Lecture 35: Concentrating Mechs

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concentrated

Formation of --- urine
Ex: water deprivation -> increased blood osmolarity -> ADH stimulated
-Goal is to prevent water loss and make concentrated urine

ADH

--- Actions:
-Stimulate Na+ K- Cl- symporters in thick limb of ascending loop of Henle -- this builds osmotic gradient in interstitial fluid
-Stimulate water reabsorption in upper collecting ducts
-Stimulate water reabsorption and urea recycling in lower collecting ducts - this builds osmotic gradient in interstitial fluid

ADH

--- Action Requirements:
-Ability of ADH to cause excretion of concentrated urine requires an osmotic gradient of solutes in the interstitial fluid of the renal medulla
-Three major solutes contribute to this high osmolarity and gradient: Na+ Cl- Urea

Factors

"Factors" affecting gradient:
-Two main factors contribute to building and maintaining the required osmotic gradient.
-Permeability differences in different sections of loop of Henle and collecting duct
-Countercurrent flow in opposite directions through tube shaped structures arranged closely and in parallel in the medulla

Countercurrent multiplication

"Countercurrent multiplication": Definition and process
-Progressively increased osmotic gradient forms in the interstitial fluid of renal medulla due to countercurrent flow.
-Involves the long loops of Henle of juxtamedullary nephrons
-Long loop of Henle is said to function as a countercurrent multiplier

Countercurrent multiplication

"Countercurrent multiplication":
1. Symporters in thick ascending limb cells of the loop of Henle cause a buildup of Na+ and Cl- in the renal medulla
2. Countercurrent flow through the descending and ascending limbs of the loop of Henle establishes an osmotic gradient in the renal medulla
3. Cells in the collecting ducts reabsorb more water and urea
4. Urea recycling causes a buildup of urea in the renal medulla

Countercurrent multiplication

Countercurrent multiplication:
1. Symporters in thick ascending limb cells of the loop of Henle cause a buildup of Na+ and Cl- in the renal medulla
Note: Water not absorbed because walls are impermeable to water. Na+ and Cl- ions build up in interstitial fluid

Countercurrent multiplication

Countercurrent multiplication:
2. Countercurrent flow through the descending and ascending limbs of the loop of Henle establishes an osmotic gradient in the renal medulla
Note: Ascending limb very permeable to water, but impermeable to solutes except urea

Countercurrent multiplication

Countercurrent multiplication
-Osmolarity of interstitial fluid outside the descending limb is higher than the tubular fluid inside
-Therefore water moves out of descending limb into interstitial fluid via osmosis
-Ascending limb cells are impermeable to water. However, cells have symporters which reabsorb Na+ and Cl- from tubular fluid into interstitial fluid of the medulla

Countercurrent multiplication

Countercurrent multiplication
3. Cells in the collecting ducts reabsorb more water and urea
- ADH increases water permeability of the principal cells. Water quickly moves via osmosis out of the collecting duct tubular fluid -> interstitial fluid -> into vasa recta
-With loss of tubule water, urea becomes increasingly concentrated. However, because duct cells deep in the medulla are urea permeable, urea diffuses into the interstitial fluid of medulla

Countercurrent multiplication

Countercurrent multiplication
4. Urea recycling causes a buildup of urea in the renal medulla
-As urea accumulates in interstitial fluid, some of it diffuses into tubular fluid in the descending and thin ascending limbs (which are urea permeable)
-However, while tubule fluid flows through thick ascending limb, DCT, and cortical portion of collecting ducts, urea remains in tubule fluid because cells are urea impermeable

Countercurrent multiplication

Countercurrent multiplication:
-As tubule fluid passes through collecting ducts (where ADH is present) water reabsorption continues via osmosis
-This reabsorption further increases urea concentration in tubular fluid, more urea diffuses into interstitial fluid of inner medulla, and the cycle repeats

Countercurrent multiplication

Countercurrent multiplication:
-Constant transfer of urea between segments of renal tubule and interstitial fluid is termed urea recycling
-Water reabsorption from tubular fluid -> promotes urea buildup in interstitial fluid -> promotes water reabsorption
-Solutes left behind in lumen become very concentrated and small volume of urine is excreted

Countercurrent Exchange

Countercurrent Exchange:
-Process by which solutes and water are exchanged between blood of vasa recta and interstitial fluid of renal medulla
-Vasa recta has descending and ascending loops parallel to each other and to the loops of Henle

Countercurrent Exchange

Countercurrent Exchange:
-Countercurrent flow through limbs allows for exchange of solutes and water between vasa recta blood and interstitial fluid in medulla
-Thus, vasa recta functions as a countercurrent exchanger

Countercurrent Exchange

Countercurrent Exchange:
-Vasa recta provides oxygen and nutrients to renal medulla without washing out the osmotic gradient
-Long loop of Henle establishes osmotic gradient in renal medulla but vasa recta maintains the gradient by countercurrent exchange

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