【正文】 he plasma glucose level increases up to a value above160~180mg/dl, glucose can first be detected in the urine. This value is called renal glucose threshold. Maximal rate of glucose transport: Maximal tubular reabsorption of glucose (Tm or TmG) normal value: male : 375 mg/min female: 300 mg/min ?HCO3 reabsorption Site: 80% reabsorbed at proximal tubule reabsorption form: CO2 depend on H+ secretion Acetazolamide(乙酰唑胺 ) inhibits carbonic anhydrase → urine amount ↑ 碳酸酐酶 ?K+ reabsorption K+ is reabsorbed by PT (65% ) and TAL(25%). K+ is reabsorbed and secreted by DCT and CT. Site K+ in the urine is mainly determined by K+ secretion， not by reabsorption 1） Secretion of K+ 2） Secretion of H+ 3） Secretion of NH3 2. Secretion of renal tubule and collecting duct 1） Secretion of K+ Site DCT CT Mechanism: K+ secretion is closely correlated to Na+ reabsorption passive process Factors affecting K+ secretion： extracellular K+ concentration， aldosterone secretion and tubular flow rate 2) Secretion of H+ Site: PT, DT and CT Mechanism: active transport PT H+ Na+ exchange DT+CT proton pump（質(zhì)子泵） H+ ATP酶 or H+ K+ ATP酶 H+ Na+ exchange Significance: promote the reabsorption of HCO3 regulate acidbase balance Promote the secretion of NH3 3) Secretion of NH3 Site: PT /TAL/DCT/ CT Source: Glutamine Mechanism: – Significance: promote the secretion of H+ promote the absorption of HCO3 regulate the acidbase balance Urinary Concentration and Dilution Compare osmolality in urine with osmolality in plasma ? Urine ? plasma ? hyperosmotic urine urine is concentrated ? Urine ? plasma ? hypoosmotic urine urine is diluted ? Urine ? =plasma ? isoosmotic – Normal value: 50 ~1200 mOsm/L 1. Urine Dilution Urine is diluted in the absence of ADH urine is diluted 2. Urine concentration 3. Generation of osmotic gradient in the medullacritical for urine concentration The interstitial fluid osmotic pressure increases gradually from outer medulla to inner medulla. How does the renal medulla bee hyperosmotic ? The Countercurrent Mechanism Mechanisms for creating osmotic gradient in the medullary interstitial fluid A. Formation of the osmolality gradient in outer medulla active transport of NaCl in TAL It is because the sodium pump is located in the basolateral membrane. Na+/K+ 2Cl active cotransport 1)Inner medullary collecting duct is highly permeable to urea. 2)The thin ascending limb of the loop is permeable to NaCl. B. Formation of osmolality gradient in inner medulla Role of the vasa recta for maintaining the high solute concentration in the medullary interstitial fluid counter current exchange of vasa recta The regulation of urine formation in the kidney 1) Renal blood flow /GFR are constant by autoregulation of the kidney 2) Regulation of Tubular Reabsorption ⑴ Glomerulotubular Balance (球管平衡 ) ? Tubular reabsorption increases/decreases automatically as the filtered load increases/ decreases. This is the second line of defense for buffering urinary output changes that could result from changes in GFR. First line is autoregulation on GFR. ? Glomerulotubular balance is a strictly local renal mechanism. It refers to the finding that the proximal