EXCHANGE OF RESPIRATORY GASES IN LUNGS

Oxygen is essential for the cells. Carbon dioxide, which is produced as waste product in the cells must be expelled from the cells and body. Lungs serve to exchange these two gases with blood.

EXCHANGE OF RESPIRATORY GASES IN LUNGS

In the lungs, exchange of respiratory gases takes place between the alveoli of lungs and the blood. Oxygen enters the blood from alveoli and carbon dioxide is expelled out of blood into alveoli. Exchange occurs through bulk flow diffusion. Exchange of gases between blood and alveoli takes place through respiratory membrane.

RESPIRATORY MEMBRANE

Respiratory membrance is a membranous structure through which exchange of respiratory gases takes

place. It is formed by epithelium of respiratory unit and endothelium of pulmonary capillary. Epithelium

of respiratory unit is a very thin layer.

Since, the capillaries are in close contact with this membrane, alveolar air is in close proximity to capillary

blood. This facilitates gaseous exchange between air and blood.

Respiratory membrane is formed by different layers of structures belonging to the alveoli and capillaries.

Layers of Respiratory Membrane

In spite of having many layers, respiratory membrane is very thin with an average thickness of 0.5 μ. Total surface area of the respiratory membrane in both the lungs is about 70 square meter.

Average diameter of pulmonary capillary is only 8 μ, which means that the RBCs with a diameter of

7.4 μ actually squeeze through the capillaries. Therefore, the membrane of RBCs is in close contact with capillary wall. This facilitates quick exchange of oxygen and carbon dioxide between the blood and alveoli.

DIFFUSING CAPACITY

Diffusing capacity is defined as the volume of gas that diffuses through the respiratory membrane each

minute for a pressure gradient of 1 mm Hg.

 Diffusing Capacity for Oxygen and Carbon Dioxide

Diffusing capacity for oxygen is 21 mL/minute/1 mm Hg. Diffusing capacity for carbon dioxide is 400 mL/minute/1 mm Hg. Thus, the diffusing capacity for carbon dioxide is about 20 times more than that of oxygen.

Factors Affecting Diffusing Capacity

1. Pressure gradient

Diffusing capacity is directly proportional to pressure gradient. Pressure gradient is the difference between the partial pressure of a gas in alveoli and pulmonary capillary blood (see below). It is the major factor, which affects the diffusing capacity.

2. Solubility of gas in fluid medium

Diffusing capacity is directly proportional to solubility of the gas. If the solubility of a gas is more in the fluid medium, a large number of molecules dissolve in it and diffuse easily.

3. Total surface area of respiratory membrane

Diffusing capacity is directly proportional to surface area of respiratory membrane. Surface area of respiratory membrane in each lung is about 70 sq m. If the total surface area of respiratory membrane decreases, the diffusing capacity for the gases is decreased. Diffusing capacity is decreased in emphysema in which many of the alveoli are collapsed because of heavy smoking or

oxidant gases.

4. Molecular weight of the gas

Diffusing capacity is inversely proportional to molecular weight of the gas. If the molecular weight is more, the density is more and the rate of diffusion is less.

5. Thickness of respiratory membrane

Diffusion is inversely proportional to the thickness of respiratory membrane. More the thickness of respiratory membrane less is the diffusion. It is because the distance through which the diffusion takes place is long. In conditions like fibrosis and edema, the diffusion rate is reduced, because the thickness of respiratory membrane is increased.

Relation between Diffusing Capacity and Factors Affecting it

Relation between diffusing capacity and the factors affecting it is expressed by the following formula:

surface area of the respiratory membrane in both the lungs is about 70 square meter.

Average diameter of pulmonary capillary is only 8 μ, which means that the RBCs with a diameter of

7.4 μ actually squeeze through the capillaries. Therefore, the membrane of RBCs is in close contact with capillary wall. This facilitates quick exchange of oxygen and carbon dioxide between the blood and alveoli.

DIFFUSING CAPACITY

Diffusing capacity is defined as the volume of gas that diffuses through the respiratory membrane each

minute for a pressure gradient of 1 mm Hg.

 

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