E XCHANGE OF G ASES

E XCHANGE OF G ASES Alveoli are the primary sites of exchange of gases. Exchange of gases also occur between blood and tissues. O and CO are exchanged in these sites by simple diffusion mainly based on pressure/concentration gradient. Solubility of the gases as well as the thickness of the membranes involved in diffusion are also some important factors that can affect the rate of diffusion. Pressure contributed by an individual gas in a mixture of gases is called partial pressure and is represented as pO for oxygen and pCO for carbon dioxide. Partial pressures of these two gases in the atmospheric air and the two sites of diffusion are given in Table . and in Figure . . The data given in the table clearly indicates a concentration gradient for oxygen from alveoli to blood and blood to tissues. Similarly, T ABLE . Partial Pressures (in mm Hg) of Oxygen and Carbon dioxide at Different Parts Involved in Diffusion in Comparison to those in Atmosphere Respiratory Atmospheric Alveoli Blood Blood Tissues Gas Air (Deoxygenated) (Oxygenated) O CO . a gradient is present for CO in the opposite direction, i.e., from tissues to blood and blood to alveoli. As the solubility of CO is - times higher than that of O , the amount of CO that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher compared to that of O . The diffusion membrane is made up of three major layers (Figure . ) namely, the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries and the basement substance (composed of a thin basement membrane supporting the squamous epithelium and the basement membrane surrounding the single layer endothelial cells of capillaries) in between them. However, its total thickness is much less than a millimetre. Therefore, all the factors in our body are favourable for diffusion of O from alveoli to tissues and that of CO from tissues to alveoli. Figure . A Diagram of a section of an alveolus with a pulmonary capillary. Figure . Diagrammatic representation of exchange of gases at the alveolus and the body tissues with blood and transport of oxygen and carbon dioxide