Final answer:
The partial pressure of oxygen at the venule end of pulmonary capillary beds should be 40 mmHg, which is the same as the venous Po2 of blood returning to the lungs.
Step-by-step explanation:
If the partial pressure of oxygen (Po2) within systemic tissues is 40 mmHg, the partial pressure of oxygen at the venule end of pulmonary capillary beds should be about the same. This is because the blood returning to the lungs through the pulmonary arteries has a venous Po2 = 40 mmHg. In the lungs, the Po2 in alveoli is approximately 104 mmHg, creating a strong pressure gradient that drives the diffusion of oxygen from the alveoli into the capillaries. As a result, venous blood leaving the lungs through the pulmonary veins has a Po2 of approximately 100 mmHg.
The correct answer to the question is therefore: C. 40 mmHg. This matches the venous Po2 of the blood returning to the lungs.