Tissues

No, I'm not going to be talking about tissues as in Kleenex and Sorbent, but rather tissues as in the tissues that make up the human body. (That was probably already pretty obvious, but I consider playing the role of Captain Obvious to be one of the aspects of my "job" here on this blog.)

There are four kinds of tissues in the human body: epithelial, connective, muscular and nervous. The different kinds of tissues are similar in that a tissue is made up of a group of similar cells and some intercellular material (called the matrix). Now that we've looked at the similarities, let's look at the differences:

Epithelial Tissue

What it does: Epithelial tissue, or epithelium, is used to cover body surfaces (e.g. skin) and line body cavities and ducts (e.g. in the small intestine). Its functions include absorption, secretion and protection, depending on the location of the tissue in the body.
Examples: Skin, lining of small intestines, secretory tissue of glands etc.
Structure: The cells are closely joined. Very little matrix between cells. Cells are classified according to their shape- cube-shaped cells are called cuboidal cells, column-shaped ones are called columnar cells, and so on. In some places there is only a single layer of cells (simple epithelium), while in other places there are multiple layers of cells (stratified epithelium). The epithelium is attached to a membrane of connective tissue, which is located beneath the epithelial cells.
Miscellaneous stuff: Cells in the glandular epithelium secrete stuff such as sweat, saliva, enzymes, hormones and so on. Sometimes the gland is only one cell, while sometimes a gland is comprised out of clusters of cells. Exocrine glands secrete stuff outside the body, or onto a body lining. Endocrine glands, on the other hand, secrete their products directly into the blood or into the tissue fluid.

Connective Tissue

What it does: Connective tissue binds and supports the other tissues of the body. There are many different types, including loose connective tissue, dense connective tissue, adipose tissue, cartilage and bone (bone has already been mentioned on this blog in my post about the skeleton) and blood. Each kind of connective tissue has different functions: loose connective tissue supports and connects other tissues; dense connective tissue does pretty much the same thing (as far as I can see) but it's stronger, making it more suited for joining tendons to bones and so on; adipose tissue cushions vital organs and restricts heat loss; and cartilage provides structural support while still providing some flexibility. (Bone and blood have been discussed in their respective posts.)
Examples: Blood, bones, cartilage (bet you didn't see those three coming, did ya?), fat (adipose tissue), tendons (dense connective tissue), fibrous membranes around various organs (also dense connective tissue) etc.
Structure: Loose connective tissue is made up of fibres and cells in a semi-fluid matrix. Dense connective tissue has large numbers of fibres, which give the tissue great strength, with cells scattered in between. Adipose tissue consists of cells that contain large globules of fat with the cytoplasm forming a thin layer on the outside. Cartilage has fibres embedded in a firm matrix which also contains spaces for the cartilage cells. Bone, as mentioned in my post about the skeleton, has inorganic salts in its matrix, increasing rigidity and strength. Blood consists of a whole bunch of cells separated by a matrix called plasma (see my post about blood for more details).

Muscular Tissue

What it does: Muscle fibres (long, thin muscle cells) are able to contract, causing movement. They can also be stretched, and are elastic enough to return to their original length after stretching. There are three types of muscular tissue: skeletal muscle, cardiac muscle and smooth muscle. (You can read more about skeletal muscle here.)
Examples: Hmm do I really have to say? Biceps, triceps, quadriceps, hamstrings, the gluteus maximus, etc. etc. etc. (there are over 600 skeletal muscles alone, don't expect me to list all of them). Cardiac muscle is found in the walls of the heart (w00t), and smooth muscle is found in the walls of many internal organs (blood vessels, bladder, uterus, alimentary canal etc.).
Structure: Structure of skeletal muscle can be found in my main post on muscles, but here's a quick overview anyway: skeletal muscle is made up of a series of parallel cylindrical muscle fibres that are up to several centimetres long. Each fibre contains several nuclei located on the edges. Skeletal muscle is also called striated muscle, as the thick and thin filaments (myosin and actin, respectively) give a striped appearance under a microscope. Cardiac muscle has cross-striations like skeletal muscle, but unlike skeletal muscle, cardiac muscles branch to form a network. Smooth muscle (or non-striated muscle) does not have striations, and is made up of long cells with tapering ends that fit into each other. Smooth muscle cells each have one nucleus in the centre of the cell (as opposed to skeletal muscle fibres, which have several nuclei around the edges of the cell).
Miscellaneous Stuff: Skeletal muscle is also called voluntary muscle as these muscles can only be contracted voluntarily. Smooth muscle (and presumably cardiac muscle as well) is known as involuntary muscle as it can't be contracted voluntarily.

Nervous Tissue

What it does: Contains neurons (nerve cells), which receive and transmit messages to the body.
Examples: Brain, spinal cord, nerves
Structure: The neurons in the tissue have a cell body which contains the nucleus as well as one or more strands of cytoplasm that extend from the cell body. There are several different types of neurons, as well as several other kinds of cells that help to support and nourish the neurons, but apparently all that is discussed in the Year 12 book. Boo!

Cellular Respiration and Protein Synthesis

Cells have lots of functions for humble little things that we can only see under a microscope. The many chemical reactions that take place within them are known as metabolism; those reactions that break down larger molecules into smaller ones are known as catabolism, while those that build up larger molecules from smaller ones are known as anabolism. Catabolism releases energy, while anabolism requires it. Hence balance here is key in keeping the cell functioning.

Cellular respiration is a metabolic process in which the organic molecules from the food are broken down to release energy for the cells. Glucose formed from the breakdown of carbohydrates, amino acids formed from the breakdown of proteins, and fatty acids and glycerol from the breakdown of lipids can all be broken down to release energy in this way. The main food material used, however, is glucose, so that's what I'm going to focus on in this post.

There are a lot of reactions that occur during respiration, each of which produces a small amount of energy, thus controlling the amount of energy released rather than releasing it all at once. Here's an equation for the overall reaction:

C₆H₁₂O₆ + 6O₂ à 6CO₂ + 6H₂O + energy

(Text equation: Glucose + oxygen --> carbon dioxide + water + energy)

60% of the energy produced is released as heat, which is used for keeping body temperature constant. The remaining energy is used to join an inorganic phosphate group to a molecule of adenosine diphosphate (ADP), which then forms adenosine triphosphate (ATP). This bond between ADP and the phosphate group is relatively easy to break, and breaking this bond releases energy. Thus ATP can transfer energy by moving around to where energy is needed before breaking down into ADP and a phosphate group (well, this is the mental image I'm getting in my head anyway). The resulting ADP can be recycled to store more energy later on.

There are two main kinds of cellular respiration: aerobic and anaerobic. Let's take a look at them in greater detail:

Aerobic Respiration

Aerobic respiration requires oxygen, but it's more effective. In the first phase, known as glycolysis, the glucose molecule is broken down in the cytoplasm over ten steps (nope, respiration isn't as simple as you might want it to be) to two molecules of pyruvic acid, simultaneously releasing two molecules of ATP.

The pyruvic acid molecules then enter a mitochondrion, where two more series of reactions occur. The first is known as the Krebs cycle (or citric acid cycle), which forms two more ATP molecules per two molecules of pyruvic acid (i.e. two more ATP molecules per original glucose molecule). The next series of reactions is called the electron transport system and can produce up to 34 molecules of ATP per original molecule of glucose.

The maximum yield from aerobic respiration of one mole of glucose is therefore 38 molecules of ATP (2 + 2 + 34). However, this is a maximum only- often the actual yield is lower than this.

Anaerobic Respiration

Anaerobic respiration is respiration without oxygen. Once glycolysis is complete, if oxygen is not present or is only present in insufficient amounts for the amount of energy required (during exercise, for instance), the pyruvic acid molecules will be converted into lactic acid, allowing cells to release some energy without oxygen. Lactic acid is what causes muscle pain and fatigue during exercise.

Lactic acid is then transported to the liver via the blood, where it recombines with oxygen to form glucose and then glycogen. I've spoken a little bit about glycogen in my second post about the digestive system. It's a molecule that can convert back and forth into glucose, and is thus a form of energy reserve for the body.

If you've been reading closely you might have noticed that, although anaerobic respiration is meant to be respiration without oxygen, the process converting lactic acid to glucose and glycogen does require oxygen! This is partly why you have to breathe heavily after exercising- the body needs lots of oxygen to repay this "oxygen debt." This extra oxygen is also known as recovery oxygen.

Uses of Energy

Why do our cells need so much energy? I've touched on the need for cells to break molecules down and build them up, but why are our cells doing this, exactly? Here's a quick list of functions that our cells need energy for:

• Building complex molecules (e.g. proteins) by combining smaller ones (synthesis)
• Cell division and growth
• Movement of cell organelles
• Movement of the whole cell
• Maintaining cell organisation
• Active transport
• Transmission of nerve impulses

Let's have a look at the first one in a little more detail.

Protein Synthesis

As mentioned in one of my earlier posts, proteins are made up of amino acids which are linked together via condensation reactions (the amino acids combine by losing molecules of water). The proteins formed are determined by genes, which are parts of the DNA. Each amino acid corresponds to a sequence of three bases on the DNA. For example, the sequence cytosine-adenine-guanine on the DNA corresponds to the amino acid valine.

Although the DNA is located in the nucleus, the proteins are assembled in the ribosomes in the cytoplasm. One type of RNA (ribonucleic acid), known as messenger RNA, is used to transfer the genetic code from the DNA to the ribosomes. RNA, as opposed to DNA, has only one strand of sugars, phosphates and nucleotide bases. Also, instead of thymine, it has a different base called uracil (which, like thymine, pairs up with alanine).

To transfer the message across, parts of the DNA molecule break apart, allowing the messenger RNA to form by matching up bases. The messenger RNA then leaves the nucleus and attaches to a ribosome.

In the cytoplasm, a different kind of RNA, transfer RNA, brings the amino acids to the ribosomes. Each transfer RNA molecule has three bases that attach to the messenger RNA molecule according to the rules of complementary bases and all that.

Now, the funky part is that there's a lot of opposites going on. Firstly, the messenger RNA is made up of bases that complement (i.e. are pretty much opposite to) those on the selected strand on DNA. Not to worry, though- the messenger RNA then matches up with transfer RNA containing "opposites" of the messenger RNA- rather, the same as what was originally on the DNA!

Confused? Here's an example...

Original DNA strand: CAGTTCCGA

Messenger RNA created: GUCAAGGCU

Transfer RNA required: CAG TTC CGA (i.e. the same as the original DNA strand. Not sure whether it should be uracil instead of thymine- the text of my book says that RNA contains uracil, not thymine, but the diagrams have thymine instead of uracil...)

CAG corresponds to valine, TTC corresponds to lysine and CGA corresponds to alanine. Hence this original DNA strand corresponds to a protein chain consisting of valine, lysine and alanine, in that order.

That's pretty much all from me on this topic. If you're still awake, though, read on for some info that might not be useful now but might be later if you decide to undertake further study:

Radioactive Tracers

By now you might be wondering how scientists know so much about the inner life of cells. This is all done through scientific experimentation (obviously), some of which involves the use of tracers. Tracers are substances that can be identified and followed. Many are made up of radioactive isotopes (forms of an element that are unstable and break down, emitting radiation). (If you want to know more about isotopes, check out one of my earlier Chemistry posts on Atomic Structure and the Periodic Table.)

Now for an example of how radioactive tracers can be used to help us learn about the body! In one experiment, amino acids were labelled with radioactive tracers and injected into the blood of rats. A few minutes later radioactive protein was found in the ribosomes of the pancreas of the rat. This helped scientists discover that proteins are made from amino acids at the ribosomes. Some other cellular processes have been discovered through this use of radioactive isotopes.

Cell Structure and Mitosis

Today, as the title implies, I'm going to be focusing on cells. Cells are the building blocks that make up tissues, which then make up organs, which then make up systems of the body (digestive system, respiratory system etc.) which then combine to form the organism, which is a word not to be confused with a word that looks somewhat similar but without the "ni" in the middle.

Much of the stuff I'm going to talk about is probably stuff that you've covered in years 8-10, so I'm just going to gloss over it. If you want more details on anything, feel free to post down in the comments!

Components of Cells

Most cells are specialised for certain purposes in the body, and hence they all have slightly different structures. There are, however, many components that most cells have in common. Let's have a look at them:

• The cell membrane, or plasma membrane, separates the cell from neighbouring cells and the external environment. It is made up of a double layer of lipid molecules as well as various proteins associated with the lipids. The cell membrane allows some substances to get through but not others.
• The cytoplasm is the fluid within the cell. It is 75% to 90% water. Inorganic materials and most carbohydrates are dissolved in the cytoplasm, while other organic compounds such as proteins and lipids are simply suspended in the cytoplasm. The term protoplasm refers to the nucleus and cytoplasm together (i.e. all of the contents of the cell).
• The cell contains a variety of small structures called organelles, which carry out various functions. (This is where I get annoyed at Blogger's apparent lack of an indented list feature.)
• One of these organelles is called the nucleus. Nearly all cells contain one, though some contain more, and some don't have a nucleus at all. It is the largest organelle, is normally oval or spherical in shape, and is surrounded by a nuclear membrane. This nuclear membrane is actually a double membrane- two membranes separated by a space. The membrane contains many gaps, or nuclear pores, which allow substances to pass through. The nucleus is often thought of as the "control centre" for the cell as it contains DNA (deoxyribonucleic acid), which determine which proteins and enzymes a cell can make, which then determine which chemical reactions can take place in the cell.
• The nucleus contains an area called the nucleolus, which mainly consists of RNA (ribonucleic acid), which plays a role in synthesising proteins.
• The nucleoplasm is the fluid inside the nucleus. Suspended in the nucleoplasm are chromatin threads (long strands of DNA) and the nucleolus.
• Ribosomes are very small, spherical organelles that can be found either floating around in the cytoplasm or attached to membranes within the cells. At the ribosomes, amino acids combine into proteins according to instructions in the DNA.
• The endoplasmic reticulum are pairs of parallel membranes that extend through the cytoplasm and connect to the cell membrane and nuclear membrane. There are two types: rough (granular) endoplasmic reticulum and smooth (agranular) endoplasmic reticulum. The main difference is that the former has ribosomes attached to it, while the latter does not. The membranes of the endoplasmic reticulum is thought to provide a surface where chemical reactions can take place, while the channels are for storing or transporting molecules.
• The Golgi apparatus, a series of flattened, membranous bags, is like the "post office" of the cell, as it is involved in packaging up proteins and so forth to prepare them for secretion from the cell. At the Golgi apparatus, molecules such as sugar, sulfate or phosphate are added, before the edges of the Golgi apparatus pinch off to form a vesicle (a liquid-filled sac) containing the proteins, which then travel through the cytoplasm to the cell membrane, where they then leave the cell.
• Lysosomes are small membrane-bounded spheres formed by the Golgi apparatus. Lysosomes contain digestive enzymes which break down material transported into the cell via vesicles as well as worn-out organelles.
• Mitochondria (singular mitochondrion) are the energy centres of the cell. They have two membranes, which contain enzymes which are involved in energy-releasing cellular reactions. The inner membrane has a series of folds (which the outer membrane lacks), increasing the surface area for chemical reactions.
• The centrioles are a pair of cylindrical structures, which are made up of rings of very fine tubules. The centrioles are located at right angles to each other. They are located near the nucleus and are involved in cellular reproduction.
• Cilia are small hair-like projections that move the whole cell, or move substances over the cell. They can be found in the respiratory tract (see my post on the respiratory system) for more details. Flagella are longer and there are only one or two per cell. In humans, only sperm cells have flagella.
• The cytoskeleton gives the cell its shape and assists the cell in moving. The cytoskeleton consists of microtubules and microfilaments. The former keeps organelles in place or moves them around the cell, while the later moves materials around the cytoplasm or moves the entire cell.
• Some cells also contain other chemical substances which aren't considered to be part of the structure of the cell. For example, red blood cells contain haemoglobin. These extra chemical substances are known as inclusions.

As well as permanent components of the cell, there are also many substances that move in and out of cells. For example, oxygen is taken into the cell to be used in cellular respiration, while carbon dioxide, a product of cellular respiration, is released from the cell. Which brings me to my next point...

Methods of Passing Through a Cell Membrane

• First up are our good ol' friends diffusion and osmosis. You can read more about them in a previous post of mine. The main idea is that the particles of a substance will move from an area of higher concentration to an area of lower concentration in order to even out the concentrations. Thus if the concentration of the given substance in the cell is lower, stuff will move into the cell; if the concentration in the cell is higher, stuff will move out of the cell.
• Active transport uses the energy of the cell in order to force particles to move from an area of lower concentration to an area of higher concentration (i.e. the opposite of diffusion).
• Endocytosis ("endo" = inwards) is yet another method of absorbing substances. The outer membrane folds around the substance in question, leaving the substance floating around the cytoplasm in a liquid-filled sac called a vesicle. (Don't worry, I'll provide diagrams in a sec.) There are two forms- if the substance is solid, it's called phagocytosis (cell-eating), whereas if the substance is liquid, it's called pinocytosis (cell-drinking).
• Exocytosis ("exo" = outwards) is the opposite of endocytosis- that is to say, the vesicles are pushed out of the cell, rather than taken into the cell.

And here's a very crude Paint diagram of what endocytosis looks like! (Or what I think it looks like...)

Basically, if you haven't already guessed, the outline is the cell membrane while the purple is the cytoplasm and all the other crap in the cell (protoplasm?). It basically folds around the little red dot, which is the particle to be "swallowed up" by the cell. Eventually the cell membrane joins up again, leaving a little bubble (called a vesicle) in the cell which contains the substance in question.

Cell Reproduction via Mitosis

The last bit I need to cover in this post is how cells reproduce via mitosis. There are at least two different ways that cells reproduce (the other being meiosis), but I'm only going to cover mitosis for now.

Cells reproduce for a variety of reasons- they reproduce to replace damaged or worn out cells as well as to help the body grow. While cells reproduce, their DNA (containing genetic information) needs to be passed along. During mitosis, this is achieved by replicating the DNA before the division process begins.

Mitosis is usually broken down into four stages (five if you include interphase), though in reality it is quite a fluid process- the cell doesn't just do one stage at a time because it looks nice in human bio books! Let's have a look at the four (five?) stages:

1. Interphase is the period between divisions. While the cell is not dividing, the DNA is in the form of long strands known as chromatin. During this phase, the DNA molecules form exact copies of themselves. This results in twice the amount of DNA, which is great, because this means that at the end you'll end up with two cells, each with the normal amount of DNA!
2. Prophase is the first actual stage of mitosis. The two pairs of centrioles become visible and move to opposite ends of the cell. Microtubules begin to radiate from them, eventually forming a framework of fibres called a spindle. Meanwhile, the nuclear membrane breaks down and the nucleus disappears. The chromatin threads become tightly coiled into roughly X-shaped chromosomes (well they're roughly X-shaped in diagrams anyway), made up of two chromatids which are joined at a point called the centromere. The two chromatids are identical DNA molecules (remember how the DNA replicated itself during interphase?). At the end of prophase, the chromatid pairs begin to move towards the centre of the cell.
3. Metaphase is the second stage of mitosis. During this stage, the chromosomes line up on the equator of the spindle. The centromere of each chromosome is attached to a spindle fibre.
4. Anaphase is the step where the chromatids actually begin to separate off into different parts of the cell, probably because of some kind of pull from the spindle fibres. Apparently the chromatids get called chromosomes once they get pulled apart (!).
5. Telophase is the final step- the two sets of chromosomes group together at opposite ends of the cell, nuclear membranes form around each group, a nucleolus appears in each new nucleus, the spindle fibres disappear, and the chromosomes uncoil to become chromatin threads again. The centrioles then duplicate, ready for the next division. The cytoplasm also begins to divide during telophase: a furrow appears between the two nuclei, which gradually deepens until the cell is split in two.

The original cell is called the parent cell; the two cells formed at the end are known as daughter cells. (Why not sons? I don't know.) The genetic information in the daughter cells is the same as that in the parent cells. However, different genes may be activated depending on whether or not the cell needs to specialise (e.g. stomach cells need to secrete digestive enzymes, nerve cells need to transmit information, and so on). This is called differentiation, and is not to be confused with finding the gradients of curves in maths.

The Structure of DNA

This is a topic I'll cover in more depth in my next post, but here's just a quick overview:

• Each DNA molecule consists of two strands, twisted into a double helix.
• The strands contain alternating sugars and phosphates.
• The strands are linked by pairs of nitrogen bases
• A sugar, phosphate and nitrogen base combined is called a nucleotide (see my post on organic compounds related to human bio)
• There are four kinds of nitrogen bases: adenine, thymine, cytosine and guanine
• The order of the bases is called the genetic code. Each gene consists of up to 1000 pairs of bases

Replication of DNA

This won't be covered in the next post, but understanding this will help you understand protein synthesis (which I am going to cover in the next post). So listen up!

As I've mentioned before, the DNA molecules undergo replication during interphase. During replication, the two strands separate. Nucleotides then come in and pair up with the nucleotides of the two strands, resulting in two full strands of DNA.

But wait! you might say. What is there to stop a different combination of nucleotides from pairing up with the two lonely strands of DNA?

The answer to that is quite simple: each nucleotide only has one other kind of nucleotide that it can pair up with. Adenine can only pair up with thymine, cytosine can only pair up with guanine, and so on.

As an example- let's just say our original strand of DNA has 5 pairs of bases like this:

A - T

G - C

G - C

T - A

C - G

When the DNA splits, you'll get these two strands:

A -

G -

G - 

T - 

C -

and

 - T

 - C

 - C

 - A

 - G

Given the rules that I just told you about which bases can pair with which, work out which nucleotides will then bond with each strand of DNA. You'll find that you'll end up with two exactly identical strands of DNA.

Now onto the next post- cellular respiration and protein synthesis!

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.)