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:
Partial pressure is the pressure exerted by an individual gas in a mixture of gases. It is represented as pO₂ for oxygen and pCO₂ for carbon dioxide, and measured in millimeters of mercury (mm Hg).

2. Gas Exchange at the Alveoli:
The alveoli in the lungs are the primary sites of gas exchange between the inhaled air and the blood. For efficient diffusion of gases to occur:

• Oxygen must diffuse from the alveoli into the deoxygenated blood. This requires the partial pressure of oxygen (pO₂) in the alveoli to be higher than in the blood. In the alveoli, pO₂ is 104 mm Hg (compared to 40 mm Hg in deoxygenated blood).
• Carbon dioxide must diffuse from the deoxygenated blood into the alveoli to be exhaled. This requires the partial pressure of carbon dioxide (pCO₂) in the alveoli to be lower than in the blood. In the alveoli, pCO₂ is 40 mm Hg (compared to 45 mm Hg in deoxygenated blood).

3. Comparison of Partial Pressures (in mm Hg):
To understand the diffusion gradients, we can look at the partial pressures of these gases at different stages:

Thus, the partial pressures of oxygen and carbon dioxide at the alveoli are 104 mm Hg and 40 mm Hg, respectively.