In humans, which among these is not a step in respiration?

Options

A. Pulmonary ventilation
B. Alveolar diffusion of `O_(2) and CO_(2)`
C. Transport of gases by blood
D. Utilisation of `CO_(2)` by cells for catabolic reactions

Answer

To solve the question "In humans, which among these is not a step in respiration?", we will analyze each option provided to determine which one does not belong to the process of respiration.

### Step-by-Step Solution:

1. **Understanding Respiration**: Respiration in humans involves several key steps: pulmonary ventilation (breathing), gas exchange in the alveoli, transport of gases in the blood, and cellular respiration (utilization of oxygen and production of carbon dioxide).

2. **Analyzing the Options**:
- **Option 1: Pulmonary Ventilation**: This refers to the process of inhalation and exhalation. It is indeed a step in respiration as it facilitates the movement of air in and out of the lungs.
- **Option 2: Alveolar Diffusion of O2 and CO2**: This step involves the exchange of oxygen and carbon dioxide between the alveoli and the blood. This is a crucial part of respiration.
- **Option 3: Transport of Gases by Blood**: This refers to how oxygen is carried by hemoglobin in red blood cells and how carbon dioxide is transported back to the lungs. This is also a key step in respiration.
- **Option 4: Utilization of CO2 by Cells for Catabolic Reactions**: This statement is incorrect. Cells do not utilize carbon dioxide; instead, they utilize oxygen for metabolic processes and produce carbon dioxide as a waste product.

3. **Identifying the Incorrect Option**: Among the options analyzed, Option 4 is the only statement that does not accurately describe a step in respiration. Cells utilize oxygen, not carbon dioxide.

4. **Conclusion**: Therefore, the correct answer to the question is Option 4: "Utilization of CO2 by cells for catabolic reactions" is not a step in respiration.

### Final Answer:
**Option 4: Utilization of CO2 by cells for catabolic reactions.**