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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.?

Urine formation begins with filtration of large amounts of fluid through the
glomerular capillaries into Bowman's capsule.
Glomerular capillary membrane
The glomerular capillary membrane is similar to that of other capillaries, except
that it has three (instead of the usual two) major layers; (1) the endothelium of the
capillary, (2) abasement membrane, and (3) a layer of epithelial cells (podocytes)
surrounding the outer surface of the capillary basement membrane. The capillary
endothelium is perforated by thousands of small holes called fenestrae .
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.
The final part of the glomerular membrane is a layer of epithelial cells that line
the outer surface of the glomerulus. These cells are not continuous but have long
footlike processes (podocytes) that encircle the outer surface of the capillaries.
The foot processes are separated by gaps called slit pores, through which the
glmerular filtrate moves.
GFR is about 20 percent of the renal plasma flow
In the average adult human, the GFR is about 125ml/min, or 180 L/day. The
fraction of the renal plasma flow that is filtered (the filtration fraction) average
about 0.2; this means that about 20 per cent of the plasma flowing through the
kidney is filtered through 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.
∎Filterability of solutes inversely related to their size.
∎ Negatively charged molecules are filtered less easily than positively charged
molecules of equal molecular size.
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.
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 . (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
GFR=Kf/(PG-PB-𝜋G+𝜋𝐵)
Net filtration pressure=60-18-32=+10mmHg