IGCSE Biology / Breathing and gas exchange
Alveoli and how they are adapted
Edexcel International GCSE Biology (4BI1), chapter 3: Breathing and gas exchange, specification point 2.40B, 2.48. Free lesson.
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Alveoli are adapted to exchange oxygen and carbon dioxide quickly by diffusion. Their structure helps keep the concentration gradients steep, so oxygen enters the blood and carbon dioxide leaves it.
How alveoli are adapted for fast diffusion
The alveoli are tiny air sacs in the lungs. There are about 300 million of them, giving the lungs a very large total surface area. Each feature helps gases diffuse faster. Many alveoli provide a large surface area, so more diffusion happens per second. The alveolus wall is one cell thick, giving a short diffusion distance. The capillary wall is also one cell thick, so oxygen crosses just two cell layers in total. A moist lining lets gases dissolve before they diffuse across the walls. A dense capillary network means blood flows past the alveoli continuously. It carries…
How oxygen and carbon dioxide move
Diffusion is the movement of particles from a higher concentration to a lower concentration. Oxygen and carbon dioxide diffuse in opposite directions, each down its own concentration gradient. Air in the alveolus has a high oxygen concentration. Blood arriving from the body has a low oxygen concentration, so oxygen diffuses into the blood. Blood arriving from the body has a high carbon dioxide concentration. Alveolar air has a low carbon dioxide concentration, so carbon dioxide diffuses into the alveolus. Both movements are passive. Ventilation and blood flow help keep both gradients…
How blood changes as it passes the alveoli
Blood arrives in the pulmonary artery and leaves in the pulmonary vein. Gas exchange changes its oxygen and carbon dioxide levels. Arriving blood has low oxygen and high carbon dioxide. It is dark red and comes from the right ventricle. Leaving blood has high oxygen and low carbon dioxide. It is bright red because it contains oxyhaemoglobin, and it goes to the left atrium. The pulmonary artery carries deoxygenated blood, while the pulmonary vein carries oxygenated blood.
What to include in an adaptation answer
For full marks, name each adaptation and link it to what it achieves. A feature on its own does not explain why gas exchange is fast. Connect large surface area to more diffusion per second. Connect thin walls to a short diffusion distance. Connect the moist lining to gases dissolving before they diffuse. Connect a good blood supply to blood flowing past and maintaining a steep concentration gradient.