Gas exchange by diffusion
Once air reaches the alveoli, gases move between the air and the blood by diffusion — the movement of molecules from where they are more concentrated to where they are less concentrated. The alveoli hold air rich in oxygen, while the blood arriving from the body is low in oxygen and high in carbon dioxide. Because of this difference in concentration, the two gases move at the same time but in opposite directions across the thin, moist alveolar wall.
Key idea
At the alveolus: oxygen diffuses from the alveolus into the blood; carbon dioxide diffuses from the blood into the alveolus to be breathed out.
How oxygen is carried
Oxygen does not simply float in the blood. It enters the red blood cells and joins with the red pigment haemoglobin to form oxyhaemoglobin. The blood then carries oxygen to every part of the body.
Exchange at the body cells
At the tissues the situation is reversed. Body cells are using oxygen and producing carbon dioxide during cellular respiration, so oxygen is low and carbon dioxide is high there. Oxygen therefore diffuses from the blood into the cells, and carbon dioxide diffuses from the cells into the blood to be carried back to the lungs.
Example
In a working leg muscle, oxygen leaves the blood and enters the muscle cells, while the carbon dioxide they make passes into the blood to be removed at the lungs.
Remember
- Diffusion goes from high to low concentration.
- Haemoglobin carries oxygen as oxyhaemoglobin.