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Learn4BI1Paper 2B · Paper 2 (all content)Structure and functions in living organismsHeart and circulation
Structure and functions in living organisms · Subtopic 6 of 21

The Heart and Circulation

Mammals have a double circulation: blood passes through the heart twice per circuit, once to the lungs (pulmonary) and once to the body (systemic), keeping body pressure high. Deoxygenated blood enters the right atrium from the vena cava, passes to the right ventricle, and is pumped through the pulmonary artery to the lungs. Oxygenated blood returns in the pulmonary vein to the left atrium, then the left ventricle, which pumps it out through the aorta. Valves stop backflow. The left ventricle wall is thickest. Coronary heart disease happens when fatty deposits block the coronary arteries.

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 45 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Describe double circulation and its advantage.
  • Trace blood through the heart.
  • Explain the structure of the heart walls and valves.
  • Explain the effect of exercise and describe coronary heart disease.
Pause and think

Why is the left side of the heart more muscular than the right?

Section 2 of 14

Core idea

Start hereTwo pumps in one

Your heart is really two pumps side by side. The right pump sends blood a few centimetres to the lungs; the left sends it all the way to your toes and back.

The big idea

In a double circulation, blood passes through the heart twice per circuit. Chambers, valves and vessels keep oxygenated and deoxygenated blood flowing one way, at the right pressure.

1. Blood flow through the heart

The double circulation

Right side to lungs; left side to body.

Worked exampleWhy the walls differ

Compare the walls of the left ventricle, right ventricle and atria.

Goal
Explain thickness.
Strategy
Thickness matches pumping distance.
  1. atria: thinnest\text{atria: thinnest}
    Pump only into the ventricles below.
  2. right ventricle: medium\text{right ventricle: medium}
    Pumps to the nearby lungs at lower pressure.
  3. left ventricle: thickest\text{left ventricle: thickest}
    Pumps to the whole body at high pressure.
AnswerMore muscle where more pressure is needed.
✓ CheckLung capillaries would be damaged by high pressure ✓.
Key takeawayCoronary heart disease Fatty deposits narrow the coronary arteries, cutting oxygen to heart muscle. Risk factors: diet high in saturated fat, smoking, inactivity, stress, genes, age.
Pause and think

Why does a hole in the septum make a baby look bluish?

Summary

  • Double circulation.
  • RA → RV → lungs → LA → LV → body.
  • Valves stop backflow.
Key words
Atrium
Upper chamber of the heart that receives blood.
Ventricle
Lower chamber that pumps blood out.
Coronary arteries
Arteries supplying the heart muscle.
HistoryWilliam Harvey. In 1628 William Harvey showed that blood circulates in a loop pumped by the heart. Before him, people thought blood was made fresh in the liver and used up by the body.
Fun factA lifetime of beats. Your heart beats around 100,000 times a day and about 3 billion times in a lifetime, without ever resting.
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