Mechanisms to adjust urine concentration Flashcards
Renal control of salt and water balance
-Normal function of these mechanisms allows?
- Water retention during dehydration
- Excretion of dilute urine when well hydrated
- Sodium excretion when blood pressure rises (Na retention when BP falls)
Failure of renal control of salt and water balance can cause?
- Edema
- Hyper/hypokalemia
- Undesirable changes in BP
- Acid/base disorders
- Neurological problems-shrinking/swelling of brain
Sodium reabsorption mechanisms
-Proximal tubule?
- Cotransport with glucose, aas, and phosphate
- Countertransport with H
Sodium reabsorption mechanisms
-Thick ascending limb?
Na, K, 2Cl cotransport
Sodium reabsorption mechanisms
-Early DCT?
-Na, Cl cotransport
Sodium reabsorption mechanisms
-Late DCT, CD?
Luminal Na membrane channels
Water and chloride follow sodium
-Water reabsorption?
- Always passive, can be transcellular or paracellular
- Follows osmotic gradients established by reabsorption of sodium
Water and chloride follow sodium
-chloride reabsorption?
- Always linked, either directly or indirectly, to Na reabsorption (Cl can balance the pos and neg charges)
- Different mechanisms in different segments
Loop of Henle-Descending limb
- Freely permeable to?
- Impermeable to?
- Freely permeable to water
- Impermeable to Na, Cl
Loop of Henle-Ascending limb
- Always impermeable to?
- Thin segment?
- Thick segment?
- Referred to as the?
- Always impermeable to water
- Thin segment-NaCl reabsorption mechanism is controversial
- Thick segment-active Na, K, 2Cl cotransport
- Referred to as the “diluting segment”
Positive potential in tubular urine
-Function?
Drives the reabsorption of solutes-if it becomes less positive, the solute reabsorption will decrease
Late DCT and CD
-Major site of?
-Major site of physiological control of salt and water balance
Late DCT and CD
-Aldosterone-function?
Stimulates Na reabsorption and secretion of K and H
Late DCT and CD
-ANP-function?
Inhibits Na reabsorption (medullary CD)
Late DCT and CD
-ADH-function?
Stimulates water reabsorption
Water permeability of CD is physiologically controlled
-Well-hydrated individuals?
- CD is impermeable to water
- Water remains in tubular lumen, dilute urine excreted
Water permeability of CD is physiologically controlled
-Dehydrated individuals?
- CD is highly permeable to water
- Water is reabsorbed, low volume of concentrated urine is excreted
ADH MOA?
ADH increases water permeability (reabsorption) of late DT/CD via V2 receptors and insertion of aquaporin channels
Solute concentrations in peritubular interstitium
-Inner medullary interstitial fluid?
Very high solute concentration
Countercurrent multiplier mechanism
-Function?
- Concentrates solute in medullary interstitium
- High solute concentrations enables kidneys to excrete highly concentrated urine, conserve water during periods of dehydration
Countercurrent Multiplier Mechanism
-Requires the integrated function of 3 components?
- Descending, ascending limbs of loop of Henle
- Vasa recta capillaries
- Collecting ducts
Components of Countercurrent Multiplier
- Na gradient that the Na, K, Cl cotransporter can establish in TAL
- Interstitium becomes hyperosmolar and pulls water out of the descending limb
- Urine in descending limb is concentrated
- Process repeats
Components of Countercurrent Multiplier
-Augmented by?
Augmented by action of ADH in CD
Get half of the gradient from Na-K-Cl cotransporter
-The other half comes from?
Get the other half of the gradient from from reabsorption of urea
- Recycling concentrates urea in the inner medulla
- Reabsorption of urea is promoted by ADH from medullary CD
Role of urea in the coutercurrent mechanism
-In the presence of ADH (cortical collecting tubule)?
- In the presence of ADH, water but NOT urea is reabsorbed in the cortical collecting tubule
- Results in an increase in the tubular fluid urea concentration
Role of urea in the coutercurrent mechanism
-In the presence of ADH (medullary collecting tubule)?
- In the presence of ADH, more water but NOT urea is reabsorbed in the medullary collecting tubule, further raising the concentration
- The inner medullary collecting tubule is relatively permeable to urea
- As a result, urea passively diffuses into the interstitium, increasing the interstitial osmolality
Countercurrent multiplier
- Vasa recta function?
- What feature allows them to do this?
- Maintain solute gradient
- Low blood flow allows them to equilibrate with surrounding interstitium-allows waste to be taken out but keep what the body needs
- Water and NaCl are exchanged between descending and ascending limbs
- Solute gradient is maintained while small amounts of NaCl and water are returned to systemic circulation
Countercurrent mechanism review figure
slide 26
Antidiuresis-high ADH
- Effect on the collecting duct?
- Characteristics of urine excreted?
- ADH makes the collecting duct epithelium highly water permeable
- Water is reabsorbed in this segment and a LOW VOLUME, HIGHLY CONCENTRATED URINE is excreted
Diuresis-Low ADH
- Effect on the collecting duct?
- Characteristics of urine excreted?
- Collecting duct epithelium is impermeable to water
- HIGH VOLUME OF DILUTE URINE is excreted
Diuresis-Low ADH
-Solute concentrations in the medullary interstitium?
LOW
Osmolar clearance
Cosm = (Uosm x V)/Posm
Osmolar clearance
-What happens to Cosm, when kidneys excrete excess solute?
Cosm increases (decreases when solute is retained)
- Free water clearance*
- Definition?
Excretion of water in excess of amount needed to excrete isosmotic urine, i.e. excretion of solute-free urine by the kidneys
- Free water clearance*
- Equation?
Cwater = V - Cosm
Free water clearance
-What is Cwater if Uosm is less than Posm?
Cwater is positive-pure water is cleared from the body (opposite if Uosm is greater than Posm)
Does ADH affect Cwater? If so, how?
Yes, it would decrease free water clearance
-water reabsorption is increased
Fractional excretion
-Definition?
The fraction (percentage) of the filtered load of a substance that is excreted in urine
Fractional excretion
-Equation?
(Ux x Pcr)/(Px x Ucr)
Normal fractional excretion?
1%
Fractional excretion
- If fractional excretion is below 1% what does it suggest about what the problem might be?
- What does it say about sodium?
- It suggests the problem is prerenal and acute glomerulonephritis
- Na avidly reabsorbed
Fractional excretion
-If fractional excretion is above 2% what does it suggest about what the problem might be?
-Acute tubular necrosis, renal
Case presentation
at the end
Renal control of salt and water balance
-Crucial for regulation of?
- Blood pressure
- Extracellular fluid concentration
- Concentrations of Na and K in body fluids
Why is the ascending limb referred to as the diluting segment?
Because it is impermeable to water so the water stays in the tubule while Na and Cl are pumped out
Which part of the countercurrent multiplier mechanism is inhibited by loop diuretics?
The Na-K-Cl cotransporter
What if a drug was given that increases blood flow to peritubular capillaries?
Concentrating ability of the kidneys is decreased (medullary interstitial osmolarity gradient is abolished)
