Which of the following statement is correct?

Options

A. The contraction of internal intercostal muscles lifts up the ribs and sternum.
B. The RBCs transport oxygen only.
C. The thoracic cavity is anatomically an air tight chamber.
D. Healthy men can inspire appromixmately `500` mL of air per minute.

Explanation

To determine which of the given statements is correct, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Analyzing the First Statement**:
- The first statement claims that "the contraction of internal intercostal muscle lifts up the ribs and sternum."
- **Explanation**: The internal intercostal muscles are primarily involved in forced expiration, not inspiration. When these muscles contract, they pull the ribs downward and inward, which aids in expelling air from the lungs rather than lifting them.
- **Conclusion**: This statement is **incorrect**.

2. **Analyzing the Second Statement**:
- The second statement states that "RBCs transport oxygen only."
- **Explanation**: Red blood cells (RBCs) contain hemoglobin, which is responsible for transporting both oxygen and carbon dioxide. Hemoglobin binds with oxygen to form oxyhemoglobin and can also bind with carbon dioxide to form carbaminohemoglobin.
- **Conclusion**: This statement is **incorrect**.

3. **Analyzing the Third Statement**:
- The third statement claims that "the thoracic cavity is anatomically an airtight chamber."
- **Explanation**: The thoracic cavity is indeed surrounded by the rib cage, diaphragm, and vertebral column, creating a sealed environment that helps in the mechanics of breathing. It is designed to maintain pressure changes necessary for inhalation and exhalation.
- **Conclusion**: This statement is **correct**.

4. **Analyzing the Fourth Statement**:
- The fourth statement says that "healthy men can inspire approximately 500 ml of air per minute."
- **Explanation**: The normal tidal volume (the amount of air inhaled or exhaled in a single breath) is about 500 ml. However, the total volume of air inspired in a minute (minute ventilation) is calculated by multiplying the tidal volume by the respiratory rate (12-16 breaths per minute), which results in approximately 6000-7200 ml per minute, not just 500 ml.
- **Conclusion**: This statement is **incorrect**.

### Final Answer:
The correct statement is the **third statement**: "the thoracic cavity is anatomically an airtight chamber."

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