Glomerular Filtration Rate

Glomerular Filtration Rate, Basic Mechanism of Absorption and Secretion in Tubules

Glomerular Filtration Rate (GFR):
It is the fluid that filtrate through the glomerulus into Bowman’s capsule each minute. It is about 125 ml/min. or 180 L/day in male (10% lower in female). The high GFR is due to very high permeability of capillaries of glomerulus. Because the inner side of the pores of glomerular membrane is negatively charged repelling other negatively charged molecules that tend to pass through pores. The glomerular membrane is completely impermeable for all plasma proteins but it is highly permeable to all other dissolved substances of normal plasma. The composition of the glumerular filtrate is the same as plasma except that it has no significant amount of proteins. The filtration fraction in normal plasma flow through both kidneys is 1/5 or 20% (125 ml/min.) it determined by:
1. The balance of hydrostatic and colloid osmotic forces acting across the capillary membrane.
2. Capillary filtration coefficient (Kf).
The glomerular capillaries have much higher rate filtration than other capillaries because of high glomerular hydrostatic pressure and large (Kf).
Glomerular capillary has 3 layers: endothelium, basement membrane and epithelial cells. The layers make up the filtration barrier. Epithelium of capillary is performed by thousands of small pores (fenestrate) similar to fenestrated capillaries found in the liver.
Factors that favoring filtration:
1. Glomeular hydrostatic pressure (60).
2. Bowman’s capsule colloid osmotic pressure (0).
Factors that opposing filtration:
1. Glomerular colloid osmotic pressure (18).
2. Bowman’s capsule hydrostatic pressure (32).