The fact that normal subjects develop stress failure of their pulmonary capillaries at extremely high but nevertheless physiological pressures immediately suggests that any pathological condition that raises the transmural pressure of the capillaries to unphysiologically high levels will result in stress failure. This is indeed the case, and Table I summarizes the pathological conditions under which stress failure of the capillaries has been described. There is insufficient space here to discuss these conditions. However, attention should be drawn to the third group, namely high states of lung inflation. It was pointed out earlier that increased longitudinal tension in the alveolar wall associated with high lung volumes would be expected to increase the wall stress of the capillaries. We have tested this in animal preparations where lung volume was increased to high levels while keeping the capillary transmural pressure constant. Such experiments show a great increase in the number of disruptions of both the capillary endothelial and alveolar epithelial layers at the high lung volumes. Consistent with this, trials of low versus traditional high tidal volumes during mechanical ventilation in intensive care units have shown reduced mortality with the low tidal volumes , although there is still some controversy in this area.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.