How does the structure of the heart and blood vessels move blood around the body?
The structure of the mammalian heart and the cardiac cycle, the structure of arteries, veins and capillaries, and the role of the circulatory system in transporting substances around the body.
An Edexcel A-Level Biology B (Salters-Nuffield) answer on the heart and circulation, covering heart structure, the cardiac cycle and pressure changes, the structure of arteries, veins and capillaries, and the mammalian double circulatory system.
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What this dot point is asking
Edexcel wants you to describe the structure of the mammalian heart and the cardiac cycle, explain how pressure changes open and close the valves, and relate the structure of arteries, veins and capillaries to their roles in the double circulatory system. Pressure-trace graphs and cardiac-output calculations are common, so practise reading where each valve opens and closes.
The heart and the double circulation
Mammals have a double circulatory system: blood passes through the heart twice per circuit. The pulmonary circuit carries deoxygenated blood from the right ventricle to the lungs; the systemic circuit carries oxygenated blood from the left ventricle to the body.
The cardiac cycle
The cardiac cycle has three stages, controlled by pressure differences.
- Atrial systole: the atria contract, pushing blood into the ventricles through the open atrioventricular (AV) valves.
- Ventricular systole: the ventricles contract, ventricular pressure rises above atrial pressure (closing the AV valves) and above arterial pressure (opening the semilunar valves), forcing blood into the aorta and pulmonary artery.
- Diastole: the heart relaxes; pressure falls, the semilunar valves close, and blood flows back into the relaxed atria.
The blood vessels
- Arteries carry blood away from the heart at high pressure; thick walls with elastic fibres and smooth muscle withstand and smooth out the pressure.
- Capillaries have walls one endothelial cell thick and a narrow lumen, giving a short diffusion distance and large surface area for exchange of substances.
- Veins carry blood back to the heart at low pressure; they have a wide lumen (reducing resistance), thinner walls and valves to prevent backflow. Skeletal muscles contracting around veins help squeeze blood back to the heart.
A double circulation is more efficient than a single one because blood can be pumped to the body at high pressure, having had its pressure restored by the heart after passing through the lungs. The heartbeat is myogenic (it originates in the heart muscle itself): the sinoatrial node sets the rhythm, the impulse spreads across the atria, is delayed at the atrioventricular node, then passes down the bundle of His and Purkinje fibres so the ventricles contract from the base upwards.
Examples in context
Example 1. Reading a pressure trace. In an exam graph of left atrial, left ventricular and aortic pressures, the AV valve closes where the ventricular curve rises above the atrial curve (start of ventricular systole), and the semilunar (aortic) valve opens where the ventricular curve rises above the aortic curve. The dip (dicrotic notch) on the aortic curve marks the semilunar valve closing. Being able to point to these crossings earns the marks.
Example 2. Heart valve replacement. When a heart valve becomes leaky or stiff (for example after rheumatic fever), surgeons replace it with a mechanical or tissue valve. The replacement still works passively, opening and closing on pressure differences, which reinforces the key idea that valves are not muscular but respond to pressure. Patients with mechanical valves take anticoagulants to prevent clots forming on the artificial surface.
Try this
Q1. Explain why the semilunar valves close during diastole. [2 marks]
- Cue. Arterial pressure becomes higher than ventricular pressure, so blood tends to flow back, pushing the semilunar valves shut.
Q2. Relate the structure of a capillary to its function. [2 marks]
- Cue. Wall one cell thick gives a short diffusion distance; narrow lumen and large total surface area allow efficient exchange.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20195 marksUsing the events of the cardiac cycle, explain how the changes in pressure cause the atrioventricular and semilunar valves to open and close, ensuring blood flows in one direction.Show worked answer →
Markers want pressure changes linked to each valve.
During atrial systole the atria contract, atrial pressure exceeds ventricular pressure, so the AV valves are open and blood flows into the ventricles. During ventricular systole the ventricles contract, ventricular pressure rises above atrial pressure so the AV valves shut (preventing backflow into the atria), then rises above arterial pressure so the semilunar valves open and blood is forced into the arteries. During diastole the ventricles relax, ventricular pressure falls below arterial pressure so the semilunar valves shut (preventing backflow from arteries), and falls below atrial pressure so the AV valves open again.
Award marks for: AV valves open when atrial pressure exceeds ventricular; AV close when ventricular exceeds atrial; semilunar open when ventricular exceeds arterial; semilunar close when arterial exceeds ventricular; valves prevent backflow.
Edexcel 20214 marksA person has a stroke volume of and a heart rate of beats per minute. Calculate their cardiac output in and explain how cardiac output would change during exercise.Show worked answer →
A worked calculation plus explanation.
Cardiac output stroke volume heart rate (dividing by ). During exercise both heart rate and stroke volume increase, so cardiac output rises, delivering more oxygenated blood and glucose to the respiring muscles and removing carbon dioxide faster.
Markers reward: correct or , the formula, and the explanation that both factors rise in exercise to meet demand.
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Sources & how we know this
- Pearson Edexcel A-Level Biology B (9BN0) specification — Pearson Edexcel (2015)