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LSS Urinary System

Alexandra Burke-Smith

These properties alone are enough to explain high osmolarity in the medulla. The figure below represents a loop of Henle with the renal cortex at the top and the bend of the loop deep in the medulla. To start with assume that there is no gradient and that the fluid everywhere in the loop and the interstitium is roughly isotonic: osmolarity = 290 mosmol/l Descending 290 290 290 290

Interstitium 290 290 290 290

Ascending 290 290 290 290

Active transport of NaCl out of the ascending limb raises interstitial osmolarity. Assume that the difference between osmolarity between the ascending limb and the interstitium at ANY HORIZONTAL LEVEL of the loop is achieved as a result of active NaCl transport is 200 mosmol/l Descending 290 290 290 290

Interstitium 390 390 390 390

Ascending 190 190 190 190

Water moves out of the descending limb down its osmotic gradient, thus raising the osmolarity of the tubular fluid in the descending limb and lowering that of the interstitium. Descending 340 340 340 340

Interstitium 340 340 340 340

Ascending 190 190 190 190

Fluid flows down the descending limb and up the ascending limb; new isotonic fluid enters the loop. Descending Interstitium Ascending 290 340 190 340 340 190 340 340 190 340 340 340 So if we go once round the cycle of: 1. Active NaCl transport out of the ascending limb 2. Osmotic equilibrium between the descending limb and the interstitium, and 3. Countercurrent flow we get the above diagram. Obviously in the real world these processes all take place simultaneously, but it makes life easier to think of them as happening one after the other. If we go around again we get this, Descending Interstitium Ascending 290 328 165 328 352 165 33

Alex's Urinary  

Alex's Urinary

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