Which of the statements below gives the best definition of gas exchange?

Options

A. Swapping oxygen for digested food in the gut capillaries.
B. Using energy to breathe.
C. Exchanging inhaled air for exhaled air in the lungs.
D. Exchanging oxygen for `CO_2` in the lung alveoli.

Answer

To determine the best definition of gas exchange from the provided options, we can analyze each statement step by step.

### Step-by-Step Solution:

1. **Understand the Concept of Gas Exchange**:
- Gas exchange refers to the process by which oxygen is taken into the body and carbon dioxide is expelled. This occurs primarily in the lungs.

2. **Evaluate Each Option**:
- **Option 1**: "Swapping oxygen for digested food in the gut capillaries."
- This statement is incorrect because gas exchange does not occur in the gut; it occurs in the lungs.

- **Option 2**: "Using energy to breathe."
- This statement is misleading. While breathing does require energy, it does not define gas exchange itself.

- **Option 3**: "Exchanging inhaled air for exhaled air in the lungs."
- This statement is partially correct but does not specifically mention the exchange of gases (oxygen and carbon dioxide), which is crucial for defining gas exchange.

- **Option 4**: "Exchanging oxygen for CO2 in the lung alveoli."
- This statement accurately describes the process of gas exchange. In the alveoli of the lungs, oxygen is absorbed into the blood, and carbon dioxide is released from the blood into the alveoli to be exhaled.

3. **Select the Best Definition**:
- Based on the analysis, **Option 4** is the best definition of gas exchange as it clearly describes the exchange of oxygen and carbon dioxide in the lung alveoli.

### Final Answer:
The best definition of gas exchange is **Option 4: "Exchanging oxygen for CO2 in the lung alveoli."**

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