Gas diffusion across the respiratory membrane
The gas diffusion coefficient depends on the solubility of the gas in the cell membrane, and it is inversely proportional to the molecular weight.
After the alveoli are ventilated, the next step in respiration is the exchange (diffusion) of O2 from the alveoli into the pulmonary blood and the diffusion of CO2 in the reverse direction. from the blood to the alveoli. Diffusion is simply the random back-and-forth diffusion of gas molecules across the respiratory membrane and adjacent fluids. However, in respiratory physiology, we are concerned not only with the basic mechanism of diffusion but also with the rate at which it occurs, which requires a great understanding of the physics of diffusion and gas exchange.
First: Thickness of the film.
Second: The surface area of the membrane.
Third: The diffusion coefficient of the gas in the substance of the membrane.
Fourth: The difference in partial pressure of the gas between the two sides of the membrane.
The thickness of the respiratory membrane is sometimes increased in the following cases:
Oedema of the interstitial space of the respiratory membrane or the alveoli itself. In addition, some pulmonary diseases that cause pulmonary fibrosis can increase the thickness of a lung, a number of parts of the cell membrane of the respiratory tract. Diffusion across the membrane is inversely proportional to the respiratory membrane thickness. Any factor that increases the thickness of the membrane by two or three times interferes with respiration.
The area of the respiratory membrane can be reduced in cases such as lobectomy. In addition, in emphysema, many alveoli are combined to lose the alveolar wall, leading to a decrease in the lung area. Sometimes it can also reduce the area up to five times. When the total lung surface area is reduced to only 1/3 or 1/4 of normal, respiration is essentially prevented.
The gas diffusion coefficient depends on the solubility of the gas in the cell membrane. And it is inversely proportional to the molecular weight. The rate of gas diffusion in the respiratory membrane is almost exactly the same as in water. Thus, the diffusion rate of CO2 is 20 times faster than O2 and O2 is 2 times faster than that of N2.
The transmembrane pressure difference is the difference between the partial pressure of gases in the alveoli and the partial pressure of gases in the pulmonary capillary blood. For O2, the partial pressure in the alveoli is higher than the partial pressure of that gas in the pulmonary capillary blood. So true diffusion will go from the alveoli to the pulmonary capillaries with CO2 in reverse.