As mentioned at the beginning of Section B, at any given moment, approximately 5 percent of the total circulating blood is flowing through the capillaries, and it is this 5 percent that is performing the ultimate function of the entire cardiovascular system—the exchange of nutrients and metabolic end products. Some exchange also occurs in the venules, which can be viewed as extensions of capillaries.
The capillaries permeate almost every tissue of the body. Since most cells are no more than 0.1 mm (only a few cell widths) from a capillary, diffusion distances are very small, and exchange is highly efficient. There are an estimated 25,000 miles of capillaries in an adult, each individual capillary being only about 1 mm long with an inner diameter of 5 ^m, just wide enough for an erythrocyte to squeeze through. (For comparison, a human hair is about 100 ^m in diameter.)
The essential role of capillaries in tissue function has stimulated many questions concerning how capillaries develop and grow (angiogenesis). For example, what turns on angiogenesis during wound healing and how do cancers stimulate growth of the new capillaries required for continued cancer growth? It is known that the vascular endothelial cells play a central role in the building of a new capillary network by cell locomotion and cell division. They are stimulated to do so
Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition
Circulation CHAPTER FOURTEEN
by a variety of angiogenic factors [for example, vascular endothelial growth factor (VEGF)] secreted locally by various tissue cells (fibroblasts, for example) and the endothelial cells themselves. Cancer cells also secrete angiogenic factors, and development of drugs to interfere with the secretion or action of these factors is a promising research area in anticancer therapy. For example, substances that inhibit blood-vessel growth have been found to reduce the size of almost any tumor (or eliminate the tumor completely) in mice; these agents are presently being studied in people with cancer.
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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.