ABSTRACT
The rate at which the glomeruli of the kidney removes catabolic products, xenobiotics, and other impurities in the blood is referred to as the glomerular ltration rate (GFR). GFR is dened as the volume of blood that is ltered per units of time. The permeability of the capillary wall of the glomerulus is due to the large pore size, which is approximately 25 times more permeable than regular capillary walls. The large capillary pore sizes in association with the cardiac contraction force facilitate the transcapillary passage of water, nutrients, ions, and ionized and unionized xenobiotics. The normal GFR is about 125 mL/min or 7.5 L/h or 180 L/day. GFR varies among healthy individuals with a range between 110 and 125 mL/min and it represents how much ltrate is removed from the blood per minute. The normal rate of plasma ow (RPF) through both kidneys is approximately 650 mL/min, and the normal rate of blood ow (RBF) is about 1100-1200 mL/min, thus about 10% of renal blood ow is ltered by the glomerular ltration (Leggett and Williams, 1991, 1995; Walton et al., 2004). The glomerular ltration of compounds that follow linear pharmacokinetics is equal to GFR multiplied by the free fraction of the compounds in plasma. The composition of the ltrate is similar to plasma, but without the large molecular weight proteins. The extent of ltration of a compound depends on the molecular weight and protein binding. All compounds of low molecular weight (<60,000 Dalton) are ltered out of the blood. The ltrate travels from the Bowman capsule toward the collecting duct and eventually the bladder during which the body reabsorbs water, nutrients and ions, and secrets other solutes. Ultimately, from the 180 L/day original ltrate, only about one liter per day reaches the bladder and leaves the body as urine.
