How the Heart Pumps Blood, Explained

Four chambers, two pumps in one organ

The heart has four chambers, a right atrium, a right ventricle, a left atrium and a left ventricle. A muscular wall called the septum runs down the middle, completely separating the right side from the left side.

This matters because the right side only ever handles deoxygenated blood returning from the body, while the left side only ever handles oxygenated blood returning from the lungs. As long as the septum is intact, these two blood supplies never mix.

Each side works as its own small pump made of two chambers: an atrium on top that receives blood, and a ventricle below it that pushes blood out. Blood always flows atrium first, then ventricle, on both sides.

Thinking of the heart as two pumps sharing one body, rather than one single pump, makes the rest of its structure much easier to follow.

The valves: one-way doors for blood

Between each atrium and its ventricle sits an atrioventricular valve, the tricuspid valve on the right and the bicuspid (mitral) valve on the left. These flap open to let blood pass from atrium to ventricle, then snap shut so blood cannot slide backward once the ventricle contracts.

A second set, the semilunar valves, sits at the exit of each ventricle, where the pulmonary artery leaves the right side and the aorta leaves the left side. They open to let blood out and close to stop it flowing back in once the ventricle relaxes.

Without these valves, every contraction would push some blood forward and some blood backward, and circulation would become inefficient almost immediately. The familiar 'lub-dub' sound of a heartbeat is actually the sound of these valves snapping shut in sequence, the atrioventricular valves closing first, then the semilunar valves a moment later.

One heartbeat, step by step

1. Both atria fill with returning blood, deoxygenated blood on the right, oxygenated blood on the left.
2. The atria contract together (atrial systole), pushing blood through the open atrioventricular valves into the ventricles below.
3. The ventricles contract (ventricular systole); the atrioventricular valves slam shut to stop blood being pushed back up into the atria.
4. The rising pressure forces the semilunar valves open, and blood is pushed out, from the right ventricle into the pulmonary artery, and from the left ventricle into the aorta.
5. The ventricles relax (diastole); the semilunar valves close to stop blood flowing back in, and the whole heart refills ready for the next beat.

Why the left ventricle wall is thicker

If you look at a cross-section of the heart, the muscle wall of the left ventricle is noticeably thicker than the wall of the right ventricle. This difference exists because the two ventricles have very different jobs.

The right ventricle only has to push blood a short distance to the nearby lungs, so it can do that job at fairly low pressure with a thinner, less powerful wall. The left ventricle has to push oxygenated blood all the way around the body, up to the brain, down to the feet, out to every organ, which needs much higher pressure to overcome the resistance of that entire network of blood vessels.

A thicker, more muscular wall lets the left ventricle generate that extra force.

Double circulation: two loops through one heart

Because the heart is really two separate pumps, blood actually passes through it twice for every complete lap of the body, this pattern is called double circulation. In the pulmonary circulation, the right side pumps deoxygenated blood to the lungs, where it picks up oxygen, before returning to the left side of the heart.

In the systemic circulation, the left side then pumps that oxygenated blood out to the rest of the body, and the deoxygenated blood that comes back afterward returns to the right side, ready to start the loop again. Passing back through the heart between these two circuits lets the blood be re-pressurised, so it reaches body tissues at high pressure rather than arriving weak after the trip through the lungs.

Which blood vessels connect to the heart, and what do they carry?

Four large blood vessels join the heart, and each one links a specific chamber to a specific destination. Two bring blood in and two take blood out, and knowing which is which, and whether the blood inside is oxygenated or deoxygenated, is a frequent source of both marks and mistakes.

What makes the heart beat without being told to?

Unlike most muscle, cardiac muscle is myogenic, which means it can contract on its own without a signal from the brain. The rhythm is set by a small patch of tissue in the wall of the right atrium called the sinoatrial node, often described as the heart's natural pacemaker.

It sends out a wave of electrical excitation that spreads across the atria and makes them contract, then passes to the ventricles so they contract a fraction of a second later, keeping the events of the heartbeat in the right order.

The heart sets its own basic rhythm, but the rate can still be adjusted to match what the body is doing. During exercise, nerve signals and the hormone adrenaline speed the heart up so that more oxygenated blood reaches the working muscles; at rest, the rate settles back down.

This is why a pulse rises within seconds of starting to run and falls again once the effort stops.

How does the heart get its own supply of blood?

Although the heart is full of blood with every beat, the blood passing through its chambers does not feed the heart muscle itself, the wall is too thick for oxygen to reach the inner muscle by diffusion from inside. Instead, the heart is supplied by its own set of vessels, the coronary arteries, which branch off the aorta and run across the surface of the heart to deliver oxygen and glucose to the cardiac muscle.

Because the heart never stops working, this supply has to be constant.

If a coronary artery becomes narrowed or blocked, the muscle it serves is starved of oxygen and can be damaged, which is the basis of coronary heart disease and heart attacks. This is why the coronary arteries appear in the syllabus alongside the heart's pumping action rather than as a separate topic.

Key facts to remember about the heart

• The heart has four chambers: two atria that receive blood and two ventricles that pump it out.
• The septum separates the right side from the left, so oxygenated and deoxygenated blood never mix.
• Valves make blood flow one way only, from atria to ventricles and out into the arteries.
• The left ventricle has a thicker wall because it pumps blood to the whole body, not just to the lungs.
• Arteries carry blood away from the heart and veins carry it back, with the pulmonary vessels as the exception.
• Blood passes through the heart twice for each full circuit of the body, which is called double circulation.

Frequently asked questions