Which statement correctly describes intrapulmonary shunting?

• A. Right-sided blood bypasses nonfunctional alveoli and returns unoxygenated to the left heart.
• B. All alveoli are functioning normally.
• C. Surfactant increases alveolar surface tension.
• D. Ventilation and perfusion are perfectly matched.

Answer: (A)

Intrapulmonary shunting means blood from the lungs passes through areas where there is no gas exchange because the alveoli are not ventilated. The best way to describe it is that right-sided blood bypasses nonfunctional alveoli and returns unoxygenated to the left heart. This causes deoxygenated blood to mix with oxygenated blood in the systemic circulation, lowering arterial oxygen levels.

Context helps: this is a form of shunt, different from a ventilation-perfusion mismatch where air reaches some alveoli but perfusion is poor, or vice versa. Surfactant’s role is to reduce surface tension in the alveoli, so stating that it increases surface tension is incorrect. A perfectly matched ventilation and perfusion (no mismatch) would yield optimal oxygenation, not shunting, so that description doesn’t describe shunt either. Large intrapulmonary shunts can limit the effectiveness of supplemental oxygen because the blood bypasses the ventilated units altogether.