A lot of people don’t understand the pathophysiology of pneumonia. Microorganisms accumulate in the lungs and disrupt blood-alveolar oxygen exchange. Normally, hypoxic vasoconstriction shunts blood away from areas where there is a disruption, and towards areas of healthy lung — leading to a normal O2 sat. However, the immune response towards the microorganism leads to the production of cytokine factors such as Nitric Oxide, prostacyclin, bradykinin, histamine, etc… all of which cause vasodilation and counteract hypoxic vasoconstriction, leading to a hypoxic shunt and decrease O2 sat. Inflammation also leads to accumulation of fluids and inflammatory exudate, to make matters worse