Glomerular filtration rate (GFR) and RBF in animals
Glomerular filtration rate (GFR)
Glomerular filtration rate (GFR) is the quantity of GF formed each minute in all the nephrons of both the kidneys/kg body weight. In humans it is about 125 ml/min. Total quantity of GFR formed /day = 180 L. Over 99% of the filtrate is reabsorbed in the tubules, the remainder passing into the urine.
Renal Blood Flow (RBF)
In the physiology of the kidney, renal blood flow (RBF) is the volume of blood delivered to the kidneys per unit time
Filtration fraction
Filtration fraction is the percentage of the renal plasma flow that becomes Glomerular Filtrate. The normal plasma flow through both the kidneys is 650 ml/min but the normal GFR in both the kidneys is 125 ml/min, hence the average filtration fraction is 19%.
Factors affecting glomerular filtration rate (GFR)
Three factors that determine the filtration pressure are- Glomerular pressure, Plasma colloidal osmotic pressure (COP) and Bowman’s capsular pressure.
Greater the glomerular pressure, greater is the filtration
Greater the plasma COP and Bowman’s capsular pressure, lesser is the filtration
Effect of renal blood flow on GFR
GFR is affected by the rate of blood flow through the nephrons. Since a very large portion of plasma is filtered through the glomerular membrane, the COP in the glomerulus is high which opposes further filtration. Therefore, a portion of plasma fluid is not filtered until new plasma flows into the glomerulus. Greater the plasma flow, greater the filtration rate.
Effect of afferent arteriolar constriction on GFR
Afferent arteriolar constriction decreases the rate of blood flow into the glomerulus and decreases GFR, causing decreased filtration rate whereas dilatation increases glomerular pressure as well as GFR.
Effect of efferent arteriolar constriction on GFR
Constriction of the efferent arteriole increases the resistance and outflow from the glomeruli, increases the glomerular pressure and also GFR initially. But when blood stagnates in the glomerulus for a prolonged period, increase in plasma COP occurs which causes a fall in GFR. Net effect is slight increase in GFR.
Effect of sympathetic stimulation on GFR
Mild to moderate stimulation of sympathetic nerves causes afferent arteriolar constriction and decreases GFR. Strong sympathetic stimulation causes great reduction in the glomerular blood flow and glomerular pressure resulting in fall of GFR to zero level.
Effect of arterial pressure on GFR
When arterial pressure increases from 100 to 200 mm Hg afferent arteriolar constriction occurs automatically by autoregulation, thus prevents a major rise in glomerular pressure (GP) and GFR increases to only 15-20%.