Structure & Function of the Heart

The heart is an organ made almost entirely of muscle. Its job is to pump blood around the body. To do this it is divided into four chambers.

The heart has a left side and a right side, completely separated by a wall of muscle (the septum) so the blood on each side does not mix. Each side has two chambers — one on top and one below:

• Atria (singular: atrium) — the two upper chambers (right atrium and left atrium). The atria receive blood that is flowing back into the heart from the veins.
• Ventricles — the two lower chambers (right ventricle and left ventricle). The ventricles pump blood out of the heart into the arteries.

A simple rule to learn:

• Blood enters the heart through an atrium (Atria → Arrives).
• Blood leaves the heart through a ventricle (Ventricles → leaVe).

Right side vs left side:

• The right side handles blood going to the lungs (it receives blood from the body and pumps it to the lungs).
• The left side handles blood going to the rest of the body (it receives blood from the lungs and pumps it around the body).

Exam tip. When you label a heart diagram, remember it is drawn as if the patient is facing you — so the heart's left side appears on the right of your diagram, and vice versa. Examiners accept the labels by their correct anatomical name regardless of which side of the page they are on.

It is much easier to remember the heart once you can picture it. The diagram below shows the four chambers, the valves between them, and the main blood vessels that carry blood in and out.

What to notice on the diagram:

• The atria are on top, the ventricles below.
• An atrioventricular valve sits between each atrium and the ventricle underneath it.
• A semilunar valve sits in each artery just as it leaves a ventricle.
• The left ventricle wall is much thicker than the right ventricle wall.

Exam tip. Practise drawing this from memory. A clear, correctly labelled heart diagram is worth several quick marks, and labelling questions appear regularly.

For the heart to work as a pump, blood must only ever flow one way. If it could slosh backwards, the heart could not build up pressure. Valves are flaps that act like one-way doors: they open to let blood through and snap shut to stop it flowing backwards (prevent backflow).

There are two types of valve you must know:

1. Atrioventricular valves

• Found between each atrium and the ventricle below it (atrio = atrium, ventricular = ventricle).
• They open when the atrium contracts, letting blood flow down into the ventricle.
• When the ventricle contracts, these valves snap shut so blood cannot be pushed back up into the atrium — it is forced out of the artery instead.

2. Semilunar valves

• Found in the arteries that leave the ventricles — the pulmonary artery and the aorta ("semilunar" means half-moon shaped).
• They open when the ventricle contracts, letting blood flow out into the artery.
• When the ventricle relaxes, they snap shut so blood already in the artery cannot flow back into the ventricle.

So the two valve types guard two different points on the journey: the atrioventricular valves stop blood going backwards into the atria, and the semilunar valves stop blood going backwards into the ventricles.

Exam tip. Whenever you mention a valve, link it to its job: "prevents backflow" or "stops blood flowing backwards". That phrase is the mark.

Look again at the diagram and you will see that the left ventricle wall is much thicker than the right ventricle wall. There is a good reason for this, and it is a favourite exam question.

The amount of muscle in a chamber's wall matches how far it has to pump blood:

• The right ventricle only pumps blood a short distance — to the lungs, which are right next to the heart. This needs a lower pressure, so its wall is thinner.
• The left ventricle has to pump blood all the way around the whole body — to the head, the toes, every organ — and back. This needs a much higher pressure, so it has a thick, muscular wall.

The link to remember is:

Thicker wall → more muscle → contracts with greater force → produces a higher pressure → pushes blood all the way around the whole body.

The atria have the thinnest walls of all, because they only need to push blood a tiny distance down into the ventricles.

Exam tip. The required idea is that the left ventricle pumps blood around the whole body (at high pressure), which is why it needs more muscle. Do not say it "pumps harder because it has more blood" — both ventricles pump the same volume; it is the distance/pressure that differs.

Humans have a double circulation, which means blood passes through the heart twice on each complete journey around the body. There are two linked loops:

1. Heart → lungs → heart (the loop that picks up oxygen).
2. Heart → rest of the body → heart (the loop that delivers oxygen).

Because the blood returns to the heart between the two loops, the heart can give it a fresh push before sending it on the long journey around the body. This keeps the blood moving at a high enough pressure.

The path of blood through the heart — learn this order:

1. Blood low in oxygen returns from the body through the vena cava into the right atrium.
2. The right atrium contracts; blood passes through the atrioventricular valve into the right ventricle.
3. The right ventricle contracts; blood is pumped through the semilunar valve into the pulmonary artery and on to the lungs (where it picks up oxygen).
4. Oxygen-rich blood returns from the lungs through the pulmonary vein into the left atrium.
5. The left atrium contracts; blood passes through the atrioventricular valve into the left ventricle.
6. The thick-walled left ventricle contracts powerfully; blood is pumped through the semilunar valve into the aorta and around the whole body.

A short version to memorise:

body → vena cava → right atrium → right ventricle → pulmonary artery → lungs
lungs → pulmonary vein → left atrium → left ventricle → aorta → body

Exam tip. Notice the pattern: blood always goes atrium first, then ventricle, and leaves through an artery. The right side deals with the lungs; the left side deals with the body. If you remember "right = lungs, left = body" you can rebuild the whole pathway.