Final answer:
The partial pressure of CO2 at the venule end of pulmonary capillary beds should be the same as in the systemic tissues, which is 46 mmHg (Option C), reflecting the CO2 levels acquired during systemic circulation before gas exchange in the lungs.
Step-by-step explanation:
If the partial pressure of CO2 within systemic tissues is 46 mmHg, the partial pressure of CO2 at the venule end of pulmonary capillary beds should be 46 mmHg (Option C). In the systemic capillaries, oxygen and carbon dioxide exchange takes place, driven by the partial pressure differences between blood and tissues. When blood is returning to the lungs through the pulmonary arteries, it has a venous Pco2 = 45 mmHg, after exchanging gases in systemic tissues where Pco2 is higher. However, at the venule end of the pulmonary capillary beds, gas exchange with alveoli has not yet occurred, so the Pco2 should be similar to what it was when it entered the pulmonary circulation, which would be close to the systemic tissue Pco2 of 46 mmHg.