Distal convoluted tubuleLess NaClis reabsorbedin the distalconvoluted tubule (only about 10% of theflitered NaC than in the proximal tubuleor the thick ascending limb.Like the thickascending limb,this segment is relativelyimpermeable towater, and theNaCireabsorption therefore further dilutes thetubularfluid
Distal convoluted tubule Less NaCl is reabsorbed in the distal convoluted tubule (only about 10% of the flitered NaCl) than in the proximal tubule or the thick ascending limb. Like the thick ascending limb, this segment is relatively impermeable to water, and the NaCl reabsorption therefore further dilutes the tubular fluid
DistalconvolutedInterstitiumLumen-urinetubulebloodPHTNatNatCIATPK+NatCa2+Ca2±The mechanismof NaCI transport in the distalconvoluted tubuleis electrically neutral Na+andClcotransport
ATP Na+ Na+ PHT ClK+ Na+ Ca2+ Lumen-urine Distal convoluted tubule Interstitiumblood Ca2+ The mechanism of NaCl transport in the distal convoluted tubule is electrically neutral Na+ and Cl- cotransport
TheNaCItransporteris blocked by diuretics of thethiazide class.BecauseK+ does not recycle across theapicalmembrane of the distal convoluted tubule asit does intheloop ofHenle,thereis no lumen-positivepotentialin thissegment, and Ca2tand Mg2+ are not driven out of the tubularlumenbyelectricalforceHowever Ca?+ is actively reabsorbedbythe distalconvoluted tubule epithelial cell via an apical Ca2+ channeland basolateral Nat/Ca2+exchanger.This process is regulatedby parathyroid hormone.Aswill beseenbelow,thedifferences in the tubule and intheloop of Henle haveimportant implications for the effects of various diuretics onCa2+transport
The NaCl transporter is blocked by diuretics of the thiazide class. Because K+ does not recycle across the apical membrane of the distal convoluted tubule as it does in the loop of Henle, there is no lumen-positive potential in this segment, and Ca2+and Mg2+ are not driven out of the tubular lumen by electrical force. However, Ca2+ is actively reabsorbed by the distal convoluted tubule epithelial cell via an apical Ca2+ channel and basolateral Na+ /Ca2+ exchanger. This process is regulated by parathyroid hormone. As will be seen below, the differences in the tubule and in the loop of Henle have important implications for the effects of various diuretics on Ca2+ transport
CollectingtubuleThe collecting tubule is responsible foronly 2-5%of NaCl reabsorption by the kidneyDespite this seemingly small contribution,thecollecting tubule plays a vital role in renalphysiology and in diuretic action. As the finalsite ofNaCl reabsorption,the collecting tubuleis responsible for determining the final Na+concentration of the urine
Collecting tubule The collecting tubule is responsible for only 2-5% of NaCl reabsorption by the kidney. Despite this seemingly small contribution, the collecting tubule plays a vital role in renal physiology and in diuretic action. As the final site of NaCl reabsorption, the collecting tubule is responsible for determining the final Na+ concentration of the urine
InterstitiumCollecting tubuleLumen-urinebloodPrincipalcellaldosteroneNatNatKATPK+H,0ADHWaterchannelmoleculesIntercalatedcellHCO3ATPHtCIThe mechanism of NaClreabsorptionin the collectingtubuleis again distinctfromthe mechanisms describedfor the othertubule segments.Theprincipalcellsarethemajorsite of Nat,Kt, andH,Otransport,and theintercalatedcellsaretheprimarysitesofprotonsecretion
ATP Na+ aldosterone K+ Na+ H2O Lumen-urine Interstitiumblood Collecting tubule Intercalated cell K+ Principal cell ADH Water channel molecules Cl H+ - ATP HCO3 - The mechanism of NaCl reabsorption in the collecting tubule is again distinct from the mechanisms described for the other tubule segments. The principal cells are the major site of Na+ , K+ , and H2O transport, and the intercalated cells are the primary sites of proton secretion