Diffusion of gases mainly takes in the alveoli, as it is the primary site of exchange of gases. Exchange of gases also occurs between blood and tissues. O₂ and CO₂ are exchanged in these sites by simple diffusion mainly based on pressure or 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 pO₂ for oxygen and pCO₂ for carbon dioxide. Number blood capillaries cover each alveolus. The capillaries that come from the pulmonary artery bring deoxygenated blood and blood capillaries, which leave the alveoli to form the pulmonary vein, carry oxygenated blood. The partial pressure of O₂ in the alveoli is 104 and in oxygenated blood it is 95 due to this variation diffusion of oxygen takes place from the alveoli to the capillaries. The partial pressure of CO₂ in the alveoli is 40 and deoxygenated blood is 45 due to this variation in pressure diffusion of CO₂ takes place from the capillaries to the alveoli. These diffusions take place in the alveoli because of the presence diffusion membrane. The diffusion membrane is made up of three major layers namely, the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries and the basement substance in between them. However, its total thickness is much less than a millimetre. Since all the factors in our body are favourable for diffusion of O₂ from alveoli to tissues through the blood capillaries and that of CO2 from tissues to alveoli through the blood capillaries diffusion takes place only in the alveoli of the respiratory system.