→ occur by diffusion Blood entering the lungs → has a lower concentration of oxygen and a higher concentration of carbon dioxide than the atmospheric air that enters the alveoli in the lungs. → concentration gradient between alveolar air and blood is set up for oxygen and carbon dioxide to diffuse.
- oxygen dissolves in the thin film of moisture on the inner walls of the alveolus
- the dissolved oxygen then diffuses through the wall of the alveolus as well as the wall of the blood capillary into the red blood cells.
- oxygen then combines with hemoglobin to form oxyhemoglobin.
- carbon dioxide also diffuses from the blood to alveolar air.
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Oxygen and carbon dioxide concentration gradients are maintained between alveolar air and blood:
- continuous flow of blood through blood capillaries
- continuous breathing in an out - which causes air in the lungs to be constantly refreshed.