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5.1 - Loop of Henle and vasa recta


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What is the osmolarity of the filtrate received by the descending loop of Henle from the PCT?
It is isosmolar (300mOsm/L)
What is the magnitude of the horizontal osmotic gradient? What establishes this gradient?
*the difference in the osmolarity of filtrate in the top of the descending and ascending limbs of the loop of Henle
Why is the filtrate leaving the ascending limb always hyposmolar?
Because the ascending limb is impermeable to water and thus always removes more solute than fluid.
What are some factors affecting the magnitude of the longitudinal gradient in the loop of Henle?
*presence of urea in the interstitium (determined by ADH)
*length of the loop
*rate of flow - affected by osmotic diuresis, water ingestion (hydration state)
How does the presence of ADH increase the magnitude of the longitudinal gradient? Does it work directly on the loop of Henle?
*reabsorption in the LOH is independent of ADH
*ADH stimulates the reabsorption of urea in the MCD - this urea then equilibrates across the wall of the descending tubule, making the filtrate at the tip of the LOH even more hyperosmotic
Describe the primary method of water and salt movement in the descending limb. In which direction does each move?
*both diffuse between leaky epithelia, although some water is moved transcellularly via AQP-1
*water is reabsorbed and salt is secreted
Is urea reabsorbed or secreted in the descending limb? What transporter is used to move this molecule?
What is the primary transporter molecule in the ascending limb? What solutes does it move and in what direction? What medication inhibits it?
*Na+ and Cl- are reabsorbed along their gradient
*K+ is reabsorbed against its gradient
How is Na+ moved from the epithelium to the interstitium in the ascending limb?
It is actively transported against its gradient via the Na+/K+ ATPase.
How does K+ leave the epithelium of the ascending tubule?
It moves passively down its gradient through ROMK channels on the apical membrane and through other leak channels on the basolateral membrane.
What is the intracellular concentration of K+ in the epithelia of the ascending tubule - low or high? How is this maintained?
*intracellular [K+] is quite high
*maintained through the action of the Na+/K+/2Cl- transporter on the apical membrane and the Na+/K+ ATPase on the basolateral membrane.
How might furosemide cause hypokalemia?
Lasix inhibits the Na+/K+/2Cl- transporter on the apical surface of the ascending limb epithelia. This limits the reabsorption of K+ in this segment, thus more is excreted with the filtrate.
What is the "single effect"?
The gradient that is set up by the separation of solute and fluid in the ascending limb.
What is the point of the countercurrent multiplication system?
It allows the amplification of the horizontal gradient into the much larger longitudinal gradient.
How can the length of the loop of Henle help determine the size of the longitudinal gradient?
They are directly proportional. The longer the loop is the greater the area over which the single effect can be multiplied.
Describe the urine one might expect to find produced by an animal with very long loops of Henle.
Because the length of the loop is directly proportional to the size of the longitudinal gradient, such a creature would be able to produce very concentrated urine.
How is urea affected by diet?
*increased in high protein diets
*decreased in malnutrition
Why is rate of fluid flow inversely proportional to the size of the longitudinal gradient?
As fluid moves faster through the descending limb, there is less opportunity for equilibration. Thus the fluid at the tip of the LOH is not as hyperosmotic and the size of the gradient is decreased.
Why might osmotic diuresis impair one's ability to concentrate urine?
The increased flow causes a reduction in the size of the longitudinal gradient.
How is water moved in and out of the vasa recta? How is urea moved?
*water via AQP-1
*urea via UT-3
Describe the movement of solutes and water across the descending arm of the vasa recta.
Solutes are taken up and water is exported. This is in response to the increasing osmolarity of the interstitium towards the tip of the LOH.
As blood moves up the vasa recta does it lose or gain water? What forces power the movement of water across this endothelium?
Water moves across the endothelium into the blood. The high osmotic and colloid pressure favors the import of water, and the osmolarity of the vasa goes from 1200 to 350 (tip to top).
Compare the osmolarity of blood entering the vasa recta with blood leaving these capillaries.
Blood at the two points is nearly isosmolar: 300 vs 350 mOsm/L. This is despite the steep gradient from the top to bottom of the structure.

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