The alveoli are the functional unit of the lungs and are the principal respiratory surface of higher animals like amphibians, reptiles, birds, and mammals. Each alveoli are composed of very thin epithelial cells and the alveolar lining is approx 1 cell thick, this thin surface allows gaseous exchange between the blood and air present within the lungs. The alveoli are richly supplied with blood capillaries and it is between these capillaries and the alveolar membrane the exchange of gases occurs by diffusion. The diffusion of gases is driven by the variations of partial pressures of oxygen and carbon dioxide between the alveolar space and blood. The partial pressure of oxygen (pO2) is 104 mm of Hg, which is higher than the pO2 in deoxygenated blood (40 mm of Hg), thus it allows oxygen to diffuse from the alveolar space into the blood, on the contrary, the pCO2 in the deoxygenated blood is (45mm of Hg) which is higher than the alveolar space where pCO2 is 40 mm of Hg thus it helps the CO2 to diffuse out of the blood into the alveolar space to be released during expiration.