Structure of the mammalian heart

Molecules, Transport and Health · The heart, blood vessels and haemoglobin · note 3 of 6

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Structure of the mammalian heartSpec 1.8

In short

The mammalian heart has four chambers: two thin-walled atria that receive blood and two thicker-walled ventricles that pump it out. The septum separates oxygenated from deoxygenated blood. Atrioventricular and semilunar valves prevent backflow, and the left ventricle has the thickest wall because it pumps blood at high pressure around the whole body.

Vertical section of the mammalian heart with the left side on the right of the page, labelled right and left atria and ventricles, septum, tricuspid and bicuspid (atrioventricular) valves, tendinous cords, papillary muscle, pulmonary and aortic (semilunar) valves, vena cava, pulmonary artery, pulmonary vein, aorta and coronary arteries. — The left ventricle has the thickest wall because it pumps blood to the whole body.

Major blood vessels of the heart
Vessel	Carries blood	From	To
Vena cava	Deoxygenated	Body	Right atrium
Pulmonary artery	Deoxygenated	Right ventricle	Lungs
Pulmonary vein	Oxygenated	Lungs	Left atrium
Aorta	Oxygenated	Left ventricle	Body
Coronary arteries	Oxygenated	Base of the aorta	Heart muscle (cardiac muscle)

The atria have thin walls because they only pump blood a short distance into the ventricles.
The ventricles have thicker, muscular walls to pump blood out of the heart. The left ventricle has the thickest wall because it pumps blood to the whole body at high pressure. The right ventricle pumps blood only to the nearby lungs at lower pressure, which protects the lung capillaries.
The septum separates the left and right sides, so oxygenated and deoxygenated blood do not mix.
Atrioventricular valves (tricuspid on the right, bicuspid on the left) stop blood flowing back from the ventricles into the atria. They are held in position by tendinous cords attached to papillary muscles, so that they are not pushed inside out when the ventricles contract.
Semilunar valves at the start of the aorta and pulmonary artery stop blood flowing back from the arteries into the ventricles.
The coronary arteries supply the cardiac muscle with oxygen and glucose. Blockage of a coronary artery causes a heart attack.

Practical:

Recommended additional practical (not a core practical): investigate the structure of a mammalian heart by dissection. Cut across the ventricles to compare wall thickness, find the valves and tendinous cords, and identify the vessels. Wear gloves and eye protection, use a sharp scalpel on a dissecting board, cut away from your hand and disinfect surfaces afterwards.

Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026

Frequently asked questions

Why do large animals need a heart and circulation?

Large animals have a small surface area to volume ratio and long diffusion distances, and active ones have high metabolic rates, so diffusion alone is too slow. A heart pumps blood in bulk (mass transport) through vessels, delivering oxygen and nutrients to all cells quickly and removing waste.

How are arteries, veins and capillaries different?

Arteries have thick, elastic, muscular walls to withstand high pressure. Veins have thin walls, wide lumens and valves to return blood at low pressure. Capillaries have walls one cell thick and a very narrow lumen, giving a short diffusion distance for exchange between blood and cells.

What happens in the cardiac cycle?

The cardiac cycle has three stages. In atrial systole the atria contract and fill the ventricles. In ventricular systole the ventricles contract, the atrioventricular valves shut and the semilunar valves open to force blood out. In cardiac diastole the heart relaxes and the atria refill.

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