Determinants of the GFR

The GFR is determined by (1) the sum of the hydrostatic and colloid osmotic forces across the glomerular membrane, which gives the net filtration pressure, and (2) the glomerular capillary filtration coefficient, Kf. Expressed mathematically, the GFR equals the product of Kf and the net filtration pressure:

GFR = Kf x Net filtration pressure

The net filtration pressure represents the sum of the hydrostatic and colloid osmotic forces that either favor or oppose filtration across the glomerular capillaries (Figure 26-12). These forces include (1) hydrostatic pressure inside the glomerular capillaries (glomerular hydrostatic pressure, PG), which promotes filtration; (2) the hydrostatic pressure in Bowman's capsule (PB) outside the capillaries, which opposes filtration; (3) the colloid osmotic pressure of the glomerular capillary plasma proteins (pG), which opposes filtration; and (4) the colloid osmotic pressure of the proteins in Bowman's capsule (pB), which promotes filtration. (Under normal conditions, the concentration of protein in the glomerular filtrate is so low that the colloid osmotic pressure of the Bowman's capsule fluid is considered to be zero.)

The GFR can therefore be expressed as

Afferent arteriole

Glomerular Glomerular hydrostatic colloid osmotic pressure pressure

Efferent arteriole

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Essentials of Human Physiology

Essentials of Human Physiology

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