Final answer:
The PCO₂ in the systemic capillary during tissue gas exchange is approximately 45 mm Hg, due to carbon dioxide diffusing from the systemic tissue to the blood based on the pressure gradient.
Step-by-step explanation:
The PCO₂ in the systemic capillary during tissue gas exchange is typically around 45 mm Hg. When blood enters the systemic capillaries, a pressure gradient is present due to the differences between the partial pressures of carbon dioxide in the tissues and the blood.
Specifically, the blood's PCO₂ is 40 mm Hg, and the systemic tissue's PCO₂ is 45 mm Hg. This pressure difference promotes the diffusion of CO₂ out of the tissue cells and into the systemic capillaries, resulting in the blood PCO₂ level rising to 45 mm Hg as it returns through the pulmonary arteries to the lungs where it will be oxygenated.