Structure of the heart

1. The heart is a double pump built almost entirely from cardiac muscle, a tissue that contracts and relaxes rhythmically for a lifetime without becoming fatigued.
2. It has four chambers: a right atrium and a right ventricle on one side, and a left atrium and a left ventricle on the other.
3. The septum is a solid wall of muscle down the middle that keeps oxygenated and deoxygenated blood completely separate.
4. The atria have thin walls because they only push blood a few centimetres down into the ventricles below them.
5. The ventricles have much thicker walls because they have to generate enough pressure to drive blood out of the heart altogether.
6. The right ventricle wall is moderately thick because it pumps only as far as the lungs, which sit alongside the heart.
7. Its pressure is deliberately kept low, because the thin capillary walls in the lungs would burst under a high pressure.
8. The left ventricle wall is the thickest of all, roughly three times the thickness of the right, because it pumps blood all the way round the body.
9. More muscle produces a more forceful contraction, which produces a higher blood pressure and so a faster flow to the tissues.
10. Four vessels are attached at the top: the vena cava into the right atrium, the pulmonary artery out of the right ventricle, the pulmonary vein into the left atrium and the aorta out of the left ventricle.
11. The cardiac muscle itself is supplied with oxygen and glucose by the coronary arteries that run across the outside of the heart.

The valves of the heart

1. A valve is a flap of tissue that opens in one direction only, so blood can pass forwards but cannot slip back.
2. Each valve opens when the pressure behind it is higher than the pressure in front of it, and snaps shut when that difference reverses.
3. The atrioventricular valves sit between each atrium and the ventricle below it.
4. The one on the right has three flaps and is called the tricuspid valve.
5. The one on the left has two flaps and is called the bicuspid valve, or the mitral valve.
6. Both are anchored to the ventricle wall by tough tendons that stop the flaps turning inside out when the ventricle contracts hard.
7. The semilunar valves sit at the base of the aorta and the base of the pulmonary artery, each made of three half-moon pockets.
8. They stop blood that has just been forced into an artery falling back into the ventricle as it relaxes.
9. Without valves the heart would push blood back and forth in the same place and no net flow would leave it.

How one heartbeat works

1. Blood flows into the two atria from the vena cava and the pulmonary vein while the heart is relaxed.
2. Both atria then contract together, raising the pressure inside them so the atrioventricular valves are pushed open.
3. Blood passes down into the ventricles, which fill and stretch.
4. Both ventricles then contract, and the pressure inside them rises sharply.
5. That pressure forces the atrioventricular valves shut and forces the semilunar valves open.
6. Blood is driven out of the right ventricle into the pulmonary artery and out of the left ventricle into the aorta.
7. The ventricles then relax, the pressure inside them falls below the pressure in the arteries and the semilunar valves close.
8. The two sets of valves closing are what produce the two sounds of a heartbeat.

Arteries

1. An artery carries blood away from the heart, and that blood leaves at high pressure in surges.
2. The wall is thick so that it can withstand that pressure without bursting.
3. The wall contains a great deal of muscle and elastic fibres, so it stretches as each surge arrives and recoils afterwards.
4. That stretching and recoil smooths the flow between beats and keeps the blood moving forwards, and it is what you feel as a pulse.
5. The lumen is narrow compared with the thickness of the wall, which helps to keep the pressure high.
6. An artery has no valves along its length, because the pressure behind the blood is high enough to keep it flowing one way.

Veins

1. A vein carries blood back towards the heart, and by this stage the blood is at low pressure and flowing steadily rather than in surges.
2. The wall is thin with little muscle or elastic tissue, because there is no high pressure to resist.
3. The lumen is wide, which reduces the resistance to flow so that low-pressure blood still returns quickly.
4. Veins contain valves along their length, which is the clearest structural difference from an artery.
5. Those valves stop blood flowing backwards, which matters most in the legs where blood has to travel upwards against gravity.
6. Many veins run between skeletal muscles, and each time those muscles contract they squeeze the vein and push blood on towards the heart.

Capillaries

1. A capillary links the smallest arteries to the smallest veins and is the only vessel where substances are actually exchanged with the tissues.
2. Its wall is one cell thick, giving the shortest possible diffusion distance between the blood and the surrounding cells.
3. Its lumen is so narrow that red blood cells pass through in single file, which presses them against the wall and shortens the distance further.
4. Capillaries are extremely numerous and branch through every tissue, so no cell is far from a supply.
5. That branching also gives a very large total surface area for exchange.
6. Blood flows slowly through a capillary bed, which allows time for diffusion to take place.
7. The wall is also slightly leaky, so plasma can seep out to bathe the tissues and white blood cells can squeeze through it.