The Circulatory System of Humans
The human circulatory system consists of a four-chambered heart, three types of blood vessels, arteries, veins and capillaries, and blood made of plasma, red blood cells, white blood cells and platelets, which together transport substances around the body.
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The four-chambered heart
The human heart has four chambers: two upper atria and two lower ventricles. The atria have thin, muscular walls and receive blood returning to the heart, while the ventricles have thicker, more muscular walls because they must pump blood out of the heart under higher pressure.
The left ventricle has the thickest wall of all four chambers, since it pumps oxygenated blood at high pressure all the way around the body, while the right ventricle only needs to pump deoxygenated blood the short distance to the lungs.
Valves keep blood flowing in one direction only. Atrioventricular valves (the bicuspid valve on the left, tricuspid valve on the right) sit between each atrium and ventricle, and semilunar valves sit at the base of the arteries leaving the heart.
Both types close to stop blood flowing backward.
Blood vessels
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Wall | Thick, muscular and elastic | Thinner, less muscular | One cell thick |
| Lumen | Narrow | Wide | Very narrow, just fits one red blood cell |
| Valves | Absent (except near the heart) | Present, to prevent backflow | Absent |
| Blood pressure | High | Low | Very low |
| Direction of blood flow | Away from the heart | Towards the heart | Between arteries and veins, at exchange sites |
Components of blood
- Plasma is the pale-yellow liquid part of blood; it carries dissolved food, hormones, carbon dioxide, and heat around the body.
- Red blood cells contain haemoglobin, which binds to oxygen to form oxyhaemoglobin, transporting oxygen from the lungs to respiring cells.
- White blood cells defend the body against pathogens, either by engulfing them (phagocytes) or by producing antibodies (lymphocytes).
- Platelets are cell fragments that help blood clot at a wound, preventing blood loss and the entry of pathogens.
How it is examined
Questions on this standard often show a diagram or cross-section of the heart or a blood vessel and ask you to name the labelled parts, explain why a chamber or vessel wall has a particular thickness, or identify a vessel from its structure. You should also be able to state the function of each blood component and link an unusual blood test result to a possible health problem.
Key terms
- Artery, a thick-walled vessel that carries blood away from the heart under high pressure.
- Vein, a wider, thinner vessel with valves that returns blood to the heart at low pressure.
- Capillary, a vessel one cell thick where substances are exchanged between blood and tissues.
- Haemoglobin, the red pigment in red blood cells that binds oxygen to form oxyhaemoglobin.
- Double circulation, blood passes through the heart twice in one complete circuit of the body.
Worked exam-style question
Question. The diagram shows a vertical section through a human heart. (a) Name the chamber that pumps blood to the lungs and the blood vessel it pumps into.
(b) Explain why the wall of the left ventricle is thicker than the wall of the right ventricle. (c) Blood leaving the left ventricle passes a valve at the base of the aorta; state the function of this valve.
(d) During exercise the tissues respire faster. Explain how the structure of a capillary suits it to supplying these tissues.
Model answer. (a) The right ventricle pumps blood into the pulmonary artery, which carries deoxygenated blood to the lungs. (b) The left ventricle pumps oxygenated blood at high pressure around the whole body, so it needs a thicker, more muscular wall to generate this force; the right ventricle pumps blood only the short distance to the lungs at lower pressure.
(c) The semilunar valve prevents the backflow of blood from the aorta into the ventricle. (d) A capillary wall is one cell thick, giving a short diffusion distance, and the capillaries form a network with a large surface area, so oxygen and glucose diffuse quickly to the respiring cells and carbon dioxide is carried away.
Practice question
A student examines two blood vessels under a microscope. Vessel P has a thick, muscular wall and a narrow lumen; vessel Q has a thinner wall, a wide lumen, and valves along its length.
(a) Identify vessel P and vessel Q. (b) Vessel Q carries blood towards the heart.
Explain how its valves help it to do this.
Exam tip
Source:SRC-DSKP-EN
Frequently asked questions
Why is the wall of the left ventricle thicker than the right ventricle?
How can you tell an artery from a vein by its structure?
What are the main components of blood and their jobs?
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