Final answer:
The oxygen partial pressure in alveolar gas drives the diffusion of O2 to the mitochondria, with a substantial difference between alveolar and capillary pressures facilitating this exchange.
Step-by-step explanation:
Only the O2 partial pressure that still prevails in the alveolar gas is available to drive the diffusion of O2 to the mitochondria. The solubility of oxygen in blood may not be high, but it is the pressure difference between the alveolar Po₂ and the blood Po₂ in the capillaries that facilitates efficient gas exchange. In the lungs, oxygen diffuses from the alveoli, which holds a partial pressure of about 104 mm Hg, into the surrounding capillaries where the blood's partial pressure is about 40 mm Hg. This considerable gradient of approximately 64 mm Hg is critical for oxygen to rapidly cross into the blood.
RBCs then transport O₂ to tissues, where it dissociates from hemoglobin and diffuses into cells. Conversely, carbon dioxide in the tissues, due to its higher solubility and the pressure gradient, diffuses into the blood to be carried back to the lungs. Factors like carbon dioxide, oxygen, and pH levels influence the dilation and constriction of pulmonary arterioles, adjusting blood flow to enhance gas exchange efficiency in response to varying conditions.