Number of R.B.C. is more in persons living at hills because

Options

A. There is less oxygen
B. Air is more purified
C. Heat is required to warm the body
D. None of these.

Answer

### Step-by-Step Solution:

1. **Understanding the Environment**: In hilly areas, the atmospheric pressure is lower than in plains, which results in lower oxygen tension (partial pressure of oxygen).

2. **Effect of Low Oxygen Tension**: The lower oxygen tension means that there is less oxygen available for the body to utilize. Hemoglobin in red blood cells (RBCs) binds to oxygen, and its ability to do so is influenced by the partial pressure of oxygen.

3. **Oxygen-Hemoglobin Dissociation Curve**: The relationship between the partial pressure of oxygen and the saturation of hemoglobin with oxygen can be represented by the oxygen-hemoglobin dissociation curve, which is sigmoid in shape. As the partial pressure of oxygen decreases, the saturation of hemoglobin also decreases.

4. **Compensatory Mechanism**: To compensate for the lower saturation of hemoglobin due to reduced oxygen availability, the body increases the production of red blood cells (RBCs). This increase in RBCs enhances the oxygen-carrying capacity of the blood.

5. **Increased Hemoglobin Content**: Along with the increase in RBCs, there is also an increase in the hemoglobin content of these cells, further aiding in oxygen transport.

6. **Increased Respiratory Rate**: Additionally, individuals living at high altitudes often experience an increase in respiratory rate, which helps to maximize the intake of oxygen and supply it to the tissues.

7. **Conclusion**: Therefore, the number of RBCs is higher in individuals living in hilly areas as a physiological adaptation to the lower oxygen levels in the atmosphere.

### Final Answer:
The number of RBCs is more in persons living at hills because there is less oxygen in the atmosphere, prompting the body to produce more RBCs to compensate for the decreased oxygen tension.

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