Gas exchange on respiratory surfaces in the body (the lungs) occurs through a process known as diffusion. Blood which is low in oxygen and high in carbondioxide (carried from cells) goes through an exchange in the lung's alveoli (where oxygen concentration is high and carbondioxide is low). The oxygen in the alveoli diffuses into the blood, while the carbondioxide in the blood diffuses into the alveoli. This diffusion is possible because of the concentration gradient across the membranes.
Pneumonia is the inflammation of the lungs due to injury or infection. Liquid (pus) accumulates in the alveoli (a natural immune response to the infection or injury), a condition known as pulmonary edema which makes it harder for gases to be exchanged between the blood and the alveoli, thereby making breathing difficult. This slows down diffusion and if the condition is severe enough, can cause a respiratory failure where oxygen levels in the blood are critically low and carbondioxide levels are very high.
