Physiological dead air space occurs when alveoli are not what?

• A. Ventilated
• B. Oxygenated
• C. Perfused with blood
• D. Filled with air

Answer: (C)

Physiological dead air space refers to portions of the respiratory system where air is present but does not participate in gas exchange due to inadequate perfusion. Specifically, this occurs when alveoli are ventilated (filled with air) but not supplied with blood flow for gas exchange. When alveoli are not perfused with blood, oxygen cannot be efficiently transferred to the bloodstream, leading to wasted effort in ventilation. This area is still filled with air but offers no effective gas exchange, thus contributing to the physiological dead space.

Understanding this concept highlights the importance of both ventilation and perfusion in the overall process of respiration. Adequate perfusion ensures that the oxygen brought into the alveoli can actually enter the bloodstream, making perfusion a critical aspect of effective gas exchange. Other options, such as ventilated, oxygenated, or filled with air, while related to the functioning of alveoli, do not accurately capture the reason why physiological dead space occurs. In essence, without adequate blood flow, the capacity for gas exchange is compromised, resulting in physiological dead air space.