Circulatory Heart Diagram

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The heart at a glance

The heart is a muscular pump about the size of a fist, sitting slightly left of centre in the chest. It has four chambers: two upper atria that receive blood and two lower ventricles that pump it out. The right side handles oxygen-poor blood and sends it to the lungs; the left side handles oxygen-rich blood and sends it to the whole body. Four valves keep blood moving in one direction. At rest, an adult heart typically beats about 60–100 times a minute.

Blood flow through the heart, step by step

1. Oxygen-poor blood from the body enters the right atrium through the superior and inferior vena cava.
2. It passes through the tricuspid valve into the right ventricle.
3. The right ventricle pumps it through the pulmonary valve into the pulmonary artery.
4. In the lungs, blood releases carbon dioxide and picks up oxygen.
5. Oxygen-rich blood returns through the pulmonary veins to the left atrium.
6. It passes through the mitral (bicuspid) valve into the left ventricle.
7. The left ventricle pumps it through the aortic valve into the aorta.
8. Arteries carry it to the body’s organs and tissues; veins bring it back to the heart.

Two circuits: pulmonary and systemic

Because blood passes through the heart twice on each complete circuit, humans have a double circulatory system. The left ventricle has a much thicker wall than the right because it must push blood around the whole body.

Arteries, veins and capillaries

The pulmonary artery and pulmonary veins are the exceptions to the “arteries carry oxygen-rich blood” rule: the pulmonary artery carries oxygen-poor blood to the lungs, and the pulmonary veins carry oxygen-rich blood back.

Why diagrams show left and right reversed

Heart diagrams are drawn as if you are facing the person, so the heart’s right side appears on the left of the page. That is why the right atrium and right ventricle are on the left in the interactive diagram and in most textbooks.

The heartbeat and valves

Each heartbeat has two main phases: systole, when the ventricles contract and push blood out, and diastole, when they relax and fill. The “lub-dub” sound comes from valves closing: “lub” as the tricuspid and mitral valves close at the start of ventricular contraction, and “dub” as the pulmonary and aortic valves close when it ends. The heart’s own electrical system, starting at the sinoatrial node, keeps the beat coordinated.

Worked example: tracing one circuit

Start at the right atrium and press “Next step”. The yellow marker moves through the tricuspid valve to the right ventricle, out through the pulmonary valve and artery to the lungs, back through the pulmonary veins to the left atrium, through the mitral valve into the left ventricle, then through the aortic valve into the aorta and on to the body, before returning through the vena cava. Saying each step aloud while the marker moves is an effective way to memorise the order.

Blood pressure and pulse

Each contraction of the left ventricle sends a pressure wave along the arteries, which you can feel as your pulse at the wrist or neck. Blood pressure is written as two numbers: the higher systolic pressure during contraction and the lower diastolic pressure during relaxation. Measuring your pulse — counting beats for 30 seconds and doubling — is a simple classroom experiment, and comparing resting and post-exercise pulse shows how the heart responds to the body’s needs.

Key vocabulary

• Atrium (plural atria) — upper receiving chamber.
• Ventricle — lower pumping chamber.
• Septum — wall dividing the left and right sides of the heart.
• Valve — flap that keeps blood flowing one way.
• Oxygenated / deoxygenated — blood rich or poor in oxygen.
• Coronary arteries — vessels supplying the heart muscle itself.

Keeping the heart healthy

• Regular physical activity strengthens the heart muscle.
• A balanced diet with plenty of fruit, vegetables and whole grains supports heart health.
• Not smoking is one of the most important steps for heart health.
• Managing blood pressure and cholesterol reduces strain on the heart.
• Sleep and stress management matter too.

This page is for education only. For symptoms such as chest pain, shortness of breath or palpitations, seek medical advice promptly.

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