`P_(50)` means

Options

A. `pO_(2)` at which the haemoglobin is half saturated with `O_(2)`
B. At `pO_(2)` of 50 mm Hg, heamoglobin is half saturated with `O_(2)`
C. Amount of `O_(2)` carried by blood does not change much even if `pO_(2)` falls to 50 mm Hg
D. All of the above

Answer

To understand what `P_(50)` means, we can break it down into several steps:

### Step-by-Step Solution:

1. **Understanding Hemoglobin**:
- Hemoglobin is a respiratory pigment found in red blood cells (erythrocytes) that binds oxygen. It is a complex iron-containing protein responsible for the red color of blood.

2. **Function of Hemoglobin**:
- Hemoglobin binds with oxygen in a reversible manner, forming a complex known as oxyhemoglobin. This process is crucial for transporting oxygen from the lungs to the tissues of the body.

3. **Oxygen Binding Affinity**:
- The ability of hemoglobin to bind with oxygen is influenced by various factors, including the partial pressure of oxygen (PO2), carbon dioxide concentration (PCO2), pH levels (H+ concentration), and temperature.

4. **Definition of P50**:
- P50 refers to the partial pressure of oxygen (PO2) at which 50% of the hemoglobin molecules are saturated with oxygen. In other words, it is the oxygen concentration at which half of the total hemoglobin has bound to oxygen.

5. **Significance of P50**:
- The P50 value is an important indicator of hemoglobin's affinity for oxygen. A lower P50 value indicates a higher affinity for oxygen, while a higher P50 value suggests a lower affinity.

6. **Conclusion**:
- Therefore, `P_(50)` is defined as the partial pressure of oxygen at which half of the hemoglobin is saturated with oxygen, which is crucial for understanding how effectively hemoglobin can transport oxygen in the blood.