B.Tubular function(Reabsorption oftubules and collectivetubes)The tubulars are involved in reabsorptionand secretion.Important components of plasmatend to be reabsorbed more or less completely:99-100%e.g.sodiumandglucose--arereabsorbed.Waste products are only partiallyreabsorbed:e.g.about45%ofisureareabsorbed
The tubulars are involved in reabsorption and secretion. Important components of plasma tend to be reabsorbed more or less completely: e.g. sodium and glucose are 99-100% reabsorbed. Waste products are only partially reabsorbed: e.g. about 45% of urea is reabsorbed. B. Tubular function (Reabsorption of tubules and collective tubes)
I.Renal tubuletransport machanismsProximal tubule1. Sodium bicarbonate, sodium chloride,glucose.amine acids,and other-organicsolute arepreferentially reabsorbed via specific transportsystems in the early proximal tubule.2.Water is reabsorberedpassivelytoSOasofnearly-osmolalitymaintainconstantproximal tubular fluid
I. Renal tubule transport machanisms Proximal tubule 1. Sodium bicarbonate, sodium chloride, glucose, amine acids, and other organic solute are preferentially reabsorbed via specific transport systems in the early proximal tubule. 2. Water is reabsorbered passively so as to maintain nearly constant osmolality of proximal tubular fluid
ProximalconvolutedInterstitiumLumen-urineABtubulebloodNatNatNatNatATPK+K+HCO3-+H+HCO3H+十HCO3H,CO3H,CO3CACA1+1CO, +H,0H,0+CO2The Nat-H+ exchange is related to carbonic anhydraseCarbonic anhydrase catalyzes the reaction of CO,and H,Oleading to H+and HCO3
H2O+CO2 CO2 + H2O H2CO3 H+ + HCO3 - Proximal convoluted tubule Lumen-urine Interstitiumblood Na+ HCO3 - Na+ CA Na+ H2CO3 HCO3 - + H+ K+ K+ Na+ B A The Na+ -H+ exchange is related to carbonic anhydrase. Carbonic anhydrase catalyzes the reaction of CO2 and H2O leading to H+ and HCO3 - . CA ATP
Loopof henleThe thickascending limb--of-the loop of Henleactively reabsorbesNaCl from thelumen (about35%of thefiltered sodium),but unlikethe proximal tubuleand the thin limb, it is extremely impermeable towater.Salt reabsorption in the thick ascendinglimbtherefore dilutes the tubularfluid,leadingtoitsdesignation as a “diluting segment.Medullaryportionsof thick ascending limbcontribute tomedullary hypertonicity and thereby also play animportantrolein concentration of urine
Loop of henle The thick ascending limb of the loop of Henle actively reabsorbes NaCl from the lumen (about 35% of the filtered sodium), but unlike the proximal tubule and the thin limb, it is extremely impermeable to water. Salt reabsorption in the thick ascending limb therefore dilutes the tubular fluid, leading to its designation as a “diluting segment”. Medullary portions of thick ascending limb contribute to medullary hypertonicity and thereby also play an important role in concentration of urine
ThickascendingInterstitiumLumen-urinelimbbloodCa2+,Mg2+Ca?+,Mg2+Ca?+,Mg2+Potential(+)KK+K+ATPK+NatNatNa+→ Nat2CI2CI→2CI2CIThe NaCl transport system in the luminal membrane ofthe thick ascending limb is aNa+/K+/2Cl-cotransporter
ATP Na+ 2ClNa+ K+ Potential(+) K+ K+ 2ClK+ Na+ 2Cl- 2ClNa+ Ca2+,Mg2+ Ca2+,Mg2+ Lumen-urine Thick ascending limb Interstitiumblood The NaCl transport system in the luminal membrane of the thick ascending limb is a Na+ /K+ /2Cl- cotransporter. Ca2+,Mg2+