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WSU BIOLOGY 251 - Topic 21

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BIO 251 1st Edition Lecture 21 Outline of Last Lecture I. Overview a. Function b. Components c. Kidney structure d. Nephron structure e. 3 basic renal processes f. Important notesII. Glomerular filtration a. Process of filtration b. Problem of GFR & BPc. Intrinsic regulation d. Extrinsic regulation Outline of Current Lecture I. Tubular reabsorption a. Processb. Reabsorption of Ba+ in proximal tubule c. Reabsorption of Na+ in distal tubule d. Reabsorption of substances via Na+ dependent secondary active transportThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.e. Reabsorption of substances via Na+ dependent passive processesf. Reabsorption of phosphateII. Tubular secretion a. H+b. K+c. Waste ions III. Urine concentration a. Countercurrent multiplication b. Controlling urine concentrationCurrent LectureI. Tubular ReabsorptionA. Introduction1. Highly selective processa) Need to get essential materials out of tubules and back in bloodb) Most wastes, other than urea, are too big to be reabsorbed and soare excreted in the urine2. Examples of reabsorption rates (Table 18.1)a) 99% of filtered water (47 gallons/day)b) 100% of filtered sugar (2.5 pounds/day)c) 99.5% of filtered salt (0.36 pounds/day)B. Process of Reabsorption: Transepithelial Transport1. Reabsorption requires molecules to cross two membranes: apical membrane and basolateral membrane (Fig 18.13)2. Two types of reabsorption (Fig 18.14)a) active: at least one of the steps requires energy (Fig 18.14a)b) passive: none of the steps requires energy (Fig 18.14b, c)C. Reabsorption of Na+ in the Proximal Tubule (Fig 19.14a)1. Na+ uses facilitated diffusion to cross apical membrane, going down concentration gradient from tubular lumen into proximal tubule epithelial cell. Can do so either by cotransport with glucose or an amino acid, or by counter transport with H+2. Na+ actively crosses basolateral membrane using the Na+-K+-ATPase pump which moves Na+ against concentration gradient from tububar cell into into interstitial fluid; then it passively diffuses into blood plasma.D. Reabsorption of Na+ in the Distal Tubule (Fig 19.14b)1. Na+ uses facilitated diffusion to cross apical membrane, going down concentration gradient from tubular lumen into proximal tubule epithelial cell. Can do so two ways:a) cotransport with Cl- ionsb) through Na+ channels. If done this way, electrical balance must be maintained by passive movement of Cl- into cell or K+ out of cell.2. Low plasma Na+ causes release of hormone aldosterone which casues increased reabsorption of Na+ by causing existing channels to open and stimulating synthesis of new channels and new Na+-K+-ATPase pumps (fig 19.15)E. Reabsorption of substances via Na+ Dependent Secondary Active Transport (Figs 19.14a & 18.15)1. Glucose and amino acids and other nutritionally important compounds aremoved against their concentration gradients by secondary active transport2. These substances are co-transported across apical membrane along with the Na+ and then diffuse across basolateral membrane.3. Note that it is actually the Na+-K+-ATPase pump in the basolateral membrane that drives this process by keeping the concentration of Na+ low in the tubular cell so that a concentration gradient exists from tubular lumen into tubular cell4. So they get a free ride with the Na+; if no Na+, the pump shuts down andnothing is transported by secondary mechanisms.5. Glucose reabsorption (Fig 18.15)a) The maximum transport rate for glucose is ~ 375 mg/minb) Under normal conditions, ~ 125 mg/ml is reabsorbedc) If you have more than 375 mg/min that is available for reabsorption, the excess ends up in the urine(1) People with diabetes mellitus have high levels of plasma glucose, and end up excreting a lot of glucose in the urine because it all can not be reabsorbedF. Reabsorption of substances via Na+ Dependent Passive Processes1. Chloride ions (Cl-) (Fig 19.14a)a) passively follow the electrical gradient of Na+2. Water molecules (in PROXIMAL TUBULES ONLY; see below for waterreabsorption in distal tubules)a) osmotically follow Na+ across the membrane from proximal tubules to interstial fluid (Fig 18.14b)3. Urea reabsorption (in PROXIMAL TUBULES ONLY; see below for urea reabsorption in distal tubules)a) follows concentration gradient established by water leaving the tubules (Fig 19.5)(1) water leaves tubules, which increase tubular concentration of urea relative tothe interstitial concentration, so it flows down its concentration gradient into theinterstitial fluid(2) although half of the urea is reabsorbed, the other half isexcreted, which in humans works just fine.(3) note: urea buildup due to renal failure is only mildly toxiccompared to buildup of H+ and K+G. Reabsorption of Phosphate (also true for Ca++ and some other electrolytes)1. The normal reabsorption rate of phosphate equals normal plasmaconcentrations of phosphate2. If you ingest excess phosphate above normal plasma concentrations, theexcess is not reabsorbed and so is excreted in the urine = very tightregulation compared to glucose3. Parathyroid hormone can alter the reabsorption rates of electrolytes toconserve them if need be.II. Tubular SecretionA. General1. Supplemental mechanism to filtration to get rid of substances2. Essentially is the reverse of tubular reabsorptionB. Secretion of H+1. When ECF is too acidic (i.e., too much H+), H+ is secreted passively (i.e., moves by diffusion from peritubular capillaries to tubular system.)C. Secretion of K+ (Fig 19.20)1. K+ must be closely regulated: if K+ too low in ECF = hyperpolarization ofnerve and muscle cell membranes (= reduced excitablity); high K+ in ECFincreases membrane excitablility, especially in the heart, which can lead tocardiac arrhythmias2. K+ actively moved in opposite directions by reabsorption in the proximaltubule and secretion in the distial tubule.3. The active transport of Na+ during Na+ reabsorption results in the secretionof K+, because Na+-K-ATPase pump moves Na+ and K+ in oppositedirections4. K+ secretion, not K+ filtration or reabsorption, is the process regulated bythe kidneys to maintain proper amounts of K+.a) If plasma K+ is too high(1) The increased plasma K+ directly increases aldosterone production, which increases K+ secretion and hence excretion in the urineb) If plasma K+ is too low(1) aldosterone production reduced, so secretion of K+ is decreased = less


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