shunt

Imagine a queue of cars meant to pass through a car wash, but a side lane lets some cars skip it entirely and rejoin the road still dirty. A shunt is the lung's version: blood that passes from the venous side to the arterial side without ever being exposed to fresh alveolar air, so it reaches the body still 'unwashed' — that is, not oxygenated.

Physiologically a shunt is the extreme end of low ventilation-perfusion mismatch: a region with blood flow but zero ventilation, a ratio of zero. This happens when air sacs are completely filled (with pus in pneumonia, water in pulmonary edema) or collapsed (atelectasis), so the blood flowing past them picks up no oxygen and dilutes the well-oxygenated blood from healthy regions. There is also a small, normal shunt — a little venous blood from the heart's own and bronchial circulations bypasses the lungs even in health.

The defining clinical feature of a true shunt is that giving extra oxygen barely helps. Because the shunted blood never meets the inhaled air, raising that air's oxygen content cannot reach it; only opening or clearing the affected lung restores oxygenation. This 'oxygen-refractory' hypoxemia is a key bedside clue that distinguishes shunt from ordinary mismatch.