A level Biology B · Edexcel 9BI0 · Topic 4D
The heart and circulation
Key points
- Mammals have a double circulation: blood passes through the heart twice per circuit, keeping high pressure to the body.
- Cardiac cycle: atrial systole, ventricular systole (AV valves shut, semilunar valves open), then diastole. Valves open and close because of pressure differences.
- The heart is myogenic: the SAN starts each beat, the AVN delays the impulse, and the bundle of His and Purkyne fibres make the ventricles contract from the apex upwards.
- Cardiac output = stroke volume × heart rate.
- Arteries have thick, elastic, muscular walls; veins have wide lumens and valves; capillaries are one cell thick for exchange.
- Atherosclerosis (fatty plaque building up under the artery lining) can lead to blood clots, heart attack and stroke; risk factors include smoking, high blood pressure and diet.
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The mammalian heart is a double pump that drives blood through a network of vessels. Exams test the cardiac cycle in detail, how the heartbeat is coordinated, and what goes wrong when arteries become blocked.
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