Those of us who work on the pulmonary circulation tend to bristle at the term "lesser circulation." After all the lung is the only organ of the body that receives the whole of the cardiac output. It is true that the pulmonary circulation has much lower pressures than the systemic circulation, but of course its functions are equally important.
A cardinal feature of a pulmonary capillary as shown in Figure 1 is that parts of its wall are extraordinarily thin. For example in the human lung the wall is of the order of 0.2 mm in thickness over much of its area of 50 to 100m2 . This extreme thinness is essential to allow the enormous amounts of gas exchange of oxygen and carbon dioxide to occur by passive diffusion. The pulmonary capillary is essentially naked in the sense that it is directly exposed to the alveolar gas.
By contrast, systemic capillaries are embedded in tissue and receive substantial mechanical support from this . Baez and colleagues measured the diameters of various small blood vessels in the exteriorized meso-appendix of rats and showed that the smallest (8 to 10 mm), that is, the capillaries, behaved essentially as rigid tubes in that their diameter remained constant over a range of pressures from 20 to 200 mmHg. Fung calculated that the surrounding tissue of capillaries in mesentery contributed over 99 percent of their rigidity with less than 1 percent coming from the endothelium and basement membrane. Pressures of the magnitude of 100 mmHg will certainly cause damage to the walls of pulmonary capillaries. It is critically important to appreciate the very different mechanical environment of pulmonary capillaries compared with systemic capillaries.
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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.