`O_2` enters the lungs when

Options

A. Positive pressure in the lungs
B. Positive pressure in the lungs
C. Negative pressure in the cells
D. Negative pressure in the lungs

Answer

To solve the question "O2 enters the lungs when," we need to understand the mechanics of breathing and how gases move in and out of the lungs.

### Step-by-Step Solution:

1. **Understanding Gas Movement**: Gases move from areas of higher pressure to areas of lower pressure. This principle is crucial in understanding how oxygen enters the lungs.

2. **Pressure in the Lungs**: For oxygen (O2) to enter the lungs, the pressure inside the lungs must be lower than the atmospheric pressure. This creates a pressure gradient that allows air to flow into the lungs.

3. **Mechanism of Breathing**: During inhalation, the diaphragm contracts and moves downward, and the intercostal muscles expand the rib cage. This expansion increases the volume of the thoracic cavity, leading to a decrease in pressure inside the lungs.

4. **Conclusion**: Since air (including O2) moves from an area of higher pressure (the atmosphere) to an area of lower pressure (the lungs), oxygen enters the lungs when there is a negative pressure in the lungs compared to the atmospheric pressure.

5. **Final Answer**: Therefore, the correct option is **negative pressure in the lungs**.