In our last video, we introduced the circulatory system, and saw how the heart acts as the system's pump. In this video though, we'll take a closer look at the blood vessels, specifically the arteries, capillaries, and veins. To quickly summarise, the arteries carry blood away from the heart, the capillaries exchange nutrients and oxygen with the tissues, and the veins carry the blood back to the heart.
As you take a closer look at each, the key thing to notice is how the structure of each type of vessel is related to its function. As arteries carry blood directly from the heart, the blood inside them is going to be at really high pressures, because it's just been pumped out of the ventricles. In order to bear these high pressures, the arteries need to be both strong and elastic. And to achieve this, they wear a thick layer of muscle tissue to keep them strong. and a thick layer of elastic tissue so that they can stretch and recoil.
These two layers though mean that the wall of the artery is quite thick when compared to its lumen, which is what we call the hole or space in the middle of the vessel. Once the arteries have transported the blood to the different parts of the body, it enters the capillaries. These vessels are really small, and come into close contact with all of the cells in our body.
They're rolled is to exchange substances with our cells, giving them useful nutrients and oxygen, and taking away waste products like carbon dioxide. If we take a closer look at a single capillary, we can see how they're adapted to this function. For example, their walls are only a single cell thick, and they're also permeable, which means that substances can easily diffuse through them. Because they're so tiny, the lumen of an individual capillary is also tiny, but there are so many of them that their total cross-sectional area, which is the area of all the capillaries added together, is actually huge, far higher than for arteries. This means that the pressure of the blood is much lower than in the arteries, and so it flows more slowly, which gives the blood time to exchange things with the tissues.
Finally, these tiny capillaries start to join up into larger vessels, called veins, which transport the blood back to the heart. Veins are relatively large, with by far the biggest lupins of any blood vessels. They also have relatively thin walls, with only small layers of elastic fibres and smooth muscle.
This works fine though, because the blood that they carry is at very low pressures, so the walls don't need to be very strong. The important thing to remember about veins though, is that they have valves. which prevent blood from flowing backwards, ensuring that it always flows towards the heart.
Before we finish, let's quickly cover how to calculate the rate of blood flow. A typical question could look something like this. 2560 ml of blood passes through the aorta, which is the big artery that leaves the heart, in 8 minutes. Calculate the rate of blood flow through the aorta in ml per minute.
So in a question like this, we would just divide how much blood has flowed, so 2560, by the time it took, so 8 minutes, which gives us 320ml per minute. And that's all for the blood vessels. If you haven't seen our other videos on the heart and the blood yet, we've put the links for those in the description below. Otherwise, cheers for watching, and we'll see you next time.