Tuberculosis
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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