5+ Year Member
Mar 23, 2014
Medical Student
Hemoglobin is a protein in RBCs that carries oxygen to the tissues. Oxygen is picked up in the lungs and dropped off in the tissues. The PO2 = 100 mmHg is when hemoglobin is in the lungs picking up O2. When hemoglobin gets to the tissues, PO2 drops (this question focuses on when its 40 mmHg).

If you focus on the PO2 = 40 mmHg reading on the x-axis, the saturation of O2 held by hemoglobin is 70% and 55% for the normal curve and right-shifted curve, respectively. For the normal curve, the saturation of O2 held by hemoglobin is 100% at PO2 = 100 mmHg. For the right-shifted curve, the saturation of O2 held by hemoglobin is 95% at PO2 = 100 mmHg.

The important thing to look at is the difference in O2 saturation at the 100mmHg and 40 mmHG PO2 levels.

For the normal curve:

100% at 100mmHg - 70% at 40 mmHg = 30% O2 drop off at the tissues.

For the right-shifted curve:

95% at 100mmHg - 55% at 40 mmHg = 40% O2 drop off at the tissues.

Even though the right-shifted curve picks up slightly less O2 in the lungs, the hemoglobin saturation at the tissues is less, resulting in a bigger O2 drop off to the tissues for the right-shifted curve. This matches with choice (A). Choice (B) says the opposite. Choice (C) is not true. Choice (D) is not true as 95% is not significantly lower than 100%. Choices (C) and (D) are also focusing on the wrong thing.
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