffer from cardiovascular diseases until around 45 years old).
• Blood cholesterol: Blood cholesterol, a measure of the fat content of the blood, can be controlled via saturated fat intake and exercise levels. Higher blood cholesterol puts you more at risk of heart disease.
• Blood pressure: Smoking, high alcohol consumption and high salt consumption are all factors that can lead to hypertension, or high blood pressure. High blood pressure is another risk factor for heart disease.
• Weight: Being overweight is another risk factor for heart disease, as there is more strain on the heart and lungs.
• Smoking: Smoking constricts blood vessels, causing hypertension. Also, the CO in smoke combines with haemoglobin, reducing the capacity of the blood to carry oxygen. Not a good combination.
• Alcohol: Consuming too much can lead to hypertension or heart muscle weakness.
• Heredity: If you have a family member with cardiovascular disease, you might be at greater risk.

So, basically, if you want the best chance at avoiding cardiovascular diseases, reduce your saturated fat and salt intake, get in a decent amount of exercise, don't smoke and don't drink excessively. Oh, and choose your parents wisely. (Not that you can. Oops.)

Next up: the respiratory system! When I can be bothered to get off the computer and do some study, that is.

Blood

Following on from yesterday's post about the circulatory system, today I'm going to talk about blood!

Blood actually has quite a lot of stuff in it for a humble red liquid that spurts out every time we get cut or scratched. 55% of our blood is plasma, which is mainly water (91%), but also has proteins (7%) and other substances (the stuff that's being transported to and from the cells) dissolved in it. The other 45% of our blood is made up of the cells in it, namely erythrocytes (red blood cells), leucocytes (white blood cells) and thrombocytes (platelets).

Blood helps maintain the internal environment of the body (for some reason the word "homeostasis" comes to mind here, but I don't know whether it's applicable in this context). It not only does this through carrying nutrients to the cells and removing their wastes, but it also maintains the pH of bodily fluids, distributes heat, maintains water content and ion concentration of bodily fluids, and protects against diseases.

Let's take a look at all of the components in blood (as listed above) and how they help to achieve these functions:

Plasma

• Water (91%)- solvent for all of the other substances contained in plasma.
• Proteins (7%)- Proteins in the plasma are known as plasma proteins, and consist of albumins (like those in egg whites), globulins (some can act as antibodies when fighting diseases) and fibrinogen (involved in blood clotting). These proteins are what makes the blood sticky.
• Ions- including sodium (Na+), potassium (K+), calcium (Ca2+), chloride (Cl-) and bicarbonate (HCO3-). Ions and proteins combined contribute to the osmotic pressure of the blood, encouraging more water to diffuse into the blood (see my earlier post about diffusion and osmosis for more info on how this works).
• Other substances- include nutrients dissolved from food, dissolved gases (oxygen and carbon dioxide), hormones, waste products from cells, etc. Proportion of these substances depend on what part of the body you're looking at, among other factors.

Formed Elements

These are basically the cells suspended in the plasma- the aforementioned red and white blood cells, as well as platelets.

Erythrocytes (a.k.a. red blood cells)

FACT FILE

• Unique features: No nucleus, contains haemoglobin
• Colour: Red (obviously)
• Shape: Circular and biconcave (i.e. each side has a concave surface), due to having no nucleus.
• Size: Very small- roughly 7.5 micrometres in diameter.
• Number: In men- about 5.4 million per cubic millimetre. In women- about 4.8 million per cubic millimetre. (We have roughly 4-6L of blood in our bodies, so just imagine how many cells we have in total- or you can calculate it yourself, if you're in that way inclined.)
• Where formed: Red bone marrow (in adults, this is the marrow of bones such as ribs, vertebrae and pelvis) at the rate of around 2 million per second!
• Lifespan: Roughly 120 days, after which the cell membrane becomes fragile due to having no nucleus.
• Main function: Transport oxygen to the cells

How is the main function of this type of cell performed? Although red blood cells are initially formed with nuclei, the nucleus is removed as the cells mature. This makes the cells biconcave in shape, while still leaving plenty of room for haemoglobin. Haemoglobin is required for carrying oxygen around the blood. It's made up of a protein called globin, combined with four haem groups, each containing an iron atom which can combine with an atom of oxygen. The combination of haemoglobin and oxygen is facilitated by the large surface area of the red blood cells, which provides more room for oxygen to diffuse inside. When the iron combines with oxygen, haemoglobin turns red. Each red blood cell can hold up to 300 million molecules of haemoglobin, making red blood cells very red indeed.

Other fun facts about this type of cell: Aging blood cells are destroyed in the liver and spleen. Iron atoms and some other parts of haemoglobin are reused, while other parts, like the bile pigments, get excreted (see my second post on the digestive system). Two million red blood cells are destroyed every second. Two million more are produced in the red bone marrow each second in order to maintain the number of red blood cells. That's a lot of cells being destroyed and created!

What happens if you don't have enough of this type of cell? Anaemia is a condition where the number of red blood cells or the concentration of haemoglobin is reduced. This results in inadequate oxygen supply to the tissues, which then results in a myriad of symptoms including fatigue and intolerance to cold. Anaemia has a variety of different causes, including loss of too much blood, iron deficiency, B12 deficiency (B12 is needed for the normal development of erythrocytes), destruction or inhibition of red bone marrow, or genetic conditions. Some of these types of anaemia have special names- for example, sickle cell anaemia is a genetic condition resulting in deformed red blood cells, and pernicious anaemia is anaemia caused by inadequate B12.

Leucocytes (a.k.a. white blood cells)

FACT FILE

• Unique features: Can shape-change and can engulf bacteria, dead cells and so on. Granular leucocytes, or granulocytes, have granules suspended in the cell contents and their nuclei are "lobed" (that is, they have several lobes connected by thin filaments rather than just being one massive lobe). Agranular leucocytes consist of monocytes and lymphocytes, which do not have granules (hence the name) and usually have spherical nuclei.
• Colour: Hmm... let me think about this one... ... ... ... ... they're white.
• Shape: Can change shape, allowing them to slide through small spaces between the cells of the walls of capillaries etc.
• Size: Larger than erythrocytes- roughly 9-14 micrometres in diameter
• Number: 5000 to 10 000 per cubic millimetre.
• Where formed: Some (granulocytes and monocytes) are formed in the red bone marrow. Another type, lymphocytes, are formed in organs such as the spleen, tonsils, thymus gland and lymph nodes.
• Lifespan: A few days. During infection, they may only live for a few hours. This is because engulfing so much crap takes the life out of them (to be more scientific, "the substances taken in interfere with normal cell functioning").
• Main function: Defend the body against invading microorganisms, remove dead or injured cells

How is the main function of this type of cell performed? Their ability to change shape allows them to get around and engulf bacteria, dead cells, cell fragments, and so on. This is known as phagocytosis.

Other fun facts about this type of cell: Although white blood cells are necessary, too many isn't necessarily a good thing, as can be seen in leukaemia, where so many abnormal white blood cells are produced that they fill the red blood marrow, inhibiting the production of red blood cells. This can lead to anaemia.

Thrombocytes (a.k.a. platelets)

• Unique features: No nucleus.
• Colour: ?
• Shape: ?
• Size: Extremely small- only around 2.5 micrometres in diameter, roughly a third of that of an erythrocyte
• Number: 250 000 - 400 000 per cubic millimetre
• Where formed: Red bone marrow, at rate of around 200 billion a day!
• Lifespan: Around 7 days
• Main function: Form blood clots when blood vessels are damaged.

How is the main function of this type of cell performed? According to http://www.med.illinois.edu/hematology/PtClotInfo.htm platelets have several functions during the clotting process, including sticking to the injured blood vessel as well as other platelets to form a "plug," and providing molecules required for the reactions that take place during clotting.

Now that we're pretty much done looking at the blood, let's look at body fluid in general and how it moves between the blood plasma and the tissues.

There are many different names for different kinds of fluid in the body:

• Intracellular fluid: Fluid inside cells.
• Extracellular fluid: Fluid outside the cells. This is broken down into two categories: tissue fluid and plasma. Extracellular fluid moves between the tissues and the blood, carrying stuff with it.
• Tissue fluid (also known as interstitial fluid or intercellular fluid): fluid found between cells in the tissues.

Extracellular fluid mainly passes back and forth via diffusion. Since capillary walls are thin, many substances can diffuse through them (with the exception of larger molecules, such as proteins), due to concentration differences. Additionally, the relatively high pressure at the arterial end of the capillaries forces some fluid out (except for, of course, the stuff that's too big to go through, like the proteins).

Once substances have passed into the tissue fluid, they can be absorbed by cells via several methods, one of which is diffusion. Cells also use similar methods to get rid of their wastes.

The veins of capillaries have relatively low pressure, as the narrowness of the capillaries resists blood flow. Also, the amount of proteins left in there create a high osmotic pressure, which causes much of the tissue fluid to return to the capillaries.

Another great thing about capillaries being so small and so restrictive of blood flow is that the slower flow provides more time and more opportunities for substances to diffuse back and forth.

Now, another thing about fluid for ya. Not all of the fluid that diffuses into the tissue cells diffuses back into the capillaries. To stop the tissues from becoming all bloated, the lymph system kicks in to save the day!

The lymph capillaries begin in the tissues (remember the lacteals from the villi in the small intestine? They're lymph capillaries) before joining up to form larger vessels, called lymphatic vessels, which then join up to become even bigger lymphatic vessels, and so on, until they eventually join into the thoracic duct and the right lymphatic duct which then open up into the subclavian veins which bring blood to the heart from the arms. Lymph nodes, located along the lymphatic vessels, remove bacteria and foreign particles from the lymph (essentially the fluid inside the lymphatic system that used to be tissue fluid. In fact the only difference between lymph and tissue fluid is its location).

The lymphatic system is comprised of the above vessels as well as three organs known as lymphoid organs- the tonsils, thymus and spleen. Lymph vessels are kind of like veins in that contractions of skeletal muscles can move the lymph along (helpful since, unlike the circulatory system, the lymph system has no pump), and in that they have valves to ensure that the lymph only moves in one direction. They are, however, more permeable than veins and other blood vessels (including the capillaries), allowing proteins and other relatively large particles to pass into the lymph capillaries. The lymphoid organs and the lymph node are involved in specific immune responses which help overcome infections.

And that's pretty much it from me on this topic. I'm going to rest now. Maybe have some churros. I dunno.

The Circulatory System

Yesterday I spoke about the digestive system, which absorbs nutrients into the body. Today I'm going to speak about the system that transports these nutrients (as well as oxygen and probably some other substances as well) throughout the body- the circulatory system!

Fortunately, the circulatory system doesn't have such a wide variety of organs to talk about, so this should be done relatively quickly. (Note the emphasis on the word "relatively.")

Basically the circulatory system is made up of:

• the heart;
• arteries and arterioles (very small arteries);
• veins and venules (very small veins); and
• capillaries.

In a nutshell, the arteries take the blood away from the heart. The arteries then branch out into smaller arteries in arterioles, which then branch out into capillaries, which take blood to each individual cell. Then the capillaries join up to venules, which then join up to larger and larger veins, before arriving back in the other side of the heart to start the process again. (The two sides of the heart send the blood to different parts of the body, as we shall soon see.)

Let's first take a look at the heart, since the heart is after all what drives the blood through all of those arteries and veins and capillaries.

The heart is located in the middle of the chest, between the lungs. Sometimes it feels like your heart is a bit more over to the left, but that's only because the left side beats harder than the right side- more on that later.

The heart is made up of four main chambers, two on each side. The two halves are separated by a wall called the septum, and the entire heart is surrounded by a membrane called the pericardium which holds the heart in place and prevents it from overbeating while still giving it some freedom to move while beating. The wall of the heart is made up of cardiac muscle (protip: words containing cardi-, cardia- or cardio- are all related to the heart).

Each side of the heart has an atrium and a ventricle. The atrium is the first chamber that the blood enters after entering the heart through the veins. It has relatively thin walls as the atrium doesn't need a lot of muscle to hold that low-pressure blood coming in from the veins. Next the blood flows to the ventricle, which has thick muscular walls which pump the blood out of the heart and to other parts of the body. Between the two chambers is a valve called the atrioventricular valve which allows blood to flow from atrium to ventricle, but not the other way. These valves consist of flaps as well as fibrous chords, or chordae tendinae. The flaps swing shut when the ventricle contracts due to blood pressure (I think), and the chordae tendinae prevent the flaps from swinging too far. Once the blood leaves the ventricle, the valve at the bottom of each artery, called the semilunar valve, which consists of three cusps, hold any blood that tries to flow back, preventing backflow.

Now that we've looked at what each side of the heart has in common, let's take a look at what's different about each side of the heart. IMO, the main difference lies in where each side pumps the blood.

The left side of the heart receives blood from the lungs and delivers blood to all over the body (except for the lungs). The circulation of blood through the body is known as systemic circulation.

The right side of the heart, on the other hand, receives blood from all over the body (except for the lungs), and delivers it to the lungs. The flow of blood through the lungs is then known as pulmonary circulation (pulmo = lungs).

Let's see how this fundamental difference relates to other differences between the two sides of the heart:

• Since the left side pumps all over the body, while the right side only pumps to the lungs, the left ventricle's muscular wall is much thicker and stronger than the right ventricle's muscular wall in order to pump blood more forcibly. This increased force required to pump blood on the left side is why, if you put your hand on your chest, your left side feels like it's beating harder than your right side.
• The names of the arteries and veins leading into and out of each side of the heart are different.
  On the left side, the vein that flows in from the lungs is called the pulmonary vein. The artery that flows to the body is called the aorta. It's the main artery in the body IIRC, and it splits into many other arteries, some of the main ones being the carotid arteries (to head and neck), the coronary artery (to the heart itself), the subclavian arteries (to the arms), the hepatic artery (to the liver. Also has a branch going to the stomach), the mesenteric artery (to the intestines), the renal arteries (to the kidneys) and the femoral arteries (which take blood to the legs).
  On the right side, on the other hand, there are two veins flowing in from the body: the superior vena cava (from the upper body) and the inferior vena cava (from the lower body). The artery flowing out towards the lungs is called the pulmonary artery, which splits into the left and right pulmonary arteries (one for each lung obviously).
• The left side's atrioventricular valve has two cusps, and is called the bicuspid valve. The right side's atrioventricular valve has three cusps, and is called the tricuspid valve. I don't know why one side has more cusps than the other. Maybe it's because the left side has more muscle to hold the blood in the ventricle?

Oh and a fun fact for you: since the blood gets pumped by the heart twice in each circuit, this type of circulation is known as double circulation. This keeps the blood moving rapidly and stops the blood from losing too much pressure.

Anyway now that I've done talking about all the bits and pieces of the heart, let's see how the heart does its job!

The heartbeat also goes by a more fancy name (just like everything in human biology)- the cardiac cycle. It consists of a sequence of events:

1. For a short time (roughly 0.4 seconds), the atria and ventricles are in diastole- that is to say, the heart muscles are relaxed, allowing the blood to flow into the atria and ventricles (the atrioventricular valves are open at this point).
2. Atrial systole occurs, lasting roughly 0.1 seconds. During this phase, the atrium contracts (that's what's meant by "systole," by the way), forcing blood into the ventricles. Meanwhile, the ventricle stays relaxed, ready to receive the blood rushing in.
3. Ventricular systole then occurs, lasting roughly 0.3 seconds. This forces blood into the arteries (aorta on left side, pulmonary artery on right side).

Although all this beating is happening in the heart, why can we feel a pulse when we put our fingers on particular points on our bodies (such as our wrists or our neck)? This is a question I'll answer very shortly.

Blood Vessels

The heart is very important and all, but equally important are the tubes that take the blood to where it's needed. I'm referring, of course, to arteries, veins and capillaries. Let's have a closer look at what each kind of blood vessel does, and how it serves its function.

Once the blood leaves the heart, it's pumped into arteries, which are branched into smaller arteries, which are branched into very small arteries called arterioles, which then supply blood to the capillaries. Arteries and arterioles have thick, strong walls complete with smooth muscle to help the arteries stretch and relax in order to maintain blood pressure as the blood is pumped through. (Take note that this stretching and relaxing does NOT force the blood along, but rather just maintains the pressure.) The expansion and contracting of the arteries occurs in time with your heartbeat, giving you the ability to feel your pulse at certain points on your body. Other great aspects of having this kind of muscle is that the stretching gives the arteries some flexibility when being assaulted with relatively high pressure blood pumped out from the heart, and that the contracting and relaxing can manipulate the diameter of arteries, which can serve to either reduce or increase blood flow to particular organs, depending on the needs of each organ. These muscles also help regulate blood flow through the capillaries. Speaking of which...

Capillaries are microscopic blood vessels which form complex networks, allowing them to carry blood to nearly every body cell. Their walls are only one cell thick, just like villi, making it easier for things to pass in and out of them. Once blood has gone through the capillaries, it goes through the...

Venules, which are small veins (like how arterioles are small arteries), which later join up to larger and larger veins. Veins carry blood back to the heart. The walls of veins are much thinner than those of arteries, since the blood flowing through them isn't high pressure any more- blood loses a lot of pressure during its journey through the capillaries. Instead, the low pressure causes backflow to be a possible problem- but fear not! Many veins have several valves that prevent backflow of blood.

But if the blood is such low pressure, when does the pressure ever increase enough for any of the valves to open? you may ask. Well, when you move around, your muscles contract, squeezing your veins and providing the pressure required to push your blood along. Due to the valves, the blood can only go in one direction.

There's also more stuff about heart sounds and how those blood pressure monitors and electrocardiograph things work. I'm not going to go into these today, as I'm already quite tired, but maybe at some other point. Or not. I really don't know.

If you really want me to add this stuff though, please let me know. If you're even reading this, that is.

Digestion and Absorption of Food- Part 2

In my last post I said that Part 2 would cover the intestines, which is true, but first I'm going to quickly talk about the liver and the pancreas.

The Pancreas

The pancreas is just beneath the stomach, in the curve of the duodenum (the duodenum is the first C-shaped curve of the small intestine. If you don't understand, just Google it so I don't have to plagiarise someone else's picture, or *shudder* go through the effort of drawing my own diagram :P). The pancreas contains cells that secrete pancreatic juice as well as other cells which secrete the hormones insulin and glucagon, which regulate sugar levels in the blood.

Pancreatic juice is important for digestion as it contains digestive enzymes, including pancreatic amylase which breaks down starch into disaccharides, particularly maltose; trypsin (a.k.a. pancreatic protease- one way to remember this one is that "protease" sounds kinda like "protein") which breaks down proteins into smaller peptide chains; ribonuclease and deoxyribonuclease, which break down RNA and DNA, respectively; and pancreatic lipases (remember, fatty acids are a kind of lipid), which break down fats into fatty acids and glycerol.

One other important attribute of pancreatic juice is that it is slightly basic with a pH of 8. This helps neutralise any HCl that might still be in the chyme from the stomach (as I said in my last post, the chyme is basically the mixture that you get after everything's been digested in the stomach).

Pancreatic juice enters the aforementioned duodenum (beginning of small intestine) via two ducts: first it goes through the pancreatic duct, which is then joined by the common bile duct, which then enters the duodenum.

The Liver

The liver is a rather large organ. It lies just below the diaphragm on the right side of the body. One thing unique about the liver is that it receives blood from two different sources: through a hepatic artery and a hepatic portal vein. (FYI, "hepa-" is a prefix that generally refers to anything to do with the liver.) The hepatic artery, which branches from the aorta (the main artery in the body), carries highly oxygenated blood to the liver. The hepatic portal vein, which passes through the capillaries of the stomach and intestines before going through the liver, takes nutrients from the alimentary canal to the liver.

The liver has a lot of functions- in fact, if my quick Google search is anything to go by, it has over 500 known functions! Fortunately, you don't have to learn about all of them for now. Here's a list of just a few of the functions of the liver:

• Blood glucose regulation: The hepatic portal vein carries glucose to the liver, which is then either used to provide energy to the liver, converted into glycogen, or converted into fat. Glycogen is basically the form in which carbohydrates are stored as an energy reserve for the body. Glycogen can be converted back into glucose and back again to maintain healthy blood glucose levels.
• Deamination: Converts excess amino acids to carbohydrates (since excess amino acids can't be stored). In the deamination process, the -NH2 amine group is removed and converted to ammonia (NH3) which is then converted to urea, which is removed by the blood by the kidneys before being excreted in urine.
• Fat conversion: Fat produced from excess glucose can be transported by the blood to fat storage tissues. Between meals, fat storage tissues release fatty acids into the blood, which are converted by the liver into substances to be used as energy sources for tissues.
• Plasma protein production: The liver produces proteins that can be found in the liquid portion of the blood, otherwise known as plasma.
• Production of blood-clotting factors: Stuff that's required to clot blood is also produced in the liver.
• Storage: The liver stores a variety of different substances, including but not limited to glycogen, iron and vitamins A and D.
• Toxin breakdown: Toxic substances, both those naturally produced in the body and those from external sources, are broken down into harmless substances in the liver.
• Hormone inactivation: Pretty self-explanatory. Some hormones can be inactivated in the liver.
• Heat production: The liver does a helluva lot of stuff, and many of those chemical processes produce heat. Thus the liver also helps to maintain constant body temperature.

Now aside from the above, why else is the liver important to the digestive system? That's because...

• the liver produces bile, which contains both bile salts and bile pigments, among other things. Bile salts are necessary for the mechanical digestion of fat, as they emulsify the fat (i.e. turn it into separate components that can't be mixed back together). This increases the surface area on which enzymes can break them down further. Bile pigments, however, aren't so useful. They're just the end result of red blood cells breaking down, and have no digestive function.
• Bile first travels from the liver to the gallbladder, a sac on the outside of the liver, where it is stored and concentrated before travelling to the duodenum via the common bile duct (the same duct that pancreatic juice travels through right before it gets to the duodenum). After being used in the intestines, the bile salts are nearly all reabsorbed into the blood to be reused, while the bile pigments just get pooped out later on.

Now that the liver and the pancreas are out of the way, it's now time to talk about...

The Small Intestine

The small intestine is only "small" in terms of diameter as compared to the large intestine. In reality, it's pretty long- around 6m. The first part, the aforementioned duodenum, is about 25cm long on its own- and that's just the top bit that goes from the stomach and circles around the pancreas in a sort of C shape before you get to the main part of the small intestine! It's the longest part of the alimentary canal, and it is where both digestion and absorption of food molecules takes place.

As if it wasn't already long enough, the inside of the small intestine has several modifications to increase its surface area and make it even better at digestion. (See Reaction Rates for a tiny bit more info on why increased surface area should make reactions more efficient.) In the small intestine, the mucosa and submucosa are not smooth, but rather have many small folds that extend into the interior. Additionally, the mucosa also has small bristle-like projections called villi (singular: villus), which are each about 1mm long, and these villi in turn have small projections called microvilli (and the whole idea of small bristles having even smaller bristles attached to them just reminds me of a certain other picture that I saw on the Internet). All these projections further increase the surface area of the insides of the small intestine.

The mucosa of the intestines, just like the stomach, contains glands that produce useful digestive juices. This time, it's not HCl-containing gastric juice, but rather intestinal juice. Other juices used in the small intestine are those produced in the liver and pancreas (namely bile and pancreatic juice, respectively). The intestinal juice, like pancreatic juice, contains many enzymes. After the pancreatic amylase in the pancreatic juice breaks down starch into disaccharides, the enzymes in the intestinal juice break it down further into monosaccharides. Each disaccharide requires a specific enzyme. To work out which enzyme you need, just take the name of the disaccharide and substitute an "a" in for the "o"- e.g. lactase breaks down lactose and sucrase breaks down sucrose. Enzymes in the intestinal juice can also further break down proteins and lipids. Proteins are broken down via peptidases (it breaks down the small peptides formed after the pancreatic protease has broken down longer chains) and lipids are broken down via lipases.

Aside from chucking a whole lot of digestive juices in, how else does the small intestine digest food? Well, you see, the circular muscle fibres alternately contract and relax in a movement called segmentation, which moves food back and forth, allowing it all to mix well. Let me explain:

Basically, the small intestine contracts at evenly spaced intervals along its length, creating small compartments. Then the middle of each "compartment" contracts, and each originally contracted muscle relaxes. This creates compartments in different places, and the food in the original compartments is sloshed around into the new compartments. Here is a terrible Paint diagram that might help you understand:

Once food has been digested enough, the small intestine begins its next job: absorbing the food! Each villus only has one layer of cells on its surface, allowing the digested food to diffuse into the blood capillaries in the villus, which surround a lymph capillary, called a lacteal. (According to InnerBody.com, a lymph capillary's primary function is to drain fluids from the tissues around it.) Some foods enter the blood capillaries, while others enter the lacteals. Aside from using diffusion to absorb food, another method of absorption is active transport, where the villi use energy to forcibly absorb nutrients against a concentration gradient (i.e. bring them from a place with a lower concentration of those nutrients to a place with a higher concentration).

Here's a quick run-down on how foods are absorbed:

• Monosaccharides- active transport. Enter blood capillaries
• Amino acids- active transport. Enter blood capillaries
• Fatty acids and glycerol- diffusion. Once in the villi, fatty acids and glycerol recombine to form triglycerides (each glycerol molecule is combined with three fatty acid molecules), before being coated with protein and entering lacteals as tiny droplets called chylomicrons.
• Fat-soluble vitamins- absorbed with the fatty acids and glycerol. Water-soluble vitamins- absorbed via diffusion into blood capillaries.
• Water- osmosis into the cells of the villi.

Food absorbed in blood capillaries then goes to the liver via the hepatic portal vein, where it's either removed for processing or retained in the blood to be carried to other body cells. Food absorbed into the lacteals are eventually emptied into the blood through veins in the upper part of the chest after being carried around by the lymph system.

Food left unabsorbed, on the other hand, continues on to...

The Large Intestine

The large intestine is only a quarter of the length of the small intestine, but it's thicker in diameter. It begins with a 6cm pouch called the caecum, which ends in the appendix. The other end of the caecum joins the colon, which has 3 parts: first is the ascending colon (which goes up), then the transverse colon (which goes from the person's right to their left), and finally the descending colon (which goes down towards the rectum and anus). Surrounding the anal opening is a circular muscle known as the anal sphincter which basically controls when you do a number 2.

The large intestine has no villi and secretes no enzymes, though it does contain bacteria, some of which break down many of the remaining organic compounds into simpler substances, releasing CO2 (carbon dioxide), methane (CH4) and hydrogen sulfide (H2S). (The book doesn't say anything but I wouldn't be surprised if this is where farts come from.) Some other bacteria produce vitamins, which are then absorbed through the walls of the large intestine into the blood. Other things that are absorbed into the blood from the large intestine include water and minerals. The absorption of water makes your poo more solid. If not enough water is absorbed, you'll end up with diarrhoea- more on this later.

The lining of the large intestine contains glandular cells which secrete a large amount of mucus. Despite this, stuff in the large intestine moves fairly slowly, taking 18 to 24 hours to pass through the colon (and remember, it's only covering a roughly 1.5m distance as opposed to the roughly 6m distance in the small intestines). Eventually peristalsis pushes it all the way to the rectum. When the outside anal sphincter relaxes and the rectum contracts (the former is voluntary, the second is automatic when the rectum is full), the body finally gets to eliminate waste in a process known as defecation. The result of this, as you well know, is a nice little pile of faeces containing all the crap (if you'll pardon the pun) that didn't get digested, such as cellulose, bacteria, bile pigments etc.

What Can Go Wrong?

Now we've covered the basics of the whole digestive system, we're going to look at a few common disorders of the alimentary canal. Make sure you're sitting down, and don't read this when eating!

Vomiting- The diaphragm and abdomen contract, forcing the stomach contents into the oesophagus and out through the mouth. Has many causes, from dizziness to sickness that causes irritation of the stomach.

Ulcers- Can occur in the walls of the stomach, duodenum or oesophagus. Usually result from pepsin and acid eroding part of the mucosa. If severe, can bleed or even make a hole right through the wall of the alimentary canal. Caused by the bacterium helicobacter pylori.

Indigestion- Has many causes, including eating too much or excessive production of HCl.

Constipation- Dry, hard faeces which are difficult to eliminate. This happens when the stuff in the large intestine moves slower than usual. Could be caused by a lack of roughage in the diet (roughage is the stuff that can't be digested but promotes the movement of food), lack of exercise or emotional problems.

Diarrhoea- An irritation in one or both of the intestines increase peristalsis, making the food move through so quickly that not enough water is absorbed, resulting in watery faeces. Can be caused by bacterial or viral infections.

Appendicitis- Inflammation of the appendix. One possible cause is blockage of the appendix by faecal matter or a foreign body, but there could be other causes.

Whew. That was a lot. I'm going to take a break now! TTFN!