Figure 23.14 shows how pressures change along the length of a glomerular capillary, in contrast to those seen in a capillary in other vascular beds (in this case, skeletal muscle). Note that average capillary hydrostatic pressure in the glomerulus is much higher (55 vs. 25 mm Hg) than in a skeletal muscle capillary. Also, capillary hydrostatic pressure declines little (perhaps 1 to 2 mm Hg) along the length of the glomerular capillary because the glomerulus contains many (30 to 50) capillary loops in parallel, making the resistance to blood flow in the glomerulus very low. In the skeletal muscle capillary, there is a much higher resistance to blood flow, resulting in an appreciable fall in capillary hydrostatic pressure with distance. Finally, note that in the glomerulus, the colloid osmotic pressure increases substantially along the length of the capillary because a large volume of filtrate (about 20% of the entering plasma flow) is pushed out of the capillary and the proteins remain in the circulation. The increase in colloid osmotic pressure opposes the outward movement of fluid.
In the skeletal muscle capillary, the colloid osmotic pressure hardly changes with distance, since little fluid moves across the capillary wall. In the "average" skeletal muscle capillary, outward filtration occurs at the arterial end and absorption occurs at the venous end. At some point along the skeletal muscle capillary, there is no net fluid movement; this is the point of so-called filtration pressure equilibrium. Filtration pressure equilibrium probably is not attained in the normal human glomerulus; in other words, the outward filtration of fluid probably occurs all along the glomerular 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.