Physiology of Capillary Beds

Glomerular Filtration

• Glomerular capillaries are uniquely situated between afferent and efferent arterioles, resulting in constant filtration of plasma by the glomerular filter.
• Hydrostatic pressure in the glomerular capillaries remains higher than the osmotic pressure exerted by their contents, causing continuous filtration.
• The glomerular filtration rate (GFR) is regulated by neural and hormonal inputs affecting the degree of constriction in two arterioles.

Glomerular Filtration Barrier

• The glomerular filtration barrier consists of three layered components: fenestrated capillary endothelium, glomerular basement membrane (GBM), and filtration slit diaphragms between pedicels.
• The GBM blocks large proteins, while filtration slit diaphragms regulate passage of small proteins.
• The glomerular filter blocks most plasma proteins, but smaller proteins, including polypeptide hormones, are removed into the filtrate.

Glomerular Function

• The total glomerular filtration area of an adult has been estimated at 500 cm2.
• The average GFR is 125 mL/min or 180 L/d.
• The glomerular filter produces a filtrate that contains very little protein.

Medical Application

• In diseases such as diabetes mellitus and glomerulonephritis, the glomerular filter becomes more permeable to proteins, leading to proteinuria.
• Proteinuria is an indicator of many potential kidney disorders.