0610 · 9.2Heart

Heart chambers and blood flow

0:00Right now, while you watch this, a muscle about the size of your fist is squeezing roughly 70 times a minute. It did it while you slept last night, and it will do it about 100,000 times today, without you ever telling it to. It never gets a rest day. So what is this pump actually made of, how does it keep blood moving in the right direction, and what happens when it goes wrong?

0:19By the end of this video you will be able to label the parts of the heart, explain why its walls are different thicknesses, describe exactly what happens during one heartbeat, explain why your heart speeds up when you exercise, and describe coronary heart disease and how to reduce the risk of it. In the last video, we saw that mammals have a double circulation. Blood goes from the heart to the lungs and back, and then from the heart to the body and back.

0:44So the heart is really two pumps joined together, a right side that pumps blood to the lungs, and a left side that pumps blood to the body. Blood leaves the heart in arteries, and it comes back to the heart in veins. That is the rule you need: arteries carry blood away from the heart, and veins return blood to the heart. Now let's open the heart up. It has four chambers.

1:05The two at the top are the atria. One of them is called an atrium. The atria receive blood coming back to the heart from the veins. The two at the bottom are the ventricles. The ventricles are the powerful pumping chambers that push blood out into the arteries. So we have the right atrium and the right ventricle on the right side, and the left atrium and the left ventricle on the left side.

1:25Down the middle, between the two sides, is a thick wall of muscle called the septum. And the whole heart is made of a muscular wall, a special kind of muscle called cardiac muscle, which can contract again and again for your entire life without getting tired. There is one trap with heart diagrams that catches almost everyone. A heart diagram is drawn as if you are looking at the person from the front, face to face.

1:47So the heart's right side is on the left of the page, and the heart's left side is on the right of the page. Picture the person standing in front of you. Their right hand is on your left. When you label a diagram, always label it from the person's point of view. Now let's follow the blood through the heart. Deoxygenated blood from the body arrives in a large vein called the vena cava, and it enters the right atrium.

2:09From the right atrium, it goes down into the right ventricle. The right ventricle pumps it out through the pulmonary artery to the lungs. In the lungs the blood picks up oxygen. The oxygenated blood comes back to the heart in the pulmonary vein, and it enters the left atrium. It goes down into the left ventricle, and the left ventricle pumps it out through the biggest artery in the body, the aorta, to the rest of the body.

2:32So, on the right side, deoxygenated blood goes from the vena cava, into the right atrium, then the right ventricle, then out along the pulmonary artery. On the left side, oxygenated blood goes from the pulmonary vein, into the left atrium, then the left ventricle, then out along the aorta.

Valves and coronary arteries

2:50The heart has one-way valves, and they are what keep the blood moving in one direction. There are two kinds you need to be able to identify. The atrioventricular valves sit between each atrium and the ventricle below it. The name tells you where they are: atrio, for atrium, and ventricular, for ventricle. They let blood flow down from the atria into the ventricles, but they stop it flowing back up.

3:12They are held in place by tough strings called tendons, which stop the flaps from turning inside out when the ventricle squeezes hard. The semilunar valves sit at the base of the two arteries leaving the heart, the pulmonary artery and the aorta. They are named after their half-moon shape. They let blood flow out of the ventricles into the arteries, but they stop it falling back into the ventricles when the ventricles relax.

3:36The valves have no muscle of their own. They are pushed open and shut by the blood pressure on each side of them, like a door that can only swing one way. The last structure is on the outside of the heart. Look at the surface of a real heart and you will see blood vessels branching over it. These are the coronary arteries. They come off the aorta, and they supply the heart muscle itself with oxygen and glucose.

3:57That might surprise you. The heart is full of blood, so why does it need its own supply? Because the walls of the heart are thick, and oxygen cannot diffuse from the blood inside the chambers deep into all that muscle quickly enough. The heart muscle needs its own delivery, just like every other muscle.

Heart wall thickness and septum

4:13Now let's explain why the walls of the heart are different thicknesses. If you cut a heart across, you notice straight away that the wall of the left ventricle is much thicker than the wall of the right ventricle. Why? Think about where each one sends its blood. The right ventricle only pumps blood to the lungs, which are right next to the heart. It does not need much force.

4:33In fact, a low pressure is better, because the capillaries in the lungs are very delicate and high pressure could damage them. The left ventricle has to pump blood all the way round the body, to the brain, the kidneys and down to your toes. That needs a much higher pressure. More muscle can contract with more force, so the left ventricle has a thicker muscle wall to produce that higher pressure.

4:53Be careful with one thing. The two ventricles pump the same volume of blood with each beat. The left one does not pump more blood. It pumps the same amount at a higher pressure. The same reasoning explains the second difference. The atria have much thinner walls than the ventricles. The atria only need to push blood a very short distance, just down into the ventricles below them. The ventricles have to push blood out of the heart and into the arteries, to the lungs or round the whole body.

5:18So the ventricles need thicker, stronger muscle walls, and the atria can be thin. Now the septum. Why is it so important? The septum keeps the two sides of the heart apart. The right side holds deoxygenated blood, and the left side holds oxygenated blood. Because of the septum, they cannot mix. So the blood the left ventricle sends to the body is fully oxygenated, and it delivers as much oxygen as possible to the cells for respiration.

5:44If the two mixed, the blood going to the body would carry less oxygen. Some babies are born with a hole in the septum, and one sign of this is that they can look slightly blue and get tired easily, because their blood is carrying less oxygen than it should.

The heartbeat and heart monitoring

5:57Now let's put it all together and describe what happens during one heartbeat. The key idea is that muscle contracts, pressure rises, and the pressure moves the blood and the valves. It begins with the heart relaxed. Blood flows in from the veins and fills both atria. Then the muscles of the atria contract. This raises the pressure in the atria, and it pushes blood down through the open atrioventricular valves, into the ventricles.

6:21The ventricles fill up. Next, the muscles of the ventricles contract. The pressure in the ventricles rises sharply. It is now higher than the pressure in the atria, so the atrioventricular valves are pushed shut. This stops blood flowing back into the atria. The pressure keeps rising until it is higher than in the arteries, and that pushes the semilunar valves open. Blood is forced out into the pulmonary artery and the aorta.

6:46Then the ventricles relax. The pressure in the ventricles falls below the pressure in the arteries, so blood starts to fall back, and it fills the cups of the semilunar valves and pushes them shut. This stops blood flowing back into the ventricles. Meanwhile the atria are filling again, and the whole cycle repeats. The left and right sides do this at the same time: both atria contract together, and then both ventricles contract together.

7:11How can a doctor check that all of this is working? The syllabus lists three ways of monitoring the activity of the heart. The first is listening to the sounds of the valves closing, using a stethoscope. A heartbeat sounds like "lub dub". The "lub" is the atrioventricular valves slamming shut as the ventricles contract. The "dub" is the semilunar valves shutting as the ventricles relax. A doctor who hears an extra swishing sound may suspect a valve that is not closing properly.

7:38The second is the pulse rate. Each time the left ventricle contracts, it sends a surge of blood along the arteries, and you can feel that surge where an artery runs close to the skin, at your wrist or at the side of your neck. Count the pulses in one minute and you have your pulse rate, in beats per minute. It is the same as your heart rate. The third is an ECG, which stands for electrocardiogram.

8:00Sticky pads called electrodes are placed on the skin, and they detect the electrical activity of the heart muscle as it contracts. The machine draws this as a trace with a spike for every beat. A doctor can use the spacing and shape of the trace to see how fast the heart is beating and whether it has a normal rhythm.

Exercise and heart rate

8:16Now let's investigate what physical activity does to the heart rate. This is a practical you might do in class, so let's think it through properly. First, sit still for a few minutes, then measure your resting pulse rate. Count the beats at your wrist for 15 seconds and multiply by 4. Then do a fixed amount of exercise, for example step-ups on a box for 3 minutes, at a steady pace.

8:37Straight after you stop, measure your pulse again, and keep measuring it every minute until it returns to your resting rate. To make it a fair test, keep the type of exercise, how long it lasts and how fast you do it the same, and repeat the whole thing to check your results are reliable. Let's work through some numbers. At rest, a student counts 18 beats in 15 seconds.

8:5818 times 4 is 72, so the resting heart rate is 72 beats per minute. Straight after exercise, they count 35 beats in 15 seconds. 35 times 4 is 140 beats per minute. So the heart rate nearly doubled. Over the next few minutes it falls gradually, 120, then 100, then back down towards 72. So the description is this: during physical activity the heart rate increases, and afterwards it gradually returns to the resting rate.

9:28Notice that it does not drop back straight away. Now, why does it happen? During exercise, your muscles are contracting much more, and contraction needs energy. Muscles get that energy from respiration. So the muscle cells respire faster, and they need more oxygen and more glucose delivered to them, and they make more carbon dioxide, which has to be taken away. The body detects the increase in carbon dioxide in the blood, and the brain sends signals to the heart to beat faster and more strongly.

9:55Blood now flows round the body faster, so oxygen and glucose reach the muscles faster, and carbon dioxide is carried away to the lungs faster. And why does the heart rate stay high after you stop? During hard exercise, the muscles cannot get all the oxygen they need, so some of their respiration is anaerobic, and that makes lactic acid. After exercise, the heart keeps beating fast to carry the lactic acid away to the liver, where it is broken down using oxygen.

10:21You will meet this idea again, called the oxygen debt, when you study respiration.

Coronary heart disease and recap

10:27Finally, what happens when the heart goes wrong? Remember the coronary arteries, which supply the heart muscle with oxygen and glucose. In coronary heart disease, fatty deposits containing cholesterol build up in the walls of the coronary arteries. The lumen, the space the blood flows through, becomes narrower, so less blood gets through. A blood clot can also form on the fatty deposit and block the artery completely. Now follow the consequence.

10:51If a coronary artery is blocked, the heart muscle beyond the blockage gets no oxygen and no glucose. It cannot respire, so it cannot release the energy it needs to contract, and that part of the muscle can die. This is a heart attack. Several things increase the risk of coronary heart disease. The syllabus lists seven risk factors. A diet high in saturated fat and salt. A lack of exercise.

11:13Stress. Smoking. Genetic predisposition, which means it runs in your family. Age, because the risk increases as you get older. And sex, because males are at greater risk than females. Notice that you can do something about some of these, but not others. You cannot change your genes, your age or your sex. But you can change your diet and how much you exercise, and that is exactly what the syllabus asks you to discuss.

11:37Start with diet. Eating less saturated fat, which is found in things like fatty meat, butter and cheese, helps to keep cholesterol levels in the blood lower, so less fatty material is deposited in the coronary arteries. Eating less salt helps to keep blood pressure down. And eating plenty of fruit, vegetables and fibre, and not eating more energy than you use, helps to avoid becoming overweight, which is itself a risk factor.

12:00Now exercise. Regular exercise strengthens the heart muscle, and it helps to lower blood pressure. It also uses up energy, which helps to keep body mass healthy, and it helps to keep cholesterol levels down. It also reduces stress. But a good discussion has balance. Diet and exercise reduce the risk. They do not remove it. A fit person with a strong family history can still develop coronary heart disease, because genetic predisposition, age and sex still matter.

12:27The best way to put it is that a healthy diet and regular exercise lower the risk, and they work best together, alongside not smoking. Let's see how this could be tested. A common question is: explain why the wall of the left ventricle is thicker than the wall of the right ventricle. A weak answer says the left side works harder. That is too vague to earn marks. A strong answer builds the chain.

12:50The left ventricle pumps blood to the whole body, which is a long way, while the right ventricle pumps blood only to the lungs, which are close by. So the left ventricle has more muscle, which contracts with more force, to produce a higher pressure. A lower pressure in the lungs also avoids damaging the delicate capillaries there. Each sentence gives a reason, and each reason is a mark. Now some common mistakes.

13:13First, labelling the diagram the wrong way round. Remember, the heart's left is on the right of the page. Second, saying the valves open and close by themselves, as if they had muscles. They do not. They are pushed open and shut by differences in blood pressure. Third, saying the left ventricle is thicker because it pumps more blood. It pumps the same volume as the right ventricle, but at a higher pressure.

13:37And fourth, thinking the heart gets its oxygen from the blood inside its chambers. It gets it from the coronary arteries, which is exactly why blocking them is so dangerous. Let's recap. The heart has four chambers: two atria at the top that receive blood, and two ventricles below that pump it out. Arteries carry blood away from the heart, and veins bring it back. The septum keeps oxygenated and deoxygenated blood apart.

14:02The atrioventricular valves stop blood flowing back into the atria, and the semilunar valves stop it flowing back into the ventricles. The left ventricle has the thickest wall because it pumps blood at high pressure all round the body, and the atria are thin because they only push blood into the ventricles. In each heartbeat, the atria contract, then the ventricles contract, and the valves are pushed open and shut by the pressure.

14:25You can monitor the heart with an ECG, the pulse rate, and by listening to the valves closing. Exercise raises the heart rate to deliver more oxygen and glucose to respiring muscles and to remove carbon dioxide faster. And coronary heart disease is the blockage of the coronary arteries, which starves heart muscle of oxygen. Its risk can be reduced by a healthy diet and regular exercise. That is the heart: two pumps side by side, one gentle and one powerful, kept working by one-way valves and its very own blood supply.