Gas Exchange
2.1 i) Know the properties of gas exchange surfaces in living organisms (large surface area to volume
ratio, thickness of surface, difference in concentration).
ii) Understand how the rate of diffusion is dependent on these properties and can be calculated using
Fick’s Law of Diffusion.
iii) Understand how the structure of the mammalian lung is adapted for rapid gaseous exchange.
Fick’s Law
• Rate of diffusion is directly proportional to surface area.
• Rate of diffusion is directly proportional to concentration gradient.
• Rate of diffusion is inversely proportional to diffusion distance.
How is the Alveolus Adapted to Gas Exchange?
1. High Surface Area
• The shape of the alveolus provides a large surface area : volume ratio.
• Quantitiy – there are millions of alveoli in the lungs.
2. Short Diffusion Distance
• Both the alveolar epithelium and capillary endothelium are one cell thick.
• The lumen of the capillary is narrow, which pushes the erythrocytes up against the capillary endothelium.
3. High Concentration Gradient
• Ventilation – inhalation brings air (oxygen) to alveolus; the alveolar air space has a higher concentration
of oxygen than the blood in the capillary, so oxygen diffuses across the membrane.
The Lungs
• Oxygen diffuses out of the alveoli, across the
alveolar epithelium and the capillary endothelium,
into the blood.
• Carbon dioxide diffuses into the alveoli from the
blood and is breathed out.
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2.1 i) Know the properties of gas exchange surfaces in living organisms (large surface area to volume
ratio, thickness of surface, difference in concentration).
ii) Understand how the rate of diffusion is dependent on these properties and can be calculated using
Fick’s Law of Diffusion.
iii) Understand how the structure of the mammalian lung is adapted for rapid gaseous exchange.
Fick’s Law
• Rate of diffusion is directly proportional to surface area.
• Rate of diffusion is directly proportional to concentration gradient.
• Rate of diffusion is inversely proportional to diffusion distance.
How is the Alveolus Adapted to Gas Exchange?
1. High Surface Area
• The shape of the alveolus provides a large surface area : volume ratio.
• Quantitiy – there are millions of alveoli in the lungs.
2. Short Diffusion Distance
• Both the alveolar epithelium and capillary endothelium are one cell thick.
• The lumen of the capillary is narrow, which pushes the erythrocytes up against the capillary endothelium.
3. High Concentration Gradient
• Ventilation – inhalation brings air (oxygen) to alveolus; the alveolar air space has a higher concentration
of oxygen than the blood in the capillary, so oxygen diffuses across the membrane.
The Lungs
• Oxygen diffuses out of the alveoli, across the
alveolar epithelium and the capillary endothelium,
into the blood.
• Carbon dioxide diffuses into the alveoli from the
blood and is breathed out.
4
