L28: Loop of Henle & Countercurrent Mechanism

Countercurrent mechanism

mechanism that involves exchanging materials/heat between fluids flowing in opposite directions

Descending limb

portion of the loop of henle that is permeable to water but impermeable to solutes, increasing osmolality

Ascending limb

portion of the loop of henle that is impermeable to water but actively transports Na, K, Cl ions into the medullary interstitium, reducing osmolality

Countercurrent multiplier

loop of henle establishes a concentration gradient in the renal medulla

Countercurrent exchanger

process in vasa recta that preserves the medullary concentration gradient

- blood flows countercurrent to tubular fluid

Vasa Recta

network of capillaries that parallel the loop of henle, acting as the countercurrent exchanger maintaining concentration gradient by passive exchange of solutes & water

Medullary osmotic gradient

progressively increasing concentration of solutes in renal medulla from the cortex to the inner medulla, drives water reabsorption from the collecting ducts under the influence of ADH

Osmolality

measure of solute concentration expressed as osmoles per kilogram of solvent, determines the direction of water movement across membranes

Urea recycling

urea is reabsorbed from the collecting & papillary ducts due to a high permeability down it concentration gradient to the medulla, increasing the osmolarity of the medulla

Hypotonic fluid

tubular fluid with a lower osmolality than the surrounding interstitial fluid, typically found in the ascending limb of the loop of henle

Hypertonic fluid

tubular fluid with a higher osmolality than the surrounding interstitial fluid, typically found in the descending limb of the loop of henle

Collecting duct

final segment of the nephron that adjusts urine concentration, passes through the medullary osmotic gradient allowing water reabsorption in the presence of ADH

Isosmotic

having the same osmolality as another solution, the fluid in the loop of henle is typically isosmotic to plasma

Aquaporins

water channels in the cell membranes of nephron segments, especially in the collecting ducts, that facilitate water reabsorption under the influence of ADH

Renal interstitium

tissue surrounding the nephron tubules & blood vessels in the kidney

Diluting segment

thick ascending limb of the loop of henle, where active transport reduces the osmolality of tubular fluid without water movement

What is a primary function of the loop of Henle in the nephron?

establishing a medullary concentration gradient through countercurrent multiplication.

What segment of the loop of Henle is water permeable?

descending limb

The thin descending limb is

permeable to water but impermeable to solutes (NaCl), increasing the osmolarity as it moves toward the medulla

The thick ascending limb is

impermeable to water but actively reabsorbs NaCl, using Na/K/2Cl symporters

- electrochemical gradient

- PTH increases permeability to Ca

By the end of the loop of Henle, the filtrate fluid

has a lower osmolarity than plasma

Countercurrent mechanism components

- countercurrent multiplier system

- recycling of urea

- countercurrent exchanger system

Countercurrent multiplier system

1. NaCl actively transported from filtrate in thick ascending limb into interstitial fluid

2. NaCl in interstitial fluid draws water out by osmosis

3. NaCl in filtrate increases at bottom of loop

4. high NaCl concentration allows NaCl reabsorption to continue

What is the function of urea recycling?

save Na to concentrate the kidney medulla

What is the nephron's primary driving force for the countercurrent multiplier mechanism?

reabsorption of NaCl in the thick ascending limb

ADH stimulus

high plasma osmolarity

ADH causes the expression & translocation of

aquaporins in the apical membrane of the collecting duct cells, promoting FACULTATIVE WATER reabsorption

What is the function of ADH?

reabsorption of water and concentration of urine

What might be the consequence on plasma and urine osmolality in the case of excessive secretion of ADH?

increased osmolarity or urine (concentrated)

decreased osmolarity of plasma (dilute)

A high osmotic pressure in the interstitial medulla is crucial in order to

concentrate the tubular fluid in the collecting duct, leaving as urine