Effect of the number of perfused capillaries on cell concentration of bloodborne molecules (dots). A, With one capillary, the left side of the cell has a low concentration. B, The concentration can be substantially increased if a second capillary is perfused. C, The perfusion of three capillaries around the cell increases concentrations of bloodborne molecules throughout the cell.
single capillary provides all the nutrients to the cell. The concentration of bloodborne molecules across the cell interior is represented by the density of dots at various locations. Diffusion distances are important; as molecules travel farther from the capillary, their concentration decreases substantially because the volume into which diffusion proceeds increases as the square of the distance. In addition, some of the molecules may be consumed by different cellular components, which further reduces the concentration.
If there is a capillary on either side of a cell, as in Figure 16.5B, the cell has a higher internal concentration of molecules from the two capillaries. Therefore, increasing the number of microvessels reduces diffusion distances from a given point inside a cell to the nearest capillary. Doing so minimizes the dilution of molecules within the cells caused by large diffusion distances. At any given moment during resting conditions, only about 40 to 60% of the capillaries are perfused by red blood cells in most organs. The capillaries not in use do contain blood, but it is not moving. Exercise results in an increase in the number of perfused cap illaries, decreasing diffusion distances. The arteriolar dilation during exercise allows arterioles to supply blood flow to nearly all of the available capillaries in muscle.
Regular exercise induces the growth of new capillaries in skeletal muscle. As shown in Figure 16.5C, three capillaries contribute to the nutrition of the cell and elevate cell concentrations of molecules derived from the blood. However, decreasing the number of capillaries perfused with blood by constricting arterioles or obliterating capillaries, as in diabetes mellitus, can lengthen diffusion distances and decrease exchange.
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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.