The oxygen tension of the mixed venous blood is:
**Core Concept**
The oxygen tension of mixed venous blood is a critical parameter in assessing the balance between oxygen delivery and consumption by tissues. It is an indicator of the oxygen extraction ratio and can be used to evaluate the adequacy of cardiac output and the oxygen-carrying capacity of the blood.
**Why the Correct Answer is Right**
Mixed venous blood is the blood that returns to the heart after passing through the systemic capillaries. The oxygen tension of mixed venous blood is typically lower than that of arterial blood due to the extraction of oxygen by tissues. The correct answer represents the average oxygen tension of mixed venous blood, which is approximately 40 mmHg. This value can be influenced by factors such as cardiac output, oxygen consumption by tissues, and the oxygen-carrying capacity of the blood.
**Why Each Wrong Option is Incorrect**
* **Option A:** This value is too high and does not accurately reflect the oxygen tension of mixed venous blood.
* **Option B:** This value is too low and may indicate severe tissue hypoxia or inadequate oxygen delivery.
* **Option C:** This value is within the normal range for arterial blood, not mixed venous blood.
**Clinical Pearl / High-Yield Fact**
A key concept in understanding the oxygen tension of mixed venous blood is the Fick principle, which states that oxygen consumption by tissues is equal to the difference between cardiac output and oxygen extraction. This principle can be used to calculate the oxygen extraction ratio and assess the adequacy of oxygen delivery to tissues.
**Correct Answer: C. 40 mmHg**