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Department
Biological Sciences
Course Code
BIOC34H3
Professor
Stephen Reid

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Lecture 19 Study Notes
Slide 1
Osmolarity in descending limb is always equal to peritubular fluid osmorality
oBoth 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
o100 mOsm as opposed to 300 mOsm
The medullary osmotic gradient helps us to produce concentrated urine (1200
mOsm)
oAs opposed to producing dilute urine of 300 mOsm
Remember also that the descending limb is only permeable to water, not ions
oWhile 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
www.notesolution.com
Slide 2
The Loop of Henle ESTABLISHES a countercurrent-multiplier: because the
fluid is flowing in opposite directions in two compartments in close proximity
oThe vasa recta (capillaries around the loop of Henle) maintain the
osmolarity gradient
Prevent pooling of solutes and disruption of medullary osmotic
gradient
Slide 3
www.notesolution.com
Blood (not tubular fluid as in Loop of Henle) enters vasa recta with 300 mOsm
(leaves with 325 mOsm)
oDescending 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
oEntering: 300 mOsm
oLeaving: 325 mOsm
oCaused by:
1.Solute (ion) reabsorption
2.Urea reabsorption
3.Colloid osmotic pressure of plasma (protein caused)
www.notesolution.com

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Description
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 www.notesolution.com 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 www.notesolution.com • 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) www.notesolution.com 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 www.notesolution.com Slide
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