0610 · 9.1Circulatory systems

Why circulatory systems are needed

0:00Imagine you are sprinting for a bus. Every muscle in your legs suddenly needs far more oxygen and far more glucose than it needed a minute ago, and it needs them right now. Your body cannot go and fetch them one cell at a time. So how does it get the right supplies to billions of cells, fast, and take the waste away again? The answer is a circulatory system.

0:19By the end of this video you will be able to describe what a circulatory system is, compare the single circulation of a fish with the double circulation of a mammal, and explain why the double circulation is such a big advantage. First, why do large animals need a circulatory system at all? A very small animal can get oxygen just by diffusion, because nothing is far from the surface.

0:41But in a large animal, most cells are a long way from the outside world, and diffusion over that distance is far too slow. So large animals need a transport system that carries substances around the body in a fluid, and that fluid is blood. The syllabus describes the circulatory system as a system of blood vessels with a pump and valves, which together make sure the blood flows in one direction only.

1:04Let's take those three parts in turn. The blood vessels are the tubes that carry the blood around the body. The pump is the heart, which squeezes and pushes the blood into those vessels. And the valves are small flaps that only open one way. When blood tries to slip backwards, the valve flaps fill with blood and close, like a door that only opens in one direction. Why does the direction matter so much?

1:25Because blood that swirls back and forth delivers nothing new. One-way flow means that fresh blood is always arriving where it is needed, and used blood is always leaving. That is what turns a bag of fluid into a working delivery service.

Single circulation in fish

1:39Now to the first system you need to describe: the single circulation of a fish. Start at the fish's heart. The heart pumps deoxygenated blood to the gills. At the gills, the blood picks up oxygen from the water, and it gives up carbon dioxide. That blood, now oxygenated, does not go back to the heart. It carries straight on around the rest of the body, delivering oxygen to the organs and muscles.

2:01Then the deoxygenated blood returns to the heart, and the journey begins again. That is why it is called a single circulation. In one complete trip around the body, the blood passes through the heart only once. The path is heart, gills, body, heart. This design has a limitation, and it is worth understanding why. The blood leaves the heart under pressure. But then it is forced through the tiny capillaries of the gills.

2:26Those capillaries are very narrow, so the blood loses a lot of pressure and slows down. By the time it leaves the gills and heads for the body, it has much less pressure than it started with. So the delivery of oxygen to the body is fairly slow. For a fish, that is fine. A fish swims in water and does not need to keep its body warm, so its demand for energy is moderate.

Double circulation in mammals

2:47Now the second system: the double circulation of a mammal. Here, the heart is really acting as two pumps side by side. One pump sends blood to the lungs, and the other sends blood to the rest of the body. Follow one red blood cell around. It leaves the right side of the heart, carrying little oxygen. It travels to the lungs, where it picks up oxygen and gets rid of carbon dioxide.

3:08Then it returns to the left side of the heart. That was the first loop, the heart to the lungs and back. Now the left side of the heart pumps that oxygen-rich blood out to the body, to the brain, the muscles, the gut and everything else. There the blood gives up its oxygen, and it comes back to the right side of the heart with little oxygen once more.

3:25That is the second loop, the heart to the body and back. So in one complete trip, the blood goes through the heart twice. The path is heart, lungs, heart, body, back to the heart. That is why we call it a double circulation. Notice the pattern: the lungs loop is called the pulmonary circulation, and the body loop is called the systemic circulation. Now the important part: why is this better?

3:49There are three advantages, and each of them follows from the same idea. The first advantage is pressure. Remember that in the fish, the blood loses pressure passing through the gills. In the mammal, the blood goes through the lungs, loses pressure there, and then goes back to the heart before it goes to the body. So the heart can give it a second push. The blood reaches the body at high pressure and travels fast.

4:11That means oxygen and glucose are delivered quickly, and waste is carried away quickly. The second advantage follows from that. Because the delivery is fast, the body can supply much more oxygen to its cells. That allows a much higher rate of respiration, which releases more energy. Mammals are warm-blooded and very active, so they need exactly that: plenty of energy to keep a constant body temperature and to run around.

4:34The third advantage is that the two loops can have different pressures. The lungs contain very thin, delicate capillaries. If the blood arrived at full body pressure, those thin walls could be damaged, and fluid could be forced into the lungs. So the right side of the heart pumps at a lower pressure to the lungs, and the left side pumps at a higher pressure to the body. Each loop gets the pressure it needs.

4:56There is also a bonus. In the mammal's heart, oxygenated blood and deoxygenated blood are kept in separate sides. They do not mix. So the blood going to the body is full of oxygen, rather than a mixture. Let's put that side by side. A fish has a single circulation. The blood passes through the heart once per trip, and the pressure to the body is low. A mammal has a double circulation.

5:20The blood passes through the heart twice per trip, and the pressure to the body is high.

Exam answers and common mistakes

5:25Now let's see how this might be tested. Suppose the question says: explain the advantage of a double circulation. A weak answer says, "It is better because blood goes round twice." That describes the system, but it does not explain anything. A strong answer says, "The blood returns to the heart after the lungs, so its pressure can be raised again. This means the blood flows to the body at high pressure, so oxygen and glucose are delivered faster.

5:50This supports a higher rate of respiration." See how the reasoning connects every step. That is what earns the marks. Another likely question is to describe the circulation of a fish. Say that the heart pumps blood to the gills, then to the body, then back to the heart, and that the blood passes through the heart only once. Then, if you are asked for the disadvantage, say the blood pressure drops after the gills.

6:13Finally, let's clear up some common mistakes. First, some students think a double circulation means two hearts. No, it does not. It is one heart, with a right side and a left side, and the blood passes through it twice. Second, some students say that arteries carry oxygenated blood and veins carry deoxygenated blood. That is usually true, but there are exceptions, and they are the ones examiners love. The pulmonary artery carries deoxygenated blood to the lungs, and the pulmonary vein carries oxygenated blood back to the heart.

6:44So the rule is about direction, not oxygen. Arteries carry blood away from the heart, and veins carry blood towards the heart. Third, when describing a fish, do not say that the blood goes to the heart twice. It goes through the heart once per trip around the body. Let's recap. A circulatory system is a set of blood vessels with a pump, the heart, and valves that keep the blood flowing one way.

7:07In a fish, the circulation is single: heart, gills, body, heart, so blood passes through the heart once, and the pressure falls after the gills. In a mammal, the circulation is double: one loop to the lungs and one to the body, so blood passes through the heart twice. And the double circulation is better, because the heart can raise the pressure again after the lungs, so the blood reaches the body quickly, supporting a high rate of respiration, while the lungs are protected from high pressure.

7:34That is the key idea to hold on to: a second push means faster delivery.