One intern calculated the concentration of O2 in blood as 0.0025 ml/ml of blood. Considering atmospheric pressure as 760mmHg, how much approximate O2 tension could have been in the blood?
**Core Concept**
The question tests the understanding of the relationship between oxygen concentration in blood and oxygen tension, which is crucial for assessing oxygenation status. This relationship is based on the **oxygen-hemoglobin dissociation curve** and the concept of **partial pressures**.
**Why the Correct Answer is Right**
To solve this, we use the fact that 1 ml of oxygen dissolves in 1 liter of blood at normal atmospheric pressure (760 mmHg) at body temperature, which corresponds to a partial pressure of oxygen (pO2) of approximately 100 mmHg when fully saturated. Given the concentration of O2 is 0.0025 ml/ml, this translates to 2.5 ml of O2 per 100 ml of blood, or 25 ml of O2 per liter of blood. This concentration corresponds to a pO2 of about 100 mmHg, considering the linear relationship between oxygen concentration and partial pressure in the range of normal atmospheric pressure.
**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it underestimates the pO2 for the given concentration.
**Option B:** This option overestimates the pO2 for the calculated concentration of oxygen in the blood.
**Option C:** Similarly, this choice does not accurately reflect the relationship between oxygen concentration and partial pressure.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that the solubility of oxygen in blood is very low, and most oxygen is transported bound to hemoglobin. The partial pressure of oxygen (pO2) is a critical measure of the oxygenation status of the blood.
**Correct Answer:** D. 100 mmHg