Question

Explain the structure and function of the respiratory membrane.

Answer 1

The respiratory membrane is the thin membrane involved in gas exchange of CO2 and O2 in the alveoli of lung. It is the air-blood barrier present in lungs. At this membrane, the alveolar wall meets the capillary walls. Thus, it consists of alveolar epithelial cells, capillary endothelial cells linked by their basement membrane. Air is present on alveolar side of membrane while blood flows on the capillary side of the membrane. The membrane is around 0.5mm thick and is semi-permeable. This membrane is folded into around 300 million alveolar air sacs. Hence, it provides a large surface area for gas exchange of oxygen and CO2.

Oxygen will move from the alveoli into the capillaries while CO2 moves from blood into the alveoli. These gases are transported across the membrane by simple diffusion. Both gases are lipid soluble. This gas exchange depends on the partial pressures of the gases in the alveoli and blood. Partial pressures indicate the concentration of the gas. Oxygen partial pressure in alveoli is 104 mmHg. Its partial pressure in blood capillaries is lower of about 40 mmHg. This difference of 64 mmHg partial pressure creates a pressure gradient that drives the simple diffusion of oxygen from alveoli into the blood capillaries.

In case of carbon dioxide, although similar difference exists in partial pressure of CO2 in capillary and alveoli, this difference is only 5 mm Hg (45 mmHg in capillaries and 40 mmHg in alveoli). The greater solubility of CO2 as compared to oxygen, however, will ensure that relative concentration of both gases that are diffusing remain the same. Diffusion can only occur if air diffusing is moist.

The diffusion rate depends on gas laws such as the Boyle’s law.