Expiration occurs due to

Options

A. Relaxation of diaphragm and external intercostal muscles
B. Contraction of abdominal intercostal muscles and diaphragm
C. Relaxation of abdominal and internal intercostal muscles
D. Contraction of diaphragm and relaxation of abdominal muscles

Answer

### Step-by-Step Solution:

1. **Understanding Expiration**:
- Expiration, also known as exhalation, is the process of releasing air from the lungs.

2. **Pressure Gradient**:
- Air moves from areas of high pressure to areas of low pressure. During expiration, the pressure inside the lungs must be higher than the atmospheric pressure for air to flow out.

3. **Decreasing Thoracic Volume**:
- To increase the pressure in the lungs, the thoracic volume must be decreased. This is achieved by the relaxation of certain muscles.

4. **Muscle Relaxation**:
- The diaphragm, which is a dome-shaped muscle at the base of the thoracic cavity, relaxes and moves upwards. This reduces the volume of the thoracic cavity.

5. **Role of Intercostal Muscles**:
- The external intercostal muscles, which are located between the ribs, also relax during expiration. This further decreases the thoracic volume.

6. **Resulting Pressure Increase**:
- As the thoracic volume decreases due to the relaxation of the diaphragm and external intercostal muscles, the pressure inside the lungs increases.

7. **Air Flow**:
- Because of the increased pressure in the lungs, air is expelled from the lungs into the atmosphere, completing the process of expiration.

### Conclusion:
- Therefore, expiration occurs due to the relaxation of the diaphragm and external intercostal muscles, which decreases the thoracic volume and increases the pressure in the lungs.