After the blood passes through the capillaries, it enters the venules, endothelial tubes usually surrounded by a mono-layer of vascular smooth muscle cells. In general, the vascular muscle cells of venules are much smaller in diameter but longer than those of arterioles. The muscle size may reflect the fact that venules operate at intravascular pressures of 10 to 16 mm Hg, compared with 30 to 70 mm Hg in arterioles, and do not need a powerful muscular wall. The smallest venules are unique because they are more permeable than capillaries to large and small molecules. This increased permeability seems to exist because tight junctions between adjacent venular endothelial cells have more frequent and larger discontinuities or pores. It is probable that much of the exchange of large water-soluble molecules occurs as the blood passes through small venules.
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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.