L28: Loop of Henle and Countercurrent Mechanism

What is the countercurrent mechanism in the kidney?

filtrate and blood flow in opposite directions (loop of Henle and vasa recta) to create and maintain the medullary osmotic gradient

Which segment of the loop of Henle is water permeable?

descending limb is permeable to water while the ascending limb is impermeable to water but actively reabsorbs NaCl.

What is the primary driving force for the countercurrent multiplier?

Reabsorption of NaCl in the thick ascending limb via secondary active transporters in the basolateral membrane to create medullary osmotic gradient

what are the components of the countercurrent mechanism?

1. countercurrent multiplier system

2. recycling of urea

3. countercurrent exchanger system

where is the countercurrent multiplier system located?

in the loop of henle of juxtaglomerular nephrons

where does recycling of urea occur?

in the collective ducts in the medulla

where is the countercurrent exchanger system?

in the vasa recta

what is the function of the countercurrent mechanism?

to reabsorb the remaining water and solutes from filtrate

countercurrent

a mechanism that involves exchanging materials or heat between fluids flowing in the opposite direction

describe the plasma osmolarity at the end of the loop of henle.

filtrate fluid has lower osmolarity than plasma and becomes more dilute “diluting limb of loop of henle”

describe the 4 steps of the countercurrent multiplier system in urine processing.

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

2. water drawn out of the filtrate in descending limb into interstitial fluid by osmosis

3. NaCl concentration of filtrate increases as it approaches bottom of loop

4. high NaCl concentration filtrate reaches thick ascending limb to allow NaCl reabsorption to occur

urea

represents the nitrogenous waste of the metabolism of proteins

urea reabsorption

from the collective ducts and papilla duct due to a high permeability and down concentration gradient towards interstitial (medulla) compartment

recycling urea

is the process by which urea is transported back into the descending limb of the loop of henle, enhancing the osmotic gradient in the medulla to aid in water reabsorption

save sodium 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

what clinical conditions lack ADH?

diabetes insipidus

what might be the consequence on plasma and urine osmolality in cases of excessive ADH?

high osmolality = increased ADH = increased water reabsorption from urine back into blood = dilute plasma

where is ADH released from?

the posterior pituitary

what stimulates the release of ADH?

Increased plasma osmolality and low blood volume

facultative water reabsorption

water reabsorbed towards a higher concentration gradient

what does ADH promote?

reabsorption of facultative water towards the high osmolarity renal medulla

formation of dilute urine

1. filtrate that exits blood and enters nephron = iso-osmotic

2. filtrate becomes more concentrated as it travels down the Loop of Henle and more dilute in ascending limb and distal tubule

3. in absence of ADH solute reabsorbed while water remains in filtrate = dilute

How does ADH influence urine concentration?

ADH promotes insertion of aquaporin-2 channels in the apical membrane of collecting duct cells, increasing water reabsorption toward the medullary gradient and concentrating urine.

What is the role of the vasa recta in the countercurrent mechanism?

A countercurrent exchanger that preserves the medullary osmotic gradient by preventing washout of solutes.

What is urea recycling and its function?

Urea is recycled from the collecting ducts into the medullary interstitium and back into the loop of Henle, increasing medullary osmolarity and aiding water reabsorption and urine concentration.

Where does aldosterone act and what does it do?

Acts on the distal nephron and collecting ducts to increase Na+ reabsorption and K+ secretion, supporting fluid balance and contributing to urine concentration.

What is the effect of PTH on the loop of Henle?

PTH increases permeability to calcium in the thick ascending limb.

How does filtrate osmolarity change from proximal tubule to end of loop of Henle?

Filtrate is ~300 mosm/L at the proximal tubule; water reabsorption in the descending limb increases osmolarity toward the medulla; end of the loop filtrate is hypoosmolar relative to plasma (roughly 220–300 mosm/L).

what does urea recycling contribute to?

increasing the osmotic pressure in the medulla

what is the pathway for urea?

1. distal collecting duct

2. diffuses into interstitial compartment of medulla

3. descending limb of the loop of Henle

what does it mean when we say urea-recycling “saves” NaCl?

part of the NaCl that should be used to increase the inner medulla’s osmolarity is substituted by urea to keep high osmolarity

What is the function of the medullary osmotic gradient?

To enable reabsorption of remaining water and solutes from the filtrate, producing concentrated urine when ADH is present.

What happens to the collecting ducts in the absence of ADH?

They remain relatively water-impermeable, leading to dilute urine.

What happens when ADH secretion is excessive?

Urine becomes highly concentrated (high osmolality) and plasma may become diluted (low osmolality) due to excess water reabsorption.

What is the difference between urine concentration and urine dilution?

Concentrated urine has high osmolality due to water reabsorption; diluted urine has low osmolality due to reduced water reabsorption.

What is the role of the thick ascending limb in solute reabsorption?

Reabsorbs Na+, K+, and 2Cl− via the NKCC transporter and is water-impermeable, contributing to the medullary gradient.