How is the heart built, and how do the blood vessels move blood around the double circulation?
The structure of the heart, the double circulatory system, and the structure and function of arteries, veins and capillaries.
A focused answer to the WJEC GCSE Science Double Award Unit 1 topic on the heart and circulation, covering the structure of the heart, the double circulatory system, and how arteries, veins and capillaries are adapted to their functions.
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What this dot point is asking
WJEC Double Award Unit 1 wants you to describe the structure of the heart, explain the double circulatory system, and describe how arteries, veins and capillaries are adapted to their functions.
The structure of the heart
The heart is a muscular pump divided into a left and a right side, each with two chambers:
- Atria (upper chambers): receive blood returning to the heart.
- Ventricles (lower chambers): pump blood out of the heart.
- Valves between the atria and ventricles, and at the exits, stop blood flowing backwards.
The right side receives deoxygenated blood from the body and pumps it to the lungs. The left side receives oxygenated blood from the lungs and pumps it to the rest of the body.
The double circulatory system
The double circulation is efficient because blood can be pumped to the lungs at low pressure (so the thin alveoli are not damaged), then returned to the heart and pumped out again at high pressure to reach the whole body quickly. This keeps oxygenated and deoxygenated blood separate.
Arteries, veins and capillaries
| Vessel | Carries | Pressure | Key features |
|---|---|---|---|
| Artery | Blood away from heart | High | Thick muscular, elastic walls; narrow lumen |
| Vein | Blood back to heart | Low | Thinner walls; wide lumen; valves |
| Capillary | Blood through tissues | Falling | Walls one cell thick; very narrow |
- Arteries have thick, elastic, muscular walls to withstand and smooth out the high pressure from the heart.
- Veins have thinner walls and a wide lumen because the pressure is low; valves stop blood flowing backwards.
- Capillaries are only one cell thick so substances such as oxygen, glucose and carbon dioxide can diffuse quickly between the blood and the body's cells.
How blood flows through the heart
Following the path of blood is a common exam task. Deoxygenated blood from the body enters the right atrium, passes into the right ventricle, and is pumped to the lungs, where it picks up oxygen. The now oxygenated blood returns to the left atrium, passes into the left ventricle, and is pumped out to the whole body. The valves between the atria and ventricles, and at the exits, keep the blood flowing one way only. After delivering oxygen to the body cells, the blood returns deoxygenated to the right side, and the cycle repeats.
How the heart muscle is supplied
The heart is made of muscle that must respire constantly, so it needs its own blood supply. This comes from the coronary arteries, which branch off and run over the surface of the heart. If these arteries become narrowed by fatty deposits, the heart muscle is starved of oxygen, which is the basis of coronary heart disease. Knowing that the heart supplies itself through the coronary arteries explains why a problem in those vessels, rather than the chambers, causes a heart attack.
Try this
Q1. Name the upper chambers of the heart. [1 mark]
- Cue. The atria.
Q2. State why capillary walls are only one cell thick. [1 mark]
- Cue. So substances can diffuse quickly between the blood and the body cells (short diffusion distance).
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksExplain why the wall of the left ventricle is thicker and more muscular than the wall of the right ventricle.Show worked answer →
A Unit 1 explain question worth 4 marks. Reward: the left ventricle pumps blood to the whole body (1), which is a long distance, so it needs to create a high pressure (1); the right ventricle only pumps blood to the lungs (1), which are nearby and need a lower pressure, so its wall is thinner (1). Markers credit the destination of each side, the distance and the pressure needed. A common error is to reverse the two sides or to say the left side carries more blood (both sides pump equal volumes).
WJEC style3 marksCompare the structure of an artery and a vein, giving two differences and linking them to function.Show worked answer →
A Unit 1 compare question. Reward: arteries have thick muscular elastic walls to withstand the high pressure of blood from the heart, while veins have thinner walls because the pressure is lower (1); arteries have a narrow lumen and veins a wide lumen (1); veins have valves to stop blood flowing backwards, which arteries do not need (1). Markers credit two clear structural differences each linked to function. A common error is to give the difference without the reason.
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