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 partial pressures of gases is correct, we can analyze each statement provided in the question. Here’s a step-by-step breakdown:

### Step 1: Understand the concept of partial pressure
Partial pressure refers to the pressure exerted by a single type of gas in a mixture of gases. In the context of respiration, it is crucial to understand how gases like oxygen and carbon dioxide behave in the lungs and blood.

**Hint:** Remember that partial pressure is related to the concentration of a gas in a mixture and plays a significant role in gas exchange.

### Step 2: Analyze the statements
1. **Statement 1:** "Inspiration can occur if intrapulmonary pressure is less than atmospheric pressure."
- This statement is true. During inspiration, the diaphragm contracts, increasing the volume of the thoracic cavity and decreasing intrapulmonary pressure below atmospheric pressure, allowing air to flow into the lungs.

**Hint:** Think about how pressure gradients work; air moves from high pressure to low pressure.

2. **Statement 2:** "Expiration can occur if intrapulmonary pressure is more than atmospheric pressure."
- This statement is also true. During expiration, the diaphragm and intercostal muscles relax, decreasing the volume of the thoracic cavity and increasing intrapulmonary pressure above atmospheric pressure, forcing air out of the lungs.

**Hint:** Consider the mechanics of breathing; what happens to pressure during expiration?

3. **Statement 3:** "Inspiration is initiated by contraction of the diaphragm which increases the volume of thoracic chamber in the anterior-posterior axis."
- This statement is partially true but misleading. While the diaphragm does contract and increases thoracic volume, it primarily affects the vertical dimension (up and down) rather than just the anterior-posterior axis.

**Hint:** Visualize the diaphragm's movement; it moves downwards, affecting the vertical space in the thoracic cavity.

4. **Statement 4:** "The contraction of external intercostal muscle lifts up the ribs and sternum causing an increase in volume of thoracic chamber in anterior-posterior axis."
- This statement is incorrect. The contraction of the external intercostal muscles primarily increases the volume of the thoracic cavity in the dorsal-ventral axis (front to back), not just the anterior-posterior axis.

**Hint:** Remember the role of the intercostal muscles; they expand the rib cage in a specific direction.

### Step 3: Identify the correct statement
Based on the analysis:
- Statement 1 and Statement 2 are correct.
- Statement 3 is misleading, and Statement 4 is incorrect.

However, since the question asks for the correct statement specifically about partial pressures, we can conclude that **Statement 1** is the most straightforwardly correct regarding the mechanics of inspiration.

**Final Answer:** The correct statement about partial pressures of gases is: "Inspiration can occur if intrapulmonary pressure is less than atmospheric pressure."