CBSE Class 12-science Answered

The partial pressures of O₂ and CO₂ in the blood vary at different points in the circulatory system.

Blood flowing through the alveolar capillaries has a lower Po₂ and a higher P_Co₂ than the air in the alveoli. As a result, CO₂ diffuses down its partial pressure gradient from the blood to the air in the alveoli. At the same time, O₂ from the air dissolves in the fluid that coats the alveolar epithelium and diffuses into the blood. By the time, the blood leaves the lungs in the pulmonary veins, its Po₂ has been raised and its P_Co₂ has been lowered. After returning to the heart, this blood is pumped through the systemic circuit.

In the tissue capillaries, gradients of partial pressure favour the diffusion of O₂ out of the blood and CO₂ into the blood. These gradients exist because of cellular respiration in the mitochondria of cells near each capillary removes O₂ from and adds CO₂ to the surrounding interstitial fluid. After the blood unloads O₂ and loads CO₂, it is returned to the heart and pumped to the lungs again.