Final answer:
Oxygen and carbon dioxide move independently and diffuse down their pressure gradients. Oxygen diffuses from the alveoli into the bloodstream, while carbon dioxide diffuses from the bloodstream into the alveoli. This continuous process of gas exchange ensures equilibrium between the air and lungs.
Step-by-step explanation:
Oxygen and carbon dioxide move independently of each other; they diffuse down their own pressure gradients. As blood leaves the lungs through the pulmonary veins, the venous Po₂ = 100 mm Hg, whereas the venous Pco₂ = 40 mm Hg. As blood enters the lungs through the pulmonary arteries, the arterial Po₂ = 40 mm Hg, whereas the arterial Pco₂ = 45 mm Hg. Because the concentrations of oxygen and carbon dioxide in the alveoli are higher than in the bloodstream, oxygen diffuses from the alveoli into the bloodstream, while carbon dioxide diffuses from the bloodstream into the alveoli. This continuous process of gas exchange ensures that oxygen and carbon dioxide reach equilibrium between the air and lungs in a similar time frame.