## **Core Concept**
The oxygen-carrying capacity of blood refers to the amount of oxygen that can be transported by the blood from the lungs to the body's tissues. This process is primarily facilitated by **hemoglobin** in red blood cells. The key factors influencing oxygen-carrying capacity include the concentration of hemoglobin, the affinity of hemoglobin for oxygen, and the number of red blood cells.

## **Why the Correct Answer is Right**
Hemoglobin (Hb) is a protein in red blood cells that binds to oxygen in the lungs and releases it in the tissues. Each molecule of hemoglobin can bind four molecules of oxygen. The oxygen-carrying capacity of blood is largely determined by the **concentration of hemoglobin** and its ability to bind oxygen. A normal hemoglobin level ranges from approximately 13.5 to 17.5 grams per deciliter (g/dL) in adult men and 12 to 16 g/dL in adult women. Therefore, the concentration of hemoglobin directly affects how much oxygen can be carried by the blood.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While the **partial pressure of oxygen (pO2)** influences how much oxygen binds to hemoglobin, it does not determine the blood's oxygen-carrying capacity. pO2 affects the loading and unloading of oxygen but is not a determinant of capacity.
- **Option B:** **Myoglobin** is a protein found in muscle cells that stores oxygen for use during high-intensity, short-duration activities. It does not play a significant role in the oxygen-carrying capacity of blood.
- **Option D:** **Plasma** can dissolve a small amount of oxygen, but its contribution to the overall oxygen-carrying capacity of blood is minimal compared to that of hemoglobin in red blood cells.

## **Clinical Pearl / High-Yield Fact**
A crucial point to remember is that the oxygen-carrying capacity of blood is primarily determined by **hemoglobin levels** and **hematocrit** (the proportion of blood volume made up by red blood cells). Conditions that alter these, such as anemia or polycythemia, directly impact the oxygen-carrying capacity of blood.

## **Correct Answer:** C.