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.