Perfusion without ventilation in atelectasis contributes to which physiologic finding?

• A. Decreased pulmonary blood flow
• B. Normal gas exchange
• C. Ventilation-perfusion mismatch with shunt
• D. Increase in alveolar oxygen tension

Answer: (C)

Perfusion without ventilation creates a ventilation–perfusion mismatch with a shunt. When alveoli collapse or are filled so they aren’t ventilated, blood continues to flow through the surrounding capillaries but doesn’t pick up oxygen. That deoxygenated blood mixes with oxygenated blood from other lung areas, lowering the overall arterial oxygen content. Atelectasis therefore increases the shunt fraction, leading to hypoxemia that is not fully corrected by oxygen therapy. This isn’t about decreased blood flow or normal gas exchange, and it isn’t about higher alveolar oxygen tension—the problem is blood flowing past nonventilated airspaces.