Final answer:
During tissue gas exchange, the Po₂ in tissue cells is approximately 40 mm Hg, allowing oxygen to diffuse down its pressure gradient from the systemic capillaries, where Po₂ is about 100 mm Hg, into the tissue cells for cellular respiration.
Step-by-step explanation:
The partial pressure of oxygen (Po₂) in the tissue cells during tissue gas exchange is approximately 40 mm Hg. In the systemic capillaries, blood has a higher Po₂ of about 100 mm Hg. This pressure difference creates a gradient that allows oxygen to diffuse from the blood and into the tissue cells. Conversely, carbon dioxide diffuses from the tissues where its partial pressure (PCo₂) is around 45 mm Hg into the systemic capillaries where PCo₂ is lower, at 40 mm Hg. This process of gas exchange is essential for supplying oxygen to cells for cellular respiration and removing the carbon dioxide produced as a waste product. After the exchange, the blood returning to the lungs carries a venous Po₂ of 40 mm Hg and a PCo₂ of 45 mm Hg to be oxygenated and rid of excess carbon dioxide in the lungs.