The following points highlight the top five processes for diffusion of gases in the lungs.
1. When the gases of the inspired air come in contact with the alveolar membrane of the lung, the exchange of gases takes place following the usual laws of diffusion. Thus, the gas passes into the blood through the membrane.
2. The gas pressure in the blood is usually expressed as gas “tensions”.
The oxygen tension (PO2) is the pressure of the dry gas with which the dissolved oxygen in the blood is in equilibrium.
3. Oxygen tension in alveolar air: 107 mm Hg.
Oxygen tension in venous blood: 40 mm Hg.
Hence, a pressure difference of 67 mm Hg drives oxygen from the alveoli of the lung into the blood.
CO2 tension in alveolar air; 36 mm Hg.
CO2 tension in venous blood: 46 mm Hg.
The difference of 10 mm Hg. is sufficient to drive CO2 from the blood into the lung. In the resting state, a difference of 0.12 mm Hg in CO2 tension causes the elimination of the gas.
Nitrogen gas is physiologically inert because its tension is the same in venous blood and lung alveoli (570 mm Hg).
4. After this exchange of gases, the blood becomes arterial.
Oxygen tension of arterial blood: 100 mm Hg.
CO2 tension of arterial blood: 40 mm Hg.
Nitrogen tension of arterial blood: 570 mm Hg.
These gases are dissolved in the blood.
5. The quantities of each of these gases which can be dissolved (calculated content in blood) is compared with the actual quantities found in the blood by the following data.