(25.7) Regulation of Urine Concentration & Volume

Name main function(s) of the Kidneys

1. Make any adjustment needed to maintain body fluid osmotic concentration at around 300 mOsm

2. Produce only small amounts of urine if the body is dehydrated or diluted urine if over-hydrated

Accomplished by using countercurrent mechanism

Define Osmolality

Number of solutes particles in 1 kg of H₂O

• 1 osmol = 1 mole of particle per kg H2O

• Body fluids have much smaller amounts → so expressed in milliosmols (mOsm) = 0.001 osmol

Define Countercurrent mechanism

Fluid flows in opposite directions in two adjacent segments of same tube with hairpin turn

Distinguish between Countercurrent multipler & Countercurrent exchanger

These countercurrent mechanisms work together to: Establish and maintain medullary osmotic gradient from renal cortex through medulla

• Gradient runs from 300 mOsm in cortex to 1200 mOsm at bottom of medulla

• Countercurrent multiplier forms gradient

• Countercurrent exchanger preserves gradient

• Collecting ducts can then use gradient to vary urine concentration

Describe the Location and Function of Countercurrent Multipler

Nephron loops of the juxtamedullary nephrons create gradient

T/F: Limbs of nephron loop are not in direct contact but close enough to influence each other’s exchanges with surrounding interstitial fluid

→ TRUE

Describe how Changes in the Permeability of the Nephron Loop to Water and Salt create Osmotic Gradients

Countercurrent Multipler

1. Descending limb of nephron loop is freely permeable to H₂O, impermeable for solutes

   • H₂O passes out of filtrate into hyperosmotic medullary interstitial fluid

   • Causes remaining filtrate osmolality to increase to ~1200 mOsm

2. Ascending limb of nephron loop is impermeable to H₂O and selectively permeable to for solutes

   • Na⁺ and Cl^- are actively reabsorbed in thick segments

     • Some passively reabsorbed in thin segment

3. Constant difference of 200 mOsm always exists between two limbs of nephron loop and between ascending limb and interstitial fluid

4. Difference is “multiplied” along length of loop (from 3000 to 1200 mOsm = difference of 900 mOsm)

Which of the following is NOT a property used to establish the medullary osmotic gradient?

A.) the filtrate flow through the ascending and descending limbs of the long nephron loops of juxtamedullary nephrons

B.) the blood flow through the ascending and descending portions of the vasa recta

C.) the descending limb's permeability to water and impermeability to salt

D.) the ascending limb's impermeability to water and permeability to salt

→ B.) the blood flow through the ascending and descending portions of the vasa recta → The vasa recta act as countercurrent exchangers to

preserve the osmotic gradient.

• A.) the filtrate flow through the ascending and descending limbs of the long nephron loops of juxtamedullary nephrons

• C.) the descending limb's permeability to water and impermeability to salt

• D.) the ascending limb's impermeability to water and permeability to salt

Explain the Countercurrent Multipler role in Countercurrent mechanism

• The MORE NaCl are ascending limb actively transport out into interstitial fluid, the more water diffuses out of the descending limb

• The MORE water that diffuses out of the descending limb, the SALTIER the filtrate becomes

• Ascending limb uses salty filtrate to further raise osmolarity of medullary interstitial fluid

Describe the Location and Function of Countercurrent Exchanger

Countercurrent exchanger utilizes vasa recta → preserve the gradeint

Role of Vasa Recta

Countercurrent exchanger utilizes vasa recta → highly permeable to water and solutes

• Flow of blood in vasa recta is also countercurrent (hairpin turn) so blood can exchange NaCl and water with surrounding interstitial fluid as it moves adjacent parallel sections of gradient

• Blood inside vasa recta remains isosmotic with surrounding interstitial fluid

• Able to reabsorb water and solutes without undoing osmotic gradient created by countercurrent multipler

Main function(s) of Countercurrent Exchanger

Preserves medullary gradient by:

1. Preventing rapid removal of slat from interstitial space

2. Removing reabsorbed water

T/F: Volume of blood at end of vasa recta is lesser than at beginning

→ FALSE

• Water is ascending vasa recta comes from descending vasa recta or is reabsorbed from nephron loop and collecting duct

• RESULT → volume of blood at end of vasa recta is GREATER than at beginning

SUMMARY of Creation of the Medullary Osmotic Gradient

Describe how the Osmotic Gradients are used to create Dilute and Concentrated Urine

• Established medullary osmotic gradeint can now be used to form dilute or concentrated urine

• WITHOUT GRADIENT would not be able to raise urine concentration >300 mOsm to conserve water

Explain Effect of Overhydration vs Dehydration in relation to Urine

Overhydration

• Produces LARGE volume of dilute urine

  • ADH production decreases; urine ~100 mOsm

  • IF aldosterone present, additional ions can be removed, causing water to dilute to ~50 mOsm

Dehydration

• Produces SMALL volume of concentrated urine

  • Maximal ADH is released; urine ~1200 mOsm

  • Severe dehydration: 99% water reabsorbed

Role of Urea

Urea helps form medullary gradient

1. Enters filtrate in ascending thin limb of nephron loop by facilitated diffusion

2. Cortical collecting duct reabsorbs water, leaving urea behind

3. In deep medullary region, now highly concentrated urea leaves collecting duct and enters interstitial fluid of medulla

   • Urea then moves back into ascending thin limb

   • Contributes to HIGH osmolality in medulla

Define Diuretics

Chemicals that enhance urinary output

List three diuretics we learned function to increase urine production

1. Caffeine (drugs for hypertension or edema)

   • Na+ reabsorption inhibitors (and resultant H₂O reabsorption)

2. Alcohol

   • ADH inhibitors

3. Osmotic diuretics

   • Substance not reabsorbed, so water remains in urine

   • EX: in diabetic patient, high glucose concentration pulls water from body