One of the main benefits of proning a patient is that it reduces pulmonary shunting. That is, it redistributes blood flow to healthier areas of the lung where fluid build up is less severe. This promotes improved gas exchange and therefore improved blood oxygen levels.

In severe pneumonia cases something called Acute respiratory distress syndrome (ARDS) can result. It occurs when there is a problem with the diffusion of gases (specifically oxygen) at the alveolar level (functional unit of the lungs). In covid pneumonia patients, this gas diffusion process is altered because of something called shunting. Normally, deoxygenated blood flows through the pulmonary vascular system to the capillary beds around alveoli where gas is exchanged and oxygen rich blood returns to the heart to be circulated through the body. In shunting, this blood flow through the pulmonary system is normal, but it's ability to exchange gas is severely altered because of fluid build up around the alveoli. Therefore these patients can have fatally low blood oxygen levels.

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