In tuberculosis, the tubercle bacilli cause a peculiar tissue reaction in the lungs, including (1) invasion of the infected tissue by macrophages and (2) “walling off ” of the lesion by fibrous tissue to form the so-called tubercle. This walling-off process helps to limit furthertransmission of the tubercle bacilli in the lungs and therefore is part of the protective process against extension of the infection. However, in about 3 per cent of all people who develop tuberculosis, if untreated, the walling-off process fails and tubercle bacilli spread throughout the lungs, often causing extreme destruction of lung tissue with formation of large abscess cavities.
Thus, tuberculosis in its late stages is characterized by many areas of fibrosis throughout the lungs, as well as reduced total amount of functional lung tissue. These effects cause (1) increased “work” on the part of the respiratory muscles to cause pulmonary venti-lation and reduced vital capacity and breathing capac-ity; (2) reduced total respiratory membrane surface area and increased thickness of the respiratory membrane, causing progressively diminished pulmonary diffusingcapacity; and (3) abnormal ventilation-perfusion ratio in the lungs, further reducing overall pulmonary diffu-sion of oxygen and carbon dioxide.
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