Which statement best defines a pulmonary shunt?

• A. Blood bypasses the alveoli and gas exchange does not occur, resulting in wasted perfusion
• B. Gas exchange occurs normally only during exercise
• C. Only oxygenated blood participates in CO2 removal
• D. Ventilation and perfusion are perfectly matched in all regions

Answer: (A)

A pulmonary shunt is when blood flows through the lungs without coming into contact with ventilated alveoli, so no gas exchange occurs for that portion of blood. In other words, perfusion happens without ventilation, and the blood that skips the alveolar air leaves the lungs not oxygenated and not properly releasing CO2. This is often described as wasted perfusion because that part of the pulmonary circulation isn’t contributing to oxygen uptake or CO2 removal.

To understand why this matters, compare it to the opposite situation: dead space, where alveoli are ventilated but not perfused, so gas exchange can’t happen there either. Ideal air–blood exchange happens when ventilation and perfusion are well matched, not when a portion of blood bypasses the alveoli.

The other statements don’t fit because gas exchange isn’t defined by exercise level, and CO2 removal isn’t limited to blood that is already oxygenated. Also, perfectly matched ventilation and perfusion would minimize shunting, not describe a shunt itself.