What are the modes of exchange of gases `(O_2 and CO_2)` at the level of alveoli and tissue respectively ?

Options

A. Active transport, active transpot
B. Diffusion, diffusion
C. Diffusion, active transport
D. Active transport, diffusion

Answer

To answer the question regarding the modes of exchange of gases (O₂ and CO₂) at the level of alveoli and tissue, we need to understand the processes involved in gas exchange in both locations.

### Step-by-Step Solution:

1. **Understanding Gas Exchange in Alveoli**:
- In the alveoli, oxygen (O₂) from the air enters the bloodstream, while carbon dioxide (CO₂) from the blood is expelled into the alveoli to be exhaled. This exchange occurs due to the difference in concentration gradients.
- The process is driven by **diffusion**, where gases move from an area of higher concentration to an area of lower concentration. In this case, O₂ moves from the alveoli (high concentration) into the blood (low concentration), and CO₂ moves from the blood (high concentration) into the alveoli (low concentration).

2. **Understanding Gas Exchange in Tissues**:
- In the tissues, the process is similar. Oxygen (O₂) is delivered from the blood to the tissues, and carbon dioxide (CO₂) produced by cellular metabolism is transported back to the blood.
- This exchange also occurs via **diffusion**. O₂ diffuses from the blood (high concentration) into the tissues (low concentration), while CO₂ diffuses from the tissues (high concentration) into the blood (low concentration).

3. **Conclusion**:
- Both the exchange of O₂ and CO₂ at the level of alveoli and tissues occurs through **diffusion**. Therefore, the correct answer is that the mode of exchange of gases at both the alveoli and tissue levels is **diffusion**.

### Final Answer:
The modes of exchange of gases (O₂ and CO₂) at the level of alveoli and tissue respectively are **diffusion** for both.