0610 · 9.3Blood vessels

Types and structures of blood vessels

0:00If you could take every blood vessel out of one human body and lay them end to end, they would stretch roughly 100,000 kilometres. That is more than twice around the Earth. Almost all of that length is made of tubes so thin you could not see one without a microscope. So why does the body need so many vessels, and why are they not all the same? By the end of this video you will be able to describe the structure of arteries, veins and capillaries, explain how each structure matches the job it does, and identify the main blood vessels going to and from the heart, the lungs, the kidneys and the liver.

0:33There are three kinds of blood vessel. Arteries carry blood away from the heart. Veins carry blood back towards the heart. And capillaries are the tiny vessels that link them. An artery divides again and again into smaller and smaller branches, until they become capillaries. The capillaries run through every tissue, and then they join up again to form veins, which take the blood back to the heart. The syllabus asks you to compare their structure using three features: how thick the wall is, how wide the lumen is, and whether there are valves.

1:03The lumen is simply the hollow space in the middle of the vessel, where the blood actually flows. Start with an artery. If you cut one across, you see a thick wall and a narrow lumen. The wall contains a thick layer of muscle and a lot of elastic tissue, the stretchy material that lets the artery expand and spring back. There are no valves along the length of an artery.

1:23Now a vein. It has a thin wall, with much less muscle and elastic tissue, and a wide lumen. Because the wall is thin, a vein is often squashed or floppy looking when you see it under the microscope, while an artery keeps its round shape. And veins have valves, which we will come back to in a moment. Finally, a capillary. Its wall is just one cell thick, the thinnest wall of all.

1:46Its lumen is very narrow, only just wide enough for red blood cells to squeeze through in single file. So here is the comparison. An artery has a thick wall, a narrow lumen and no valves. A vein has a thin wall, a wide lumen and valves. A capillary has a wall one cell thick, a very narrow lumen and no valves.

Arteries and veins: pressure adaptations

2:06Now for the more interesting part. Why is each vessel built like that? The answer for arteries and veins comes down to one thing: the pressure of the blood inside them. Arteries receive blood straight from the ventricles. Every time the ventricles contract, a surge of blood is forced into the arteries at high pressure. So arteries need a thick, strong wall that can withstand that high pressure without bursting.

2:29The elastic tissue in the wall matters too. When the surge of blood arrives, the elastic wall stretches. Then, between beats, it springs back and pushes on the blood. This keeps the blood moving forward and keeps the pressure high between heartbeats, so the flow is smoother. That stretch and recoil is actually what you feel as your pulse. The narrow lumen also helps to keep the blood at high pressure.

2:51By the time blood reaches the veins, it has passed through the narrow capillaries, and it has lost most of its pressure. So veins carry blood at low pressure. They do not need a thick wall to withstand it, so their walls are thin. Their wide lumen means there is less resistance to the flow, so the blood can still move easily, even at low pressure. But low pressure causes a problem.

3:12Blood in the veins of your legs has to travel upwards to your heart, against gravity. At low pressure it could easily flow backwards and pool in your feet. That is what the valves are for. Valves in veins let blood flow only towards the heart. If blood starts to flow backwards, it fills the pocket-shaped flaps of the valve and pushes them shut. And how does the blood get pushed along in the first place?

3:34Many veins run between your skeletal muscles. When you walk and your leg muscles contract, they squeeze the veins, pushing blood along them. Because of the valves, the only way it can go is up, towards the heart. So that is the pattern for arteries and veins. High pressure, so arteries have thick, muscular, elastic walls. Low pressure, so veins have thin walls, a wide lumen and valves to stop the blood flowing backwards.

Capillaries and exchange

3:58Now the capillaries. The arteries and veins are really just transport. The capillaries are where the real work happens. Their function is exchange: they allow substances to pass between the blood and the cells of the tissues. Oxygen and glucose move out of the blood in the capillaries, into the cells, where they are used for respiration. Carbon dioxide and other waste products move out of the cells, into the blood, to be carried away.

4:23Capillaries also carry things like hormones and nutrients to the cells that need them. Now, how is the structure of a capillary related to this function? First, the wall is only one cell thick, so the diffusion distance between the blood and the cells is very short, and substances can diffuse across quickly. Second, capillaries are very narrow and they branch into a huge network. This means there is a capillary very close to almost every cell in the body, so no cell is far from a supply.

4:50And all those vessels together give a very large surface area for exchange. Third, the narrow lumen means the red blood cells squeeze through slowly, in single file, pressed close against the wall. That slow flow gives time for substances to diffuse in and out. The walls of capillaries are also slightly leaky. Some of the plasma, the liquid part of the blood, is pushed out between the cells. This fluid bathes the cells and carries dissolved substances straight to them.

Main blood vessels and organs

5:18Now let's name the main blood vessels, starting with the heart. There are four you need to know. The vena cava brings deoxygenated blood from the body back to the right atrium. The pulmonary artery takes deoxygenated blood from the right ventricle to the lungs. The pulmonary vein brings oxygenated blood from the lungs back to the left atrium. And the aorta takes oxygenated blood from the left ventricle out to the rest of the body.

5:43That gives you the lungs as well. The vessel that carries blood to the lungs is the pulmonary artery, and the vessel that carries blood away from the lungs is the pulmonary vein. The word pulmonary always means to do with the lungs. Next, the kidneys. The word for kidney is renal. The renal artery branches off the aorta and carries blood into the kidney. The renal vein carries blood away from the kidney and back into the vena cava.

6:09Notice the naming pattern, because it will save you a lot of memorising. The first word tells you the organ, pulmonary for the lungs and renal for the kidneys. The second word tells you the direction. An artery takes blood to the organ, away from the heart. A vein takes blood away from the organ, back towards the heart. The liver is the special one, because it has blood arriving from two places.

6:29The word for liver is hepatic. Like any organ, the liver needs oxygen, and it gets oxygenated blood through the hepatic artery, which branches off the aorta. But the liver also receives blood from the gut. After a meal, the small intestine absorbs digested food, such as glucose and amino acids, into its blood. That blood does not go straight back to the heart. Instead, it travels along the hepatic portal vein, directly to the liver.

6:55This lets the liver deal with the absorbed nutrients first, for example storing some of the glucose, before the blood carries on to the rest of the body. The hepatic portal vein is unusual. Most veins take blood back towards the heart, but this one carries blood from the capillaries of the gut to the capillaries of the liver. Finally, the hepatic veins carry blood away from the liver and into the vena cava, to go back to the heart.

7:19So the liver has three vessels to remember. The hepatic artery brings oxygenated blood in. The hepatic portal vein brings blood rich in absorbed nutrients in from the gut. And the hepatic veins take blood out, to the vena cava.

Identifying vessels in exam questions

7:32Let's see how this can be tested. A common question gives you a photomicrograph or a table, and asks you to identify the vessels. Suppose vessel A has a wall 1 millimetre thick and a lumen 4 millimetres wide. Vessel B has a wall half a millimetre thick and a lumen 5 millimetres wide. And vessel C has a wall one cell thick, and a lumen of about 8 micrometres, just wide enough for one red blood cell.

7:57C is easy. A wall one cell thick is a capillary. For A and B, don't just look at the size of the vessel. Compare the wall with the lumen. In A, the wall is thick compared with its narrow lumen, 1 millimetre of wall around a 4 millimetre lumen. So A is an artery. In B, the wall is thin compared with its wide lumen, only half a millimetre of wall around a 5 millimetre lumen.

8:19So B is a vein. If the image shows a valve, that is another clue that it is a vein. Comparing the wall with the lumen matters because a big vein can have a thicker wall than a small artery. What gives it away is the thickness of the wall relative to the lumen. If the question then asks you to explain the structure, always link it to pressure. For example: the artery has a thick wall with elastic tissue because it carries blood at high pressure from the heart, so the wall withstands the pressure and stretches and recoils with each heartbeat.

8:49Now some common mistakes. First, saying that arteries always carry oxygenated blood. The rule is about direction, not oxygen. The pulmonary artery carries deoxygenated blood, and the pulmonary vein carries oxygenated blood. Second, saying that arteries have valves along their length. They do not. The high pressure from the heart keeps blood moving forward. The only valves near arteries are the semilunar valves where they leave the heart. It is the veins that have valves along their length.

9:19Third, thinking that the blood in veins is actually blue. Deoxygenated blood is a dark red. Diagrams colour it blue just to show which blood has less oxygen. And fourth, when explaining capillaries, don't just say the wall is thin. Say it is one cell thick, so the diffusion distance is short. The link to diffusion is what earns the mark. Let's recap. Arteries carry blood away from the heart at high pressure, so they have a thick wall with muscle and elastic tissue, a narrow lumen and no valves.

9:49Veins carry blood back to the heart at low pressure, so they have a thin wall, a wide lumen, and valves to stop the blood flowing backwards. Capillaries are where substances are exchanged between the blood and the cells, and their walls are one cell thick, so the diffusion distance is short. The main vessels of the heart are the vena cava, the aorta, the pulmonary artery and the pulmonary vein.

10:10The lungs are served by the pulmonary artery and the pulmonary vein, and the kidneys by the renal artery and the renal vein. And the liver has three: the hepatic artery, the hepatic portal vein bringing blood from the gut, and the hepatic veins. The big idea to hold on to is that every vessel is built for the pressure it carries and the job it does.