What is the expected mixed venous oxygen tension, in mm Hg, in a normal adult after breathing 100% oxygen for 10 minutes?
**Core Concept**
The partial pressure of oxygen in arterial blood (PaO2) and mixed venous oxygen tension (PvO2) are crucial parameters that indicate the balance between oxygen supply and demand in the body. In a normal adult, breathing 100% oxygen for a short duration can significantly alter the PvO2 due to the increased PaO2 and subsequent increase in oxygen extraction by the tissues.
**Why the Correct Answer is Right**
When a person breathes 100% oxygen for 10 minutes, the PaO2 reaches its maximum value, approximately 600-700 mm Hg. However, the body's tissues are adapted to extract a certain amount of oxygen from the blood, which is around 25% of the total oxygen content. This oxygen extraction results in a decrease in PvO2. In a normal adult, the PvO2 is expected to be around 40-45 mm Hg after breathing 100% oxygen for 10 minutes. This is because the body's tissues continue to extract oxygen from the blood, even when the PaO2 is at its maximum.
**Why Each Wrong Option is Incorrect**
**Option A:** 150 mm Hg is too high to be a normal mixed venous oxygen tension, even after breathing 100% oxygen. The PvO2 would be expected to decrease due to oxygen extraction by the tissues.
**Option B:** 740 mm Hg is an excessively high value for mixed venous oxygen tension and is not physiologically plausible. The PvO2 would be expected to be lower than the PaO2 due to oxygen extraction.
**Option D:** 573 mm Hg is also an excessively high value for mixed venous oxygen tension and is not consistent with the expected effects of breathing 100% oxygen on PvO2.
**Clinical Pearl / High-Yield Fact**
When a patient is breathing 100% oxygen, it is essential to monitor the PvO2 to assess the balance between oxygen supply and demand in the body. A PvO2 of less than 40 mm Hg may indicate inadequate tissue oxygenation, whereas a PvO2 above 50 mm Hg may indicate excessive oxygen supply.
**✓ Correct Answer: C. 45**