Intrapulmonary shunting occurs when:

the presence of pulmonary surfactant causes a decrease in alveolar surface tension, thus impairing the exchange of gases in the lungs.

a decrease in respiratory rate and depth causes carbon dioxide accumulation in the alveoli and an overall decrease in blood oxygen levels.

blood coming from the right side of the heart bypasses nonfunctional alveoli and returns to the left side of the heart in an unoxygenated state.

any impairment in circulatory function causes a reduced ability for oxygen and carbon dioxide to diffuse across the alveolar-capillary membrane.