The Heart

Explained

Four chambers, and the route through them

The heart is a double pump. The right side handles deoxygenated blood and sends it to the lungs; the left side handles oxygenated blood and sends it to the rest of the body.

Each side has two chambers. The atria are the upper chambers, which receive blood arriving at the heart. The ventricles are the lower chambers, which pump blood out of it. Atria receive, ventricles pump, and the singular of atria is atrium.

The route

Deoxygenated blood arrives from the body in the vena cava and enters the right atrium.

It passes into the right ventricle, which pumps it out through the pulmonary artery to the lungs.

Oxygenated blood returns from the lungs in the pulmonary vein and enters the left atrium.

It passes into the left ventricle, which pumps it out through the aorta to the whole body.

Learn it as a loop and say it aloud: vena cava, right atrium, right ventricle, pulmonary artery, lungs, pulmonary vein, left atrium, left ventricle, aorta.

Left and right are the patient's

On a diagram, the left side of the heart appears on the right of the page, because you are looking at the person facing you. Labelling from your own point of view reverses everything, and it is the most common diagram error in this topic.

Why the left ventricle wall is thicker

Both ventricles pump the same volume of blood with each beat, so the difference is not about quantity.

The right ventricle only has to push blood as far as the lungs, which are next to the heart. The left ventricle has to push blood all the way round the body, to the brain, the fingers and the toes.

A thicker wall contains more muscle, so it contracts more powerfully and generates a higher pressure, which is what sends the blood that distance.

The lungs also need the lower pressure. Their capillaries are delicate, and blood at systemic pressure would damage them and force fluid into the air spaces.

Note also that the ventricle walls are thicker than the atrium walls, because the atria only push blood into the ventricles a few centimetres below.

The valves

Valves make sure blood flows one way only. They open when the pressure behind them is higher and are pushed shut when the pressure in front is higher.

Valves between each atrium and its ventricle stop blood returning to the atrium when the ventricle contracts. Valves at the base of the pulmonary artery and the aorta stop blood flowing back into the ventricles when they relax.

The heart sounds are those valves closing, which is why there are two of them per beat.

The coronary arteries

The heart is a muscle and needs its own blood supply, which it cannot take from the blood passing through its chambers. The coronary arteries branch from the aorta and supply the heart muscle itself.

When fatty deposits narrow them, less oxygen reaches the muscle, so it respires less and pain follows during exertion. A complete blockage starves that region of muscle altogether. That is coronary heart disease, and it is the reason these small vessels get their own name.