Which gas maintains a constant partial pressure in the alveoli during normal conditions?

• A. N2O
• B. O2
• C. CO
• D. CO2

Answer: (D)

The gas that maintains a constant partial pressure in the alveoli during normal conditions is carbon dioxide (CO2). This is primarily due to the fact that the human body has mechanisms in place to regulate the concentration of CO2 in the blood and, consequently, in the alveoli.

During respiration, CO2 is produced as a byproduct of metabolism and is transported in the blood to the lungs, where it diffuses into the alveoli and is exhaled. The body strives to maintain a stable level of CO2 in the blood, as significant deviations can lead to respiratory and metabolic imbalances. Because the body regulates CO2 levels through ventilation and other control mechanisms, the partial pressure of this gas remains relatively constant in the alveoli, typically around 40 mmHg under normal conditions.

In contrast, the partial pressures of other gases such as oxygen (O2) and nitrous oxide (N2O) can vary significantly due to factors like environmental changes, activities such as exercise, and differences in individual respiratory patterns. Carbon monoxide (CO) is present in very low concentrations and does not have a stable partial pressure in the same way that CO2 does. Thus, in the context of normal physiological conditions, carbon dioxide is the gas that