The adult heart is not only smaller relative to body size and has left and right ventricles of unequal mass, but also shows differences at the cellular from level from that of the fetus (Table 3.19).
Myocytes now make up only 25% of the cells in the heart by number (2,000,000,000), although they occupy 75-78% of the heart volume. The myocytes are cylinders of complex morphology, 15-17 um in diameter and 75-100 mm in length, with an L/W ratio of 5.5.
The non-muscle cells are all smaller, such as fibroblast, endothelial, smooth muscle, nerve, and adipose cells. The fibroblasts which synthesize collagen struts and the elastin network holding the myocytes in place, number 4-6,000,000,000.
The endothelial cells of course line all the vascular surfaces and the smooth muscle cells control vaso- and veno-motion by contraction or relaxation.
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