Indiana University School of Medicine, Indianapolis, Indiana
Basic lung anatomy is simple. Branching airways terminate in alveoli, and the arterial and venous trees interconnect via capillaries that are embedded in alveolar walls. The complexity of the structure comes from its ~3 x 108 alveoli and their accompanying ~3 x 1010 capillaries. These very large numbers have caused anatomists and physiologists to simplify the lung to a one-alveolus, one-capillary model. With time, this model has tended to be thought of as the real lung; some senior scientists express considerable surprise when they realize that pulmonary capillary pathways cross more than one alveolus.
Although pulmonary microvascular anatomy and its effect on perfusion are extremely complex, they are not hopelessly so. We have considerable information about how the pulmonary circulation utilizes the extensive reserve in the pulmonary capillary bed when the demand for oxygen increases. In trying to understand how these reserves come into play when the lung reaches its full potential as a gas exchanger during heavy exercise, we consider three kinds of changes that occur in the capillary bed as cardiac output increases from rest to exercise.
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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.