Deoxygenated blood entering the arterial circulation is known as what phenomenon?

• A. V/Q mismatch
• B. Perfusion defect
• C. Shunting
• D. Dead space ventilation

Answer: (C)

Shunting occurs when blood bypasses the gas-exchange units of the lung and returns to the arterial system without being oxygenated. This can happen inside the heart (intracardiac shunt) or in the lungs (intrapulmonary shunt) when perfused alveoli are not ventilated. The key consequence is deoxygenated blood mixing with oxygenated blood, producing hypoxemia that is not fully reversed by supplemental oxygen because part of the blood never gets a chance to pick up oxygen in the lungs.

V/Q mismatch involves areas where ventilation and perfusion are not matched, but gas exchange still occurs and some oxygenation can be improved with oxygen therapy. A perfusion defect means reduced blood flow to ventilated regions, leading to wasted ventilation but not the same direct mixing of unoxygenated blood into the arterial stream. Dead space ventilation is ventilation without perfusion, so it does not result in deoxygenated blood entering the arterial circulation.