Final answer:
The partial pressure of oxygen within blood passing through the metarterioles of systemic capillaries is generally 40 mmHg, which is established by the pressure gradient during the oxygen exchange process.
Step-by-step explanation:
The partial pressure of oxygen within blood passing through the metarterioles of systemic capillaries is generally 40 mmHg. This is because in systemic capillaries, the blood has a partial pressure of oxygen (Po₂) of about 100 mmHg, but in the surrounding tissue cells, Po₂ is approximately 40 mmHg. This gradient is what drives oxygen out of the capillaries and into tissue cells.
Seeing as the blood returning to the lungs through the pulmonary arteries has a venous Po₂ of 40 mmHg, this indicates that after the oxygen exchange process, the remaining partial pressure of oxygen in the blood is about 40 mmHg.