Glomerular Filtration RateContents 1Introduction 2Regulation of the GFR 3Regulation of Renal Blood Flow and Capillary Hydrostatic Pressureo 31Constriction of the !fferent and "fferent !rterioles #Physiological Regulators of GFRo #1!utoregulationo #2!ngiotensin 2o #3$ympathetic %er&ous $ystemo ##%itrous '(ide and Prostaglandins ##1%itrous '(ide ##2Prostaglandins ##3"ffects of Bloc)ing %itric '(ide or ProstaglandinsIntroduction*he glomerular filtration rate or GFR is the amount of fluid filtered from the capillaries into the Bowmans capsule of the )idneys per unit time *he GFR can +e e(pressed as the following formula,GFR = Kf x net filtration pressure-f . the filtration coefficent-f can furthermore +e e(pressed +y the following formulaKf = membrane permeability x filtration area*he GFR is practically proportional to meta+olic +ody mass *herefore the +igger the animal the greater the GFRRegulation of the GFRThe following formula helps us to understand GFR and how various factors affect it. hilst reading this article you may find it useful to refer bac! to it"# = $%& ' %() * R# . Flow/ %& . Pressure in afferent arteriole/ %( . Pressure in efferent arteriole/ R . Resistance*here are two ma0or forces opposing GFR *hese are the hydrostatic pressure in the Bowmans space and the plasma protein osmotic pressure *hese are not under physiological control *he filtration coefficient is also +eyond the realms of physiological control 'n the other hand the hydrostatic pressure in the capillaries and the renal +lood flow are under physiological regulation andad0ust filtration according to the +odies needsRegulation of Renal Blood Flow and Capillary Hydrostatic Pressure*hese two factors are determined +y the arterial +lood pressure coupled with the contraction of +oth the afferent and efferent arterioles *he total resistance of the afferent and efferent arterioles/ which is determined +y the contraction of them/ determines the renal +lood flow and any particular arterial pressure *herefore it is important that they change with arterial pressure in order to maintain a steady renal +lood flowConstriction of the &fferent and (fferent &rterioles%ormally the afferent arteriole is of larger diameter than the efferent *his means there is high resistance as the +lood is forced from a wider &essel to a narrower one and this promotes filtration Ifthe arterial +lood pressure remains constant then contracting either &essel reduces +lood flow as it increases resistance Howe&er contracting either has opposite effects on the filtration pressure If you contract the afferent arteriole there will +e less of a pressure difference +etween the afferent andefferent arteriole so there will +e reduced filtration pressure Howe&er if you constrict the efferent arteriole you are increasing the pressure difference +etween the two and filtration pressure increase'&erall the constriction of the afferent arteriole decreases +oth +lood flow and filtration pressure where as constricting the efferent arteriole decreases +lood flow +ut increases filtration pressure 1Both of these statements are assuming a constant +lood pressure2 *he fact that +oth can +e altered allows independent regulation of +oth GFR and +lood flowPhysiological Regulators of GFR*he main systems which regulate renal +lood flow and GFR are,&utoregulation# = $%& ' %() * R# . Flow/ %& . Pressure in afferant arteriole/ %( . Pressure in efferant arteriole/ R . ResistanceIf renal resistance to +lood flow was constant then any change to mean arterial +lood pressure would alter +lood flow/ glomerular hydrostatic pressure and therefore filtration Howe&er if +lood pressure is changed +y a small amount o&er a short period of time +lood flow and filtration to the )idneys is not really affected *his is due to autoregulatory feed+ac) mechanisms which allow the )idney to &ary the resistance in the afferent arteriole If it wasn3t for these autoregulatory mechanisms then a small increase in arterial +lood pressure would drastically increase the e(cretionof salt and water leading to a drastic reduction in the concentration of %aCl in the "CF%ressure &utoregulationIf arterial +lood pressure increases then resistance in the afferent arteriole increases also and the opposite occurs if +lood pressure falls *he role of pressure autoregulation is to ensure that during transient changes in +lood pressure there is little effect on renal +lood flow and therefore filtration pro&iding the +odies need for the e(cretion of water and solutes remains the same *his is essential as only a small change in renal +lood flow and thus filtration rate can ha&e a massi&e change on urine output *he mechanisms for this response are found within the )idneys,+yogenic Response $tretching of +lood &essels due to increased +lood pressure results in the +lood &essel decreasing it3s diameter *his results in an increased resistance to +lood flow *hus )eeping the GFR constantTubuloglomerular Feedbac! $TGF) 4hen +lood pressure increases for a short amount of time more +lood flows through the glomerulus and therefore more filtrate is produced *his results in a decrease in pro(imal tu+ule rea+sorption *his increases the concentration of %aCl in the distal tu+ule which is detected +y the 5acula 6ensa *his structure releases local factors resulting in the &asoconstriction of the afferent arteriole If +lood pressure decreases the opposite occursThe ,imitations of &utoregulation6espite the efforts of the autoregulatory system an increase in +lood pressure still leads to an increased secretion of salt and water *his is +ecause e&en a small percentage change in GFR leads to large percentage change in the e(cretion of salt and water *his e(cretion is howe&er far less drastic than would +e the case without autoregulation and actually helps to restore pressure to normal *his increase in urinary output as a result of an increase in arterial +lood pressure is termed pressure diuresis&ngiotensin -.ympathetic /ervous .ystem4hen the animal is in a situation of crisis or stress +lood flow to the )idneys is reduced for the sa)e of other organs such as the +rain/ heart and s)eletal muscles *he sympathetic ner&ous system and a heightened le&el of adrenalin in the plasma cause the contraction of +oth the afferant and efferant arterioles !s the efferant arteriole is contracted alongside the afferant one there is still a pressure differance allowing for filtration to still occur and reducing the impact on filtration compared to the impact on +lood flow !t times when sympathetic tone is &ery high the renal +lood flow could +e reduced to 178379 of normal *his practically stops filtration occuring and thus stops urine productionResting .ympathetic &ctivity:nli)e many other organs the )idneys ha&e a low resting sympathetic tone *herefore the sympathetic ner&ous system cannot effecti&ely decrease the resistance +y decreasing itself *his suggests that its main aim is to compensate for a fall in +lood pressure or to prepare the +ody for thefight or flight response(ffect on Reabsorption!s GFR is less affected than +lood flow the filtration of what +lood does pass through the glomerulusis more efficent *his means that the +lood entering the pertitu+ular capillaries will ha&e a higher protein osmotic pressure and a lower hydrostatic pressure as more of the plasma will ha&e +een filtered *his causes greater rea+sorption of water and salt from the tu+ules *his causes urine &olume to fall It also stimulates renin/ *he Renin !ngiotensin !ldosterone $ystem 1R!!$2 and aldosterone which all in turn ha&e their effect on rea+sorption/et (ffect of 0ncreased .ympathetic /ervous &ctivity on the Kidneys Reduced renal +lood flow $mall decrease in e(creted waste Increased conser&ation of water and sodium/itrous 1xide and %rostaglandins%itrous '(ide and Prostaglandins ha&e an impact on arteriolar resistance *heir role in the regulationof renal +lood flow and filtration is howe&er uncertain/itrous 1xide 5ediates dilation in the cortical circulation%rostaglandins 5ediates dilation in medullary circulation 1cortical in e(tremes2 PG"2 is in&ol&ed in the regulation of the rea+sorption of sodium Prostacyclin increases the secretion of potasium +y stimulating renin secretion thus acti&ating the Renin8!ngiotensin8!ldosterone $ystem and as a result increasing the amount of !ldosterone secreted %rostacyclin increases renal blood flow and gfr when circulating volume is decreased. *his results in increased tu+ular flow and increased potasium secretion In healthy;hydrated indi&iduals these compounds do not play a significant role in sodium;water homeostasis(ffects of 2loc!ing /itric 1xide or %rostaglandins