Final answer:
Physiologic right to left shunts can occur due to factors such as infection or edema in the lungs, which obstruct an area and decrease ventilation but not perfusion. Anatomical shunts can also occur due to the effect of gravity on the lungs.
Step-by-step explanation:
A physiologic right to left shunt occurs when there is a decrease in ventilation but not in perfusion, leading to a mismatch in the V/Q ratio.
This can be caused by factors such as infection or edema in the lung that obstructs an area, reducing ventilation.
The decreased ventilation impairs gas exchange and leads to a lower V/Q ratio.
An example of an anatomical shunt is the effect of gravity on the lungs, where the ventilation of airways does not match the perfusion of arteries surrounding those airways.