Final answer:
The partial pressure gradient of oxygen decreases (option A) as blood moves through the pulmonary capillaries due to oxygen diffusing into the blood, causing the pressure gradient between the alveolar air and the blood to diminish as oxygenation occurs.
Step-by-step explanation:
As blood moves along the length of the pulmonary capillary, the partial pressure gradient of oxygen decreases. This is because as blood flows through the pulmonary capillaries, it picks up oxygen from the air in the alveoli. The oxygen diffuses from the alveoli into the blood because of the difference in partial pressure—the partial pressure of oxygen in the alveoli is higher than that in the deoxygenated blood entering the pulmonary capillaries. As the blood becomes more oxygenated, the partial pressure of oxygen in the blood rises, thus reducing the gradient between the alveolar air and the blood, leading to a decrease in the gradient as the blood moves through the capillary.
For instance, oxygen and carbon dioxide move independently of each other down their own pressure gradients. Oxygen enters the blood where its partial pressure is lower, and carbon dioxide diffuses out where its partial pressure is higher compared to the tissue cells. This process of gas exchange continues until the blood leaving the capillaries has a venous Po₂ approximately equal to the Po₂ in the alveoli and a reduced gradient.