USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

A physiological shunt occurs when blood is directed to the lungs but does not participate in gas exchange due to certain conditions. In the context of respiratory physiology, this typically involves situations where there is blood flow through the pulmonary circulation where alveoli are not adequately ventilated (for instance, due to obstruction or fluid). As a result, the blood that passes through the lungs does not become oxygenated effectively.

The physiological shunt can be observed in conditions like pneumonia or atelectasis, where some areas of the lung are perfused but not ventilated. This leads to blood returning to the systemic circulation with a lower oxygen saturation than normal.

In contrast, an anatomic shunt refers to the direct mixing of venous and arterial blood due to physical connections, such as heart defects; a right-to-left shunt typically leads to deoxygenated blood entering systemic circulation; and a left-to-right shunt involves oxygenated blood being redirected from systemic circulation back to the pulmonary circulation without any impairment in gas exchange in the lungs. Thus, physiological shunting is specifically related to the scenario where blood reaches the lungs but cannot participate in gas exchange effectively.