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?
…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.
Arteries and veins contain squamous endothelium (flat, single-celled layer), smooth muscle tissue alongside elastic and fibrous tissue.
From the diagram it is 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 squamous endothelium, making them very thin and permeable.
*except for the pulmonary artery and the aorta
Atheroma, aneurysm, thrombosis, myocardial infarction?
Read on to learn what these words mean. Coronary heart disease is a major cause of death in the UK and much of the rest of the world. Risk factors associated with CHD are diet, blood cholesterol, cigarette smoking and high blood pressure.
This is the build up of fatty material in the walls of arteries. It is often the underlying cause that leads to heart disease.
As you can see, it leads to the narrowing of arteries, causing a lowering of blood supply. Atheroma is associated with an increased risk of aneurysm and thrombosis. Aneurysm is a ballooning of the artery which weakens the affected area. This requires urgent treatment, otherwise it is fatal if the balloon “pops”. Thrombosis is a blood clot stuck in a vessel which results in less blood supply to a specific area, and the subsequent affected tissues may be starved of blood and die.
If the blood supply to the heart muscle is stopped, then a myocardial infarction occurs. This is the scientific name for a heart attack. The heart muscle (or part of it) dies as a result of a lack of oxygen from the blood.
These are different kinds of aneurysm (you don’t need to learn the names).
Stuff like eggs and meat contain high levels of cholesterol which can lead up to atheroma. Plants on the other hand have little cholesterol (Disclaimer: I’m a vegetarian ^_^), so by far the easiest way to cut on cholesterol is to remove meat from the diet, especially fatty meats. Cholesterol levels are also genetically inherited, in which case diet is even more important in preventing CHD. There are two kinds of cholesterol, HDL and LDL. HDL has a positive impact on health by removing blood cholesterol and sending it to the liver; LDL has a negative impact by doing the exact opposite – carrying cholesterol from the liver to other cells in the body.
Plot twist however, the sugar industry might have paid scientists to publish fat-related studies to take away any emphasis on the role of sugar in disease. It turns out that excess sugar is far more troublesome than fat. Excess sugar leads to inflammation which in turn primes these tissues for malfunction and disease.
The mechanism by which smoking causes CHD (specifically atherosclerosis = hardening of the arteries) is complex and not fully mapped out yet. However, it is known that certain substances contained in tobacco lead to artery constriction, which in turn raises blood pressure.