Question

With the help of labelled diagram explain the exchange of gases between alveolus and capillary.

Answer 1

External respiration is the process of exchange of gases at the alveolar level. Alveoli consist of a layer of simple squamous epithelium resting on a basement membrane. They are intimately associated with a dense network of capillaries. The capillary wall is also made up of simple squamous epithelium resting on a thin basement membrane. Both the layers are thin walled and have similar structure. Together they form the respiratory membrane through which gaseous exchange occurs i.e. between the alveolar air and the blood. Diffusion of gases will take place from an area of higher partial pressure to an area of lower partial pressure until the partial pressure in the two regions reaches equilibrium. The partial pressure of CO₂ of blood entering the pulmonary capillaries is 45 mmHg while partial pressure of CO₂ in alveolar air is 40 mmHg.

This difference in partial pressure causes diffusion of CO₂ from the capillaries into the alveolus. Similarly, partial pressure of O₂ of blood in pulmonary capillaries is 40 mmHg while in alveolar air it is 104 mmHg. Due to this difference O₂ diffuses from alveoli to the capillaries.