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 42–5. A common type of pneumonia is bacterial pneu-monia, caused most frequently bypneumococci.This disease begins with infection in the alveoli; the pulmonary membrane 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.
In pneumonia, the gas exchange functions of the lungs change 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 results in two major pulmonary abnormalities: (1) reduction in the total available surface area of the res-piratory membrane and (2) decreased ventilation-perfusion ratio. Both these effects cause hypoxemia (low blood oxygen) and hypercapnia(high blood carbon dioxide).
Figure 42–6 shows the effect of the decreased ventilation-perfusion ratio in pneumonia, showing that the blood passing through the aerated lung becomes 97 per cent saturated with oxygen, whereas that passing through the unaerated lung is about 60 per cent saturated. Therefore, the average saturation of the blood pumped by the left heart into the aorta is only about 78 per cent, which is far below normal.
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