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The Heart
Written by Tim Sheppard MBBS BSc. Last updated 9/11/10

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What is the circulatory system?

The circulatory system is the system that gets substances to where they're needed. Using blood, oxygen and nutrients are transported to the cells of the body and metabolism releases energy for the cells to use in their various processes. In order for metabolism to work, a steady supply of oxygen and nutrients must be maintained. Well, the nutrients (obtained by eating) can be relatively easily stored, but the oxygen must be constantly obtained from the air.

For this reason, the blood has to go to the lungs to pick up oxygen, then to the body to deliver it, and then to the lungs again to get more oxygen, then to the body ... and so on and so on. Every time the blood comes back from the body, it has to go to the lungs again to pick up more oxygen. It is said that this is a double circulation.

The direction and location of blood is most easily defined by considering if there's any oxygen about. If there is, the blood binds to the oxygen, and is said to be oxygenated. If the blood has delivered the oxygen and is returning to the body, it is said to be deoxygenated. The double circulation requires a 'double-pump', known as the heart; one pump pumps blood to the lungs, the other pumps blood to the body. The pump which pumps blood to the lungs will be full of deoxygenated blood from the body, which needs to go and get oxygenated. The blood in the pump which pumps blood to the body will be full of oxygenated blood from the lungs, which needs to go and deliver its load.

What is the heart?

The heart is a double-pump made up of four chambers, two atria and two ventricles. The atria receive blood, and the ventricles pump this blood out of the heart again, according to the way in which the circulatory system works.

The animation describing the circulatory system is right, but the pump doesn't quite follow in that direction. A more accurate representation is shown on the left.

Imagine we're following blood from the body. Having become deoxygenated, it enters the right hand side of the heart, received into a chamber called the right atrium. The received blood is then pumped into a chamber named the right ventricle. This has thicker walls, and is able to pump the blood further - all the way through the lungs into the left side of the heart. Here the newly oxygenated blood is again received into an atrium - the left atrium - from which blood is pumped into the left ventricle. The left ventricle has even thicker walls than the right ventricle, and is able to pump blood all around the body, back to the right side of the heart.

The atria are positioned at a more superior end of the heart, while the ventricles are more inferior, but the blood pumped out of the ventricles (into the lungs or the rest of the body) is pumped upwards. The position of the heart in the chest means that there are places that are higher up - the brain, for instance - and if the blood were pumped downwards, it would struggle to reach the height of the brain. The blood is, instead, pumped upwards through arteries that lead to the lungs or different parts of the bodies, and returns in large vessels that lead to the atria.

Since all the chambers pump blood into the next section, it is in a sense a quadruple pump, in that it has four separate pumping areas. However, the pumping mechanism is a little more complicated than that. First of all it's important to consider the features of the different chambers, and the vessels relating to each.

Blood enters the right atrium through veins called the vena cava. These are actually two veins - one comes from the top of the body, and is predictably therefore the superior vena cava, the other comes from below and is therefore the inferior vena cava.

The blood then travels through a valve to stop back flow. The tricuspid valve separates the right atrium and right ventricle, and it closes when the pressure in the right ventricle is greater than that of the left ventricle. This makes sure that when blood is pumped out of the right ventricle, it leaves through the pulmonary valve (also known as a semilunar valve due to its shape) and into the pulmonary arteries, taking the blood to the lungs. The valve here serves the same function of preventing backflow.

The left side works in much the same way; blood enters the left atrium through one of four pulmonary veins, and passes through the mitral (or biscuspid) valve into the left ventricle, where the blood is pumped through the aortic valve (or again, semilunar valve) into the massive artery, the aorta.

The right side of the heart has the most interesting features - and most especially the right atrium. Arguably most important is the sino-atrial node - also known as the pacemaker of the heart. It is the sino-atrial node that stimulates a heart beat by sending an electrical impulse across the heart.

The blood from the body enters the heart via the superior and inferior vena cava, and after swilling around in the right atrium, is forced through the tricuspid valve. This, as previously mentioned, ensures unidirectional flow - it allows blood to travel in only one direction. When the atria contracts, the pressure is higher in the atria so the blood flows into the ventricles.

The last two features here are the coronary sinus and the crista terminalis. The coronary sinus is another entrance to the heart. The deoxygenated blood from most parts of the body comes through either the superior vena cava or the inferior vena cava. However, the heart has it's own supply, and there's no point in making that return via the superior or inferior vena cava. Instead, it drains straight back into the right atrium, through this coronary sinus.

The crista terminalis is effectively a mark of division. When a baby is growing in the womb, its one heart is developed from two different starting points. When the heart is fully developed, this becomes apparent as the crista terminalis divides smooth and rough areas of the right atrium. Passing from the entrances of the superior vena cava and the inferior vena cava, the crista terminalis has the smooth sinus venarum on the back part of the heart, and the rough ridges known as musculi pectanati on the front.

The features of the right atrium can obviously not be seen from the outside of the heart, but the crista terminalis sometimes can be seen as a groove following the same route. This is known as the sulcus terminalis, but it is not found in everyone.

The right ventricle is relatively typical of the two ventricles, and is essentially very simple. It of course has the tricuspid valve where the blood enters, just as the left ventricle has the mitral valve to receive blood from the left atrium. Blood leaves the right ventricle via the pulmonary valve, so when the pressure in the ventricle is higher than that in the pulmonary artery, blood will leave, and when the ventricle stops contracting, the pressure will fall but the valve will shut, preventing backflow.

The papillary muscles and chordae tendineae are there to make sure the tricuspid valve remains competent. When the pressure rises in the right ventricle, the 'flaps' of the valve snap shut, preventing blood from going into the right atrium. However, the pressure has to rise very high in order to pump the blood as far as it needs to go; the flaps could quite easily blow through.

To prevent this happening, the chordae tendineae are attached. They are like strings which attach the valve 'flaps' to the ventricle wall, making sure they can't go too far back. They still allow movement, because the valve still needs to snap shut; however, the prevent it going too far. The papillary muscles are there to make sure the chordae tendineae still work. When the papillary muscles contract, they pull on the chordae tendineae, and help them to stop the valve flaps blowing through. If for some reason there is a problem with the valve and blood does flow back into the right atrium, the valve is said to be incompetent (because it's not 'doing it's job') and it can cause significant problems.

Now the function of the heart can seem quite complicated, but it's essentially very logical - it just takes a while to understand. The best plan is to take it step by step, following each movement as it comes. The important thing to consider is the heart is a double pump - and both sides are doing much the same thing, it's just that their targets are different - the left side of the heart, full of oxygenated blood, is sending blood out to the body; the right side of the heart has just received its blood from the body, and is sending it out to the lungs.

How is blood pumped through the heart?

Step 1 - Everything relaxed

When everything is relaxed, the walls are not exerting a lot of pressure on the blood, so it can happily flow in through the vena cava. Because the pressure in the ventricles is no greater than in the atria, the blood can also flow straight through into the ventricles.

Since the ventricles are relaxed, this is called diastole.

Step 2 - Atrial Systole

When the sino-atrial node starts a heart beat, the atria contract. This forces blood into the ventricles; the tricuspid and mitral valves do not prevent blood flowing in this direction, so it can happily flow through.

The atria are contracting, and this is known as systole. However, the ventricles are still relaxed, allowing blood in, and the condition of the heart is therefore still, strictly speaking, diastole.

Step 3 - Everything Contracting

Eventually the electrical signal gets through to the ventricles, and they contract. The pressure rises in the ventricles, and when it is greater than in the atria, the blood will try to flow back into the atria. Fortunately, there are valves to stop this taking place, so the blood instead will flow through the pulmonary and aortic valves into the pulmonary artery and the aorta.

Since the ventricles are contracting, the state of the heart is now, quite definitely, systole.

Step 4 - Ventricular Systole

Eventually the atria stop contracting, and it's just the ventricles forcing blood out. Since the pressure in the atria is reduced, blood can start flowing back into the atria. The pressure in the ventricles is still higher, so the mitral and tricuspid valves will still be shut, but filling of the atria sets us up to return to step 1....

Since the ventricles are still contracting, the heart is said to still be in systole.

What is the cardiac cycle?

The cardiac cycle is the name given to the cycle of events which occurs during each heart beat. It is often used when describing the activity of the heart, or how particular aspects change during the course of a heart beat.

Pressure is a typical example. On the right is an animation showing how the pressure in different parts of the heart change according to time.

Starting with the left atrium, the pressure rises as the atria contract. When the mitral valve closes, continued contraction of the atria leads to pressure increasing still, until atrial contraction ceases. The atria stop contracting, so pressure decreases, and then as atrial filling occurs (with blood returning to the heart), pressure increases again until the mitral valve opens, allowing blood to flow into the ventricles. The pressure then increases as atria begin to contract again...

The pressure in the left ventricle rises as it filled, then rises sharply as the strong, muscular ventricular walls contract. As the pressure exceeds that in the atria, the mitral valve closes. Eventually the ventricular walls stop contracting and the pressure falls, to a point where the pressure in the atria exceeds that in the ventricles, at which point the mitral valve opens again, and the pressure in the left ventricle is the same as the pressure in the left atrium.

The aorta, into which blood leaving the left side of the heart is pumped, will have pressure that is initially decreasing as the blood passes away from the aorta towards the rest of the body. The pressure is usually high, but when the blood flows through to the periphery, the pressure drops to such an extent that, with the pressure risen in the contracting ventricle, the pressure is higher in the ventricle than in the aorta. At this point, the aortic valve opens. The pressure in the aorta then follows the pressure in the ventricle, but will only drop to a certain point. Just after the aortic valve closes, there is something called the dicrotic notch, which is produced by the flow of blood back to the aortic valve leading to a slight, brief increase in pressure. The high pressure in the aorta then drops again as the blood flows out...

Strictly speaking the cardiac cycle can be split into 6 convenient sections, found by extending the four steps shown higher up. These six sections are as follows:

1. Atrial systole - where the atria are contracting
2. Isovolumetric contraction - where everything is contracting
3. Rapid ejection - where the increase in pressure in the ventricles is sufficient to exceed that in the large arteries, and the blood is forced out into the circulation
4. Slow ejection - where the decrease in pressure in the ventricles, still greater than that in the large arteries, forced blood out of the heart but at a slightly slower rate
5. Isvolumetric relaxation - where everything is relaxed
6. Ventricular loading - where the ventricles are passively being filled in response to blood returning from the circulation

This is often represented on something called a pressure-volume loop, which shows how the changes in ventricular pressure affect ventricular volume.

During atrial systole, the volume in the ventricles increases without increase pressure much; then the ventricles contract (isvolumetric contraction), leading to increase in pressure but no change in volume. When the semilunar valves (aortic and pulmonary) open, the blood is forced out, leaving to a rapid decline in volume, while pressure continues to increase. Eventually the pressure decreases a little, while volume also decreases. Finally the volume is back to its bottom value, and the relaxation of the ventricles (isovolumetric relaxation) leads to reduced pressure. As the blood returns to the heart and passes straight into the ventricles as well as the atria, the ventricular volume increases without adjusting pressure.

The pressure-volume loop is a great way of seeing how effective a heart is working, but it isn't always easy to measure. In practice, doctors can learn a lot by looking at the electrical activity of the heart, using something called an ECG; but that's a considerably more complicated issue.

What is systole?

Systole is the name given to the time when the heart is beating - or, more specifically, when the ventricles are contracting. The heart happily pumps blood through, with the atria pushing blood into the ventricles, and then the ventricles firing it off into the rest of the body. While the ventricles are contracting, the blood is forced out into the circulation (preventing from going back by the mitral and tricuspid valves). During this period, the heart is said to be 'in systole'.

What is diastole?

Diastole is the name given to the time when the heart is not beating - or, more specifically, when the ventricles are not contracting. The heart happily pumps blood through, with the atria pushing blood into the ventricles, and then the ventricles firing it off into the rest of the body. While the ventricles are relaxed, the force behind the blood that has just been thrown into the system is no longer there; blood is able to flow into the ventricles, and the pressure in the circulatory system is reduced. This is why blood pressure has a systolic pressure and a diastolic pressure.

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