Which of the following statements about partial pressures of gases is CORRECT ?

Options

A. Inspiration can occur if the intra- pulmonary pressure is less than the atmospheric pressure . i.e., there is a positive pressure in the lungs with respect to atmospheric pressure.
B. Expiration can occur if the intra- pulmonary pressure is more than the atmospheric pressure, i.e., there is a negative pressure in the lungs with respect to atmospheric pressure.
C. Inspiration is initiated by the contraction of diaphragm which increases the volume of thoracic chamber in the antero - posterior axis.
D. The contraction of external intercostal muscles lifts up the ribs and the sternum causing an increase in the volume of the thoracic chamber in the antero - posterior axis.

Answer

To determine which statement about the partial pressures of gases is correct, we need to analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding Intrapulmonary Pressure**:
- Intrapulmonary pressure refers to the pressure within the lungs. For inspiration to occur, the intrapulmonary pressure must be lower than the atmospheric pressure. This creates a pressure gradient that allows air to flow into the lungs.

2. **Analyzing Statement A**:
- Statement A claims that inspiration occurs if intrapulmonary pressure is less than atmospheric pressure, stating there is positive pressure in the lungs. This is incorrect because if the intrapulmonary pressure is less than atmospheric pressure, it indicates a negative pressure in the lungs, allowing air to flow in.

3. **Analyzing Statement B**:
- Statement B states that expiration can occur if intrapulmonary pressure is more than atmospheric pressure, claiming there is negative pressure in the lungs. This is also incorrect because expiration occurs when the intrapulmonary pressure is greater than atmospheric pressure, indicating positive pressure in the lungs.

4. **Analyzing Statement C**:
- Statement C states that inspiration is initiated by the contraction of the diaphragm, which increases the volume of the thoracic chamber in the anterior-posterior axis. This is correct. When the diaphragm contracts, it moves downwards, increasing the volume of the thoracic cavity and allowing air to flow into the lungs.

5. **Analyzing Statement D**:
- Statement D claims that the contraction of external intercostal muscles lifts up the ribs and sternum, causing an increase in volume of the thoracic chamber in the anterior-posterior axis. This is incorrect. The contraction of external intercostal muscles primarily increases the volume of the thoracic cavity in the dorsal-ventral axis, not the anterior-posterior axis.

### Conclusion:
After analyzing all the statements, the correct answer is **C**: "Inspiration is initiated by the contraction of the diaphragm, which increases the volume of the thoracic chamber in the anterior-posterior axis."

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