The term pneumonia includes any inflammatory condition of the lung in which some or all of the alveoli are filled with fluid and blood cells, as shown in Figure 1. A common type of pneumonia is bacterial pneumonia, caused most frequently by pneumococci. This disease begins with infection in the alveoli; the pulmonary mem brane becomes inflamed and highly porous so that fluid and even red and white blood cells leak out of the blood into the alveoli. Thus, the infected alveoli become progressively filled with fluid and cells, and the infection spreads by extension of bacteria or virus from alveolus to alveolus. Eventually, large areas of the lungs, sometimes whole lobes or even a whole lung, become “consolidated,” which means that they are filled with fluid and cellular debris.
Fig1. Lung alveolar changes in pneumonia and emphysema.
In persons with pneumonia, the gas exchange functions of the lungs decline in different stages of the disease. In early stages, the pneumonia process might well be localized to only one lung, with alveolar ventilation reduced while blood flow through the lung continues normally. This condition causes two major pulmonary abnormalities: (1) reduction in the total available surface area of the respiratory membrane and (2) a decreased ventilation-perfusion ratio. Both of these effects cause hypoxemia (low blood O2) and hypercapnia (high blood CO2).
Figure 2 shows the effect of the decreased ventilation-perfusion ratio in pneumonia. The blood passing through the aerated lung becomes 97 percent saturated with O2, whereas that passing through the unaerated lung is about 60 percent saturated. Therefore, the average saturation of the blood pumped by the left heart into the aorta is only about 78 percent, which is far below normal.
Fig2. Effect of pneumonia on percentage saturation of oxygen (O2) in the pulmonary artery, the right and left pulmonary veins, and the aorta.
