Final answer:
option c,The partial pressure of CO2 within blood passing through the systemic capillaries is generally 45 mm Hg, reflecting carbon dioxide uptake from the tissues.
Step-by-step explanation:
The partial pressure of CO2 within blood passing through the metarterioles of systemic capillaries is generally about 45 mm Hg. This is because, as blood enters the systemic capillaries, it generally has a partial pressure of oxygen (Po₂) of 100 mm Hg and a partial pressure of carbon dioxide (PCO₂) of 40 mm Hg. As blood flows through systemic capillaries, it loses oxygen and gains carbon dioxide due to the pressure difference with the surrounding tissue cells that have a Po₂ of approximately 40 mm Hg and a PCO₂ of around 45 mm Hg.
This pressure gradient causes carbon dioxide to diffuse from the tissues, where it is higher, across the interstitial fluid and into the capillaries, increasing blood PCO₂. Thus, by the time the blood returns to the lungs via the pulmonary arteries, the venous Po₂ is lower, at about 40 mm Hg, while the PCO₂ has risen to approximately 45 mm Hg. The answer to the question is C. 45 mm Hg.