1 12 2 Osmoregulation The Kidney BIO 361T Fall 2014 1 Draw a mammalian nephron labeling each section a Indicate how it differs from the nephron of a reptile No loop of Henle b Label where filtration secretion and reabsorption occur including solute examples for each c Label where Na is reabsorbed and whether it is passive or active d Label where H2O is reabsorbed Explain this mechanism Follows solute concentration through aquaporins e 2 Label where the filtrate is being concentrated and diluted The diagram above now illustrates the single effect in the nephron How is this multiplied by the loop of Henle Give a detailed mechanism This is a good opportunity to practice drawing Single effect osmolarity gradient generated between ascending limb and medullary interstitium Multiplied As filtrate flows up the ascending limb of the loop it is progressively diluted by NaCl reabsorption This reabsorbed solute then draws water from the nearby descending limb so that the filtrate flowing down through the descending limb is progressively concentrated by this osmotic removal of water Via this recycling mechanism the osmotic concentration at the tip of the longest loops may reach levels of many thousands of mOsm kg H2O and it is this concentration that limits the concentration of the urine Greenwald 1989 2 3 Why does length of the loop of Henle when examined across a wide range of animals correlate to maximum urine concentration As animals get larger so do their kidneys As kidneys get larger their loops of Henle get longer If the countercurrent multiplier of the loop is longer it has a greater effect by the tip of the loop the urine is extremely concentrated in solutes that the body needs to get rid of This sets up an extensive gradient in the interstitial fluid of the kidney that will drive H2O reabsorption in the collecting duct concentrating the urine 4 Do you think larger kidneys will always produce more concentrated urine Explain your answer Based on the countercurrent mechanism alone one would predict that absolute loop length would correlate with maximal urine concentration However there are other mechanisms that concentrate the urine Further measurements of loop length and maximal urine concentration between small and large mammals shows that this is not absolutely true For example small desert rodents whose loops of Henle are quite short in absolute terms 7 14 mm produce the most osmotically concentrated of mammalian urines with some concentrations exceeding 8 000 mosm kg H20 Larger mammals for example cows sheep and horses with much longer loops 37 mm for the horse produce urines of much lower maximal concentrations e g 1 800 mosm kg H20 for horse Umax see Greenwald and Stetson 1988 for other examples These data which show that Umax decreases with increasing loop length are exactly the opposite of what we are generally led to expect from countercurrent theory Greenwald 1989 5 Suggest two other mechanisms that may differ between mammalian taxa to explain differences in urine concentration collecting ducts in larger mammals might be less sensitive to hormonal regulation larger space between ascending and descending tubes might lessen the impact of the countercurrent multiplier ascending limbs in larger mammals might be permeable to water more blood flow in larger mammals might wash out the osmotic gradient in the interstitial fluid of the kidney More basolateral membrane infolding which may correlate to Na K ATPase concentration Higher concentration of mitochondria which means more ATP for pumping Na out These last two differences are evident in electron micrograph photos of the thick ascending limbs of horses vs big brown bats Greenwald and Stetson 1988 6 Why did doctors used to taste a patient s urine to test for diabetes Include a molecular mechanism for what is going wrong in the kidney Diabetes is a failure to properly metabolize sugar either because insulin is not produced Type I or the body does not recognize it Type II Sugar is higher than normal in the blood Whereas 100 of glucose is normally reabsorbed by secondary active transport through a Na Glc symporter that is driven by the Na gradient set up by the basolateral Na K ATPase an accumulation of sugar in the blood will decrease the concentration gradient for Glc Instead it will accumulate in the urine because it can no longer be reabsorbed
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