4 Gas Exchange at the Alveoli

Youtube video on gas exchange at the alveoli. URL: http://www.youtube.com/watch?v=XTMYSGXhJ4E Visited on 10 August 2013

Oxygen gas (highest in the alveolar air)

• Oxygen in the alveolar air dissolves in the layer of moisture that lines the alveoli wall.

• Dissolved oxygen gas diffuses down the concentration gradient from the alveolar space into the red blood cells.

• Within the red blood cells, oxygen combines with haemoglobin to form oxyhaemoglobin.

oxygen + haemoglobin <----> oxyhaemoglobin

Carbon dioxide gas (highest in the blood that just enters the alveoli)

• Carbon dioxide gas produced by tissue cells diffuse into blood and enters red blood cells.

• Reaction occurs in red blood cells (RBCs) and is catalysed by carbonic anhydrase.

• Hydrogencarbonate ions (HCO₃^-) formed diffuse out of RBCs into blood plasma.

• Once (HCO₃^-) ions reach the lungs, they diffuse back into RBCs where they are converted into carbonic acid and then into CO₂ and H₂O.

• CO₂ then diffuses out of blood capillaries into alveoli where it is removed when being breathed out.

4.1 Adaptations of alveolus for gas exchange

1. Each alveolus is well supplied with blood capillaries to increase the rate of exchange of oxygen and carbon dioxide gas between the alveolar air space and blood capillaries.

2. Continuous flow of blood through the blood capillaries so as to maintain a steep diffusion gradient of oxygen and carbon dioxide gas between the alveolar air space and the blood.

1. One cell thick alveolar wall separating the blood capillaries from the alveolar air. Allows fast exchange of gases between the blood and alveolar air space.

1. Layer of moisture lining the alveoli. Enables oxygen gas to dissolve in the layer of moisture before diffusing into the blood.

4.2 Maintenance of oxygen and carbon dioxide concentration gradients between alveolar air and blood

1. Continuous flow of blood through the blood capillaries.

2. Breathing of air in and out of the alveoli, caused by breathing.