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The site of gas exchange in mammals is the epithelium of the alveoli
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Exchange of gasTo enable efficient transfer of materials across them, ex- es in the lungs
change surfaces are thin, partially permeable and have
a large surface area
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Role of the alveoli in gas exchange:
There are about 300 million alveoli in each human lung
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Around each alveolus is a network of pulmonary capillaries, so narrow that red
blood cells are flattened against the thin capillary walls in order to squeeze through
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Diffusion of gases between the alveoli and the blood will be very rapid because:
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Red blood cells are slowed as they pass through the pulmonary capillaries, allowing
more time for diffusion
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Being thin, these specialised exchange surfaces are easily damaged and
so are often located inside an organism for protection
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Where an exchange surface (like the lungs) is located in the body, the
organism has some means of moving the external medium over the surface, like a means of ventilating the lungs in a mammal
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Breathing movements constantly ventilate the lungs, and the action of the heart constantly circulates blood around the alveoli
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Blood flow through the pulmonary capillaries maintains a concentration gradient
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Risk factors for lung disease

Smoking: 90% of people suffering from COPD (chronic obstructive
pulmonary disease—lung disease) are, or have been heavy smokers
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Genetic Make-up: some people are genetically more likely to get
lung disease = this explains why some lifelong smokers never get
lung disease while others die early
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Occupation: people working with harmful chemicals, gases and
dusts that can be inhaled have an increased risk

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