WSU BIOLOGY 251 - Topic 21 (6 pages)

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Topic 21

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Topic 21-the urinary system: reabsorption, secretion, and urine concentration


Lecture number:
21
Pages:
6
Type:
Lecture Note
School:
Washington State University
Course:
Biology 251 - Introductory Human Physiology
Edition:
1
Unformatted text preview:

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 II Important notes Glomerular filtration a Process of filtration b Problem of GFR BP c Intrinsic regulation d Extrinsic regulation Outline of Current Lecture I Tubular reabsorption a Process b Reabsorption of Ba in proximal tubule c Reabsorption of Na in distal tubule d Reabsorption of substances via Na dependent secondary active transport These 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 processes f II Reabsorption of phosphate Tubular secretion a H b K c Waste ions III Urine concentration a Countercurrent multiplication b Controlling urine concentration Current Lecture I Tubular Reabsorption A Introduction 1 Highly selective process a Need to get essential materials out of tubules and back in blood b Most wastes other than urea are too big to be reabsorbed and so are excreted in the urine 2 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 Transport 1 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 ions b 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 are moved against their concentration gradients by secondary active transport 2 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 cell 4 So they get a free ride with the Na if no Na the pump shuts down and nothing is transported by secondary mechanisms 5 Glucose reabsorption Fig 18 15 a The maximum transport rate for glucose is 375 mg min b Under normal conditions 125 mg ml is reabsorbed c 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 reabsorbed F Reabsorption of substances via Na Dependent Passive Processes 1 Chloride ions Cl Fig 19 14a a passively follow the electrical gradient of Na 2 Water molecules in PROXIMAL TUBULES ONLY see below for water reabsorption 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 to the interstitial concentration so it flows down its concentration gradient into the interstitial fluid 2 although half of the urea is reabsorbed the other half is excreted which in humans works just fine 3 note urea buildup due to renal failure is only mildly toxic compared 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 plasma concentrations of phosphate 2 If you ingest excess phosphate above normal plasma concentrations the excess is not reabsorbed and so is excreted in the urine very tight regulation compared to glucose 3 Parathyroid hormone can alter the reabsorption rates of electrolytes to conserve them if need be II Tubular Secretion A General 1 Supplemental mechanism to filtration to get rid of substances 2 Essentially is the reverse of tubular reabsorption B 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 of nerve and muscle cell membranes reduced excitablity high K in ECF increases membrane excitablility especially in the heart which can lead to cardiac arrhythmias 2 K actively moved in opposite directions by reabsorption in the proximal tubule and secretion in the distial tubule 3 The active transport of Na during Na reabsorption results in the secretion of K because Na K ATPase pump moves Na and K in opposite directions 4 K secretion not K filtration or reabsorption is the process regulated by the 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 urine b If plasma K is too low 1 aldosterone production reduced so secretion of K is decreased less K in the urine 5 Problems with K secretion a Because low plasma Na also stimulates aldosterone production to increase Na reabosrption conservation of Na can inadvertently eliminate K via the


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