At high altitude the RBCs in the human blood will

Options

A. increase in size
B. decrease in size
C. increase in number
D. decrease in number

Explanation

### Step-by-Step Solution:

1. **Understanding High Altitude Effects**: At high altitudes (generally above 300 meters), the atmospheric pressure decreases, leading to lower oxygen availability. This condition can lead to altitude sickness, characterized by symptoms like nausea, fatigue, and dizziness.

2. **Body's Response to Low Oxygen**: To adapt to lower oxygen levels, the body initiates several compensatory mechanisms. One of the primary responses is an increase in the production of red blood cells (RBCs).

3. **Increase in RBC Count**: The normal range of RBCs in human blood is approximately 4.5 to 5.5 million per cubic millimeter. At high altitudes, the body increases this number to enhance oxygen transport.

4. **Increase in Hemoglobin Production**: Along with an increase in RBC count, the body also boosts hemoglobin production. Hemoglobin is the protein in RBCs that binds to oxygen, and having more hemoglobin allows for greater oxygen transport.

5. **Increased Respiratory Rate**: The body also compensates by increasing the respiratory rate. This helps to take in more oxygen to counteract the lower oxygen availability in the environment.

6. **Altered Hemoglobin Affinity**: Interestingly, the binding affinity of hemoglobin for oxygen decreases at high altitudes. This means that hemoglobin will release oxygen more readily to the tissues, which is crucial for preventing hypoxia (oxygen deficiency in tissues).

7. **Conclusion**: Therefore, at high altitudes, the RBC count in human blood will increase as a compensatory mechanism to ensure that tissues receive adequate oxygen despite the lower atmospheric oxygen levels.

### Final Answer:
At high altitude, the RBCs in human blood will **increase in number**.

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