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The Blood System

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Cells in mammals require a constant supply of nutrients and oxygen, and a way to remove waste products. Blood is great, as it does all that. Blood needs a way of getting to all cells of the body, a way to… circulate. Without that, blood would just get pulled by gravity towards the centre of the earth. Not a pretty sight I’m afraid.


There are two circulations in the body:


1. The pulmonary circulation takes blood from the heart, pumping it to the lungs in order to oxygenate it.


2. The systemic circulation takes blood from the heart to everywhere else. Eyes, legs, hand, bum, you name it.


Key point: the oxygen-rich blood vessels entering an organ are called arteries, while the oxygen-depleted blood vessels leaving an organ are called veins.


So a blood vessel entering the liver or kidneys would be an artery. A blood vessel leaving the liver or kidneys would be a vein.


The liver attribute is hepatic ( for example, the working cell unit in the liver is the hepatic cell), while the kidney attribute is renal (for example, renal failure). So what would the blood vessel entering the liver be called?


…pressing question.

…pressing on.


…the hepatic artery! Same principle applies to the rest: the hepatic vein, the renal artery and the renal vein.


There’s a catch (welcome to biology). In the case of the blood vessels leaving or entering the lungs, the rules are reversed. The pulmonary vein carries oxygenated blood to the heart, while the pulmonary artery carries deoxygenated blood into the lungs.



You also need to learn the blood vessels entering and leaving the heart.


1. The aorta is the main artery which carries oxygen-rich blood to the rest of the body.


2. The coronary arteries supply blood to the heart itself (and they are the affected arteries in coronary heart disease).


3. The superior vena cava and the inferior vena cava bring deoxygenated blood from the upper half of the body, and the lower part of the body respectively.


It’s all really logical… apart from the bit on the lungs.


There are 4 types of blood vessels: arteries, arterioles, capillaries and veins. Each type has a different function, and therefore a different structure. Here is a diagram of how arteries branch off into arterioles, then into capillaries, and eventually into veins as the blood becomes deoxygenated.


So what do they do?


Arteries must be able to counteract the pressure created by every heart beat by recoiling, so that the stream of blood is smoothened.


Arterioles are able to direct blood supply to certain parts of the body, so must be able to constrict or dilate.


Capillaries are the site of substance exchange as well as diffusion, so their walls must be thin enough for this to happen quickly.


Veins are unique as they contain valves which prevent backflow of blood.



From the above picture is it clear that there are important structural differences between arteries and veins, which reflect their different functions. Firstly, veins have valves while arteries do not*. Secondly, arteries have a narrower lumen (hollow diameter) than veins. Thirdly, arteries have a thicker wall of muscle and elastic tissue.


Arteries and arterioles are similar. The key difference is that arteries have more elastic tissue than muscle, while arterioles have more muscle than elastic tissue.


Capillaries are 1-cell thick, making them very thin and permeable.


Tissue Fluid

Tissue fluid is what surrounds all respiring cells. This is where they draw their nutrients from, and where they eliminate waste products into. Tissue fluid movement back and forth between cells and blood is directed by one of two things: either the hydrostatic pressure exerted by the blood rushing through arteries; or by osmosis caused by the proteins within blood.


Therefore, as blood goes through the artery to respiring tissues, the tissue fluid is forced out of the blood, into the tissues. As blood passes through it loses pressure, so that the tissue fluid now enters the subsequent vein due to osmosis. This occurs because the water potential in the vein is lower than outside due to the proteins in the blood which reduce it. This sums up the circulation of tissue fluid.

*except for the pulmonary artery and the aorta

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