Urine formation begins with filtration of large amounts of fluid through the
glomerular capillaries into Bowman's capsule.Determinant 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/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.The filtration fraction is
calculated as follows:
Filtration fraction=GFR/Renal plasma flow
Despite the high faction rate, the glomerular filtration barrier selective in
determining which molecules will filter, based on their size and electrical charge.These forces include:
(1)hydrostatic pressure inside the glomerular capillaries (glomerular hydrostatic
pressure,PG) which promotes filtration(60);
(2)the hydrostatic pressure in Bowman's capsule (PB) outside the capillaries,
which opposes filtration(18);
(3)the colloid osmotic pressure of the glomerular capillary plasma proteins (pG),
which opposes filtration(32), and
(4) the colloid osmotic pressure of the proteins in Bowman's capsule (pB), which
promotes filtration .surrounding the endothelium is the basement membrane, which consists of a
meshwork of collagen and proteoglycan fibrillae that have large spaces through
which large amounts of water and small solutes can filter.?Filterability of solutes inversely related to their size.?