Explanation

Gaseous exchange between blood and alveolar air across respiratory membrane occurs by simple diffusion. The blood drained from lungs includes not only oxygenated blood but also some deoxygenated blood that has supplied its oxygen to tissue cells. The$\ p_{O_{2}}$ of this blood is about 95-97 mm hg. After receiving this blood from the lungs, the heart pumps it into the arteries, which carry it to all parts of the body, while flowing through the capillary networks in various tissues, his blood supplies oxygen to all cells in exchange of carbon dioxide. The average $p_{O_{2}}$ in tissue fluids is about 40mm Hg, whereas the $p_{O_{2}}$ in arterial blood supplying the tissues is 95 mm Hg. This pressure difference ensures vary rapid deoxygenation of the unstable oxyhaemoglobin in the tissue and diffusion of released oxygen into tissue fluid and then into the cells. The arterial blood normally supplies about 25% of its $O_{2}$ to the tissue.