Biology
Evaluating gas exchange in humans
We are learning how to: • Describe the features of the human gas exchange system. • Explain how the features enable gases to be exchanged. • Evaluate how well adapted the human gas exchange system is to its function.
Gases pass from the lungs to the blood and vice versa across the alveoli. These must have certain design features to enable this to happen efficiently. Without well-adapted alveoli we would be deprived of oxygen. Our gas exchange system
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air in, oxygen rich
Once in the lungs, air travels through the bronchioles to the alveoli. Gas exchange then takes place across the walls of the alveoli. •
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alveoli in the lungs
Oxygen passes from the alveoli into the blood, which carries it to all the body cells where it is used in respiration.
oxygen carried by red blood cells
Carbon dioxide is a waste product of respiration in cells. It is carried by blood back to the lungs. Carbon dioxide passes from the blood into the alveoli. The carbon dioxide is breathed out.
carbon dioxide carried, dissolved in blood
1. Name the gas that passes back into the alveoli after respiration in the cells of the body. 2. Explain why oxygen is taken to all the cells in the body.
Designed to perfection?
air out, carbon dioxide rich
cells in the body
FIGURE 1.2.15a: Gas exchange across alveoli. Why do you think the alveoli have such a large surface area?
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Our gas exchange system has specific features that allow it to work efficiently. Tiny blood vessels called capillaries run over the surface of the alveoli. Gases travel between the blood in the capillaries and the air in the alveoli. This exchange is made as effective as possible by the thin lining of the alveoli and the very thin lining of the capillary walls. The surfaces of the alveoli are ‘bumpy’ (similar to cauliflower florets) and the walls are moist.
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KS3 Science Book 1: Eating, Drinking and Breathing
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