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University of Toronto Scarborough
Biological Sciences
Stephen Reid

Lecture 19 Study Notes Slide 1 • Osmolarity in descending limb is always equal to peritubular fluid osmorality o Both are higher by 200 mOsm to the ascending limb of the Loop of Henle • Tubular fluid in the loop of Henle leaves with lower mOsm it had when it came in o 100 mOsm as opposed to 300 mOsm • The medullary osmotic gradient helps us to produce concentrated urine (1200 mOsm) o As opposed to producing dilute urine of 300 mOsm • Remember also that the descending limb is only permeable to water, not ions o While the ascending limb is permeable to ions not water • Water diffusing out of the descending loop of henle follows ions diffusing out of the ascending limb osmotically Slide 2 • The Loop of Henle ESTABLISHES a countercurrent-multiplier: because the fluid is flowing in opposite directions in two compartments in close proximity o The vasa recta (capillaries around the loop of Henle) maintain the osmolarity gradient Prevent pooling of solutes and disruption of medullary osmotic gradient Slide 3 • Blood (not tubular fluid as in Loop of Henle) enters vasa recta with 300 mOsm (leaves with 325 mOsm) o Descending and ascending portions of vasa recta are both permeable to water and ions Descending portion: Loses water; Gains solutes Ascending portion: Gains water; Loses solutes Slide 4 • Blood leaving the vasa recta has a higher mOsm than blood that enters vasa recta o Entering: 300 mOsm o Leaving: 325 mOsm o Caused by: 1. Solute (ion) reabsorption 2. Urea reabsorption 3. Colloid osmotic pressure of plasma (protein caused) Slide 5 • Watch this section in lecture Slide 6, 7, and 8 • Fluid leaving ascending loop of Henle enters distal tubule of juxtaglomerular nephron at mOSm of 100 • Some water can be reabsorbed by collecting duct or distal tubule IF permeable to water o Permeability is regulated by hormones Antidiurectic Hormone (ADH)/ Vasopressin (Reduces urination, but concentrates it as it causes reabsorption of water into blood [vasa recta]) • Synthesized by Paraventricular Nucleus of Hypothalamus • Stored in posterior pituitary • ADH/Vasopressin stimulates water reabsorption from collecting duct/distal tubule (where urine is concentrating) into vasa recta o Released in response to osmolarity o Stimulates insertion of Aquaporin 2 on apical membrane facing tubular lumen o Aquaporin 3 is always on the basolateral membrane and not under hormonal control o If distal tubule and collecting duct are permeable to water (i.e. water going out) urine has Low volume High concentration (max of 1400 mOsm) o If distal tubule and collecting duct are not permeable to water (i.e. water going out) urine has High volume Low osmolarity Slide
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