Expiration is the result of

Options

A. contraction of internal intercostal muscles.
B. relaxation of internal intercostal musceles and diaphragm.
C. contraction of external intercostal muscels and relaxation of diaphragm.
D. relaxation of external intercostal muscles and diaphragm.

Answer

To solve the question "Expiration is the result of?", we will analyze each option provided and determine which one correctly describes the process of expiration.

### Step-by-Step Solution:

1. **Understanding Expiration**:
- Expiration, also known as exhalation, is the process of breathing out air from the lungs. It involves the relaxation of certain muscles and the decrease in the volume of the thoracic cavity.

2. **Analyzing the Options**:
- **Option 1: Contraction of internal intercostal muscles**:
- This option is incorrect. The contraction of internal intercostal muscles aids in forced expiration but is not the primary mechanism for normal expiration.

- **Option 2: Relaxation of the internal intercostal muscles and diaphragm**:
- This option is partially correct but not complete. While relaxation of the diaphragm is involved, the internal intercostal muscles do not primarily relax during normal expiration.

- **Option 3: Contraction of external intercostal muscles and relaxation of diaphragm**:
- This option is incorrect. The contraction of external intercostal muscles is associated with inhalation (inspiration), not expiration.

- **Option 4: Relaxation of the external intercostal muscles and diaphragm**:
- This option is correct. During expiration, the external intercostal muscles relax, allowing the ribcage to move downwards and inwards, while the diaphragm also relaxes, leading to a decrease in thoracic volume and expulsion of air from the lungs.

3. **Conclusion**:
- Based on the analysis, the correct answer to the question "Expiration is the result of?" is **Option 4: Relaxation of the external intercostal muscles and diaphragm**.