What is the name for an area where alveoli are ventilated but not perfused?

• A. Alveolar dead space
• B. Anatomic dead space
• C. Physiologic shunt
• D. Diffusion defect

Answer: (A)

Ventilation without perfusion describes alveoli that receive air but lack blood flow, so gas exchange cannot occur. This situation is called alveolar dead space. It arises when perfusion to some lung units is reduced or blocked (for example, by a pulmonary embolus), leaving ventilated but nonperfused alveoli.

Anatomic dead space refers to airways that do not participate in gas exchange because they lack alveoli, so no gas exchange happens there despite ventilation. Physiologic shunt is the opposite pattern: blood passes through lungs with little or no ventilation of those units, so it returns poorly oxygenated to the circulation. Diffusion defect means impaired movement of oxygen across the alveolar-capillary membrane, not a ventilation-perfusion mismatch.