The partial pressures (in mm Hg) of oxygen (O₂ ) and carbon dioxide (CO₂ ) at alveoli (the site of diffusion) are :

The correct option is: pO₂ = 104 and pCO₂ = 40

Step-by-Step Explanation:

1. Understanding Partial Pressure:
The pressure contributed by an individual gas in a mixture of gases is called its partial pressure. It is represented as pO₂ for oxygen and pCO₂ for carbon dioxide. Gases diffuse from an area of higher partial pressure to an area of lower partial pressure.

2. Partial Pressures in Alveoli:
Alveoli are the primary site of gas exchange in the human lungs. The exchange of gases occurs by simple diffusion based on a concentration or pressure gradient.
For oxygen to diffuse from the alveoli into the blood, and for carbon dioxide to diffuse from the blood into the alveoli, specific partial pressure gradients must exist:
- The partial pressure of oxygen (pO₂) in the alveoli is 104 mm Hg.
- The partial pressure of carbon dioxide (pCO₂) in the alveoli is 40 mm Hg.

3. Comparison with Deoxygenated Blood:
To facilitate diffusion, let's look at the partial pressures in the deoxygenated blood arriving at the alveoli:
- pO₂ in deoxygenated blood is 40 mm Hg (lower than the alveolar pO₂ of 104 mm Hg, allowing O₂ to diffuse into the blood).
- pCO₂ in deoxygenated blood is 45 mm Hg (higher than the alveolar pCO₂ of 40 mm Hg, allowing CO₂ to diffuse out of the blood into the alveoli).

Therefore, the partial pressures of O₂ and CO₂ at the alveoli are pO₂ = 104 mm Hg and pCO₂ = 40 mm Hg, which corresponds to the first option.