How are exchange surfaces adapted, and how does the circulatory system transport substances?
The need for exchange surfaces and a transport system, the structure of the lungs and gas exchange, and the human circulatory system including the heart and blood.
A focused answer to Edexcel GCSE Combined Science Topic 8 (CB8), covering the need for exchange surfaces and a transport system, the structure of the lungs and gas exchange in the alveoli, and the human circulatory system including the heart and blood.
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What this dot point is asking
Edexcel wants you to explain why large organisms need exchange surfaces and a transport system, describe the structure of the lungs and how gas exchange happens in the alveoli, and describe the human circulatory system including the heart and the blood.
Why exchange surfaces are needed
Good exchange surfaces share features: a large surface area, a thin barrier (short diffusion distance), a moist surface, and a way to maintain a steep concentration gradient (such as a blood supply or ventilation).
The lungs and gas exchange
Air enters through the trachea, which branches into two bronchi, then smaller bronchioles, ending in tiny air sacs called alveoli. Gas exchange happens across the alveoli:
- Oxygen diffuses from the air in the alveoli into the blood in the surrounding capillaries.
- Carbon dioxide diffuses from the blood into the alveoli to be breathed out.
The alveoli are adapted with a large surface area, walls one cell thick, a moist lining and a rich blood supply, all of which speed up diffusion.
The circulatory system
The heart has four chambers. 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 body. The left ventricle has a thicker muscular wall because it pumps blood further, at higher pressure.
Three types of blood vessel carry the blood: arteries (carry blood away from the heart at high pressure, thick walls), veins (return blood to the heart at low pressure, with valves), and capillaries (tiny, one-cell-thick vessels where exchange with cells happens).
The blood
Blood is made of:
- Plasma: the liquid that carries cells, dissolved substances, carbon dioxide and urea.
- Red blood cells: carry oxygen using haemoglobin; they have no nucleus and a biconcave shape for a large surface area.
- White blood cells: defend against pathogens.
- Platelets: small fragments that help the blood to clot.
Try this
Q1. State two adaptations of the alveoli for gas exchange. [2 marks]
- Cue. Any two of: large surface area, thin (one-cell-thick) walls, moist surface, rich blood supply.
Q2. Name the part of the blood that carries oxygen. [1 mark]
- Cue. Red blood cells (using haemoglobin).
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 20194 marksExplain three ways the alveoli in the lungs are adapted for efficient gas exchange.Show worked answer →
A 4-mark explain question linking structure to function.
The alveoli give a very large total surface area, so more gas can be exchanged at once (1 mark). They have walls that are only one cell thick, giving a short diffusion distance for oxygen and carbon dioxide (1 mark). They have a rich blood supply from many capillaries, which maintains a steep concentration gradient by removing oxygen and bringing carbon dioxide (1 mark). They are also moist, so gases dissolve before diffusing (1 mark, any three).
Markers reward three valid adaptations, each linked to why it speeds up diffusion (large area, short distance, steep gradient, moist surface).
Edexcel 20213 marksDescribe the path taken by blood through the heart, starting from the blood returning from the body.Show worked answer →
A 3-mark describe question on the double circulatory system.
Deoxygenated blood from the body enters the right atrium, then passes to the right ventricle, which pumps it to the lungs to be oxygenated (1 mark). Oxygenated blood returns from the lungs to the left atrium, then to the left ventricle (1 mark). The left ventricle pumps the oxygenated blood out to the rest of the body through the aorta (1 mark).
Markers reward the correct order through the four chambers and the idea that the right side serves the lungs and the left side serves the body.
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Sources & how we know this
- Edexcel GCSE (9-1) Combined Science (1SC0) specification — Pearson (2016)