6 - Urine Concentration and Dilution Flashcards
What is the normal response of a human to ingestion of 1 liter of water? What was the urine flow rate prior to consumption?
Prior to consumption: UR = 0.1 ml/min of hypertonic urine (600mOscm/L)
After ingestion: initially, small decrease in plasma osmolarity.
Minutes later: marked increase in urine flow rate, and urine osmolarity drops significantly and becomes hypotonic (<200 mOsm/L).
What does not change during the process of reestablishing plasma osmolarity after ingestion of 1L water?
Urinary solute excretion rate was barely changed.
This is because the kidney has the ability to separate solute from water through altering the solute concentration (osmolarity) of urine.
How much of the filtered load of water is reabsorbed in the PT? What type of reabsorption is this?
~2/3.
This is isosmotic reabsorption
What is the osmolarity in the interstitial space in the renal cortex?
300 mOsm/L.
What occurs under conditions when concentrated urine is being formed in terms of osmolarity?
A gradient is formed from the border of the cortex and medulla (300 mOsm/L) with a progressive increase in osmolarity in the deeper (further from cortex) portions of the medulla to about ~1200 mOsm/L
What happens when fluid descends the thin descending limb?
The fluid is isotonic to the plasma enters a water permeable segment where the interstitial space has a greater and greater osmolarity.
Water therefore leaves, and the tubular fluid reaches an osmotic equilibrium with the interstitial space at each level.
What happens when fluid makes the turn into the thin ascending loop of henle?
Sodium passively diffuses out and the tubular fluid in the lumen begins to equilibrate with that in the interstitium.
What happens when the fluid reaches the thick ascending loop of henla?
Impermeable to water but has an active Na/K/2Cl transporter.
This segment generates an osmotic gradient of about 200 mOsm/L between the lumen and interstitial space.
Where is the thick ascending limb located? What does the luminal osmolarity be reduced to in this segment?
Starts in the outer medulla but transitions into the cortex.
Since it can generate a 200mOsm/L gradient in the cortex where the interstitial, then the luminal osmolarity can be reduced to approximately 100 mOsm/L.
What happens to the tubular fluid in the early distal tubule?
It’s impermeable to water, and demonstrates active Na reabsorption by the thiazide-sensitive Na/Cl so-transporter.
Further dilution of tubular fluid can occur here.
What happens to tubular fluid in the late distal tubular and collecting duct?
In the presence of ADH, which increases water permeability, water follows the osmotic gradient and reaches osmotic equilibrium at each level of the medulla.
By the end, the fluid (urine) is hypertonic with an osmolarity of 1200 mOsm/L.
What does the process of urine concentration achieve?
It turns a large volume of isotonic fluid into a small volume of concentrated fluid.
Enables body to excrete solute while retaining water.
What is countercurrent multiplication? What does it depend on?
A method used by the kidney to establish the medullary concentration gradient. Depends on two closely associated segments with differing permeability.
Removal of water in the descending limb and NaCl in the ascending limb to achieve equilibrium causes an accumulation of Na in the medulla.
What else does the medulla have a high concentration of?
Urea.
What is the interstitial osmotic gradient composed of?
Approximately equal amounts of urea and NaCl.
What happens to urea in the PT?
It is passively reabsorbed.
In the distal portions of the PT, urea concentration is equal to that of plasma.
How does the concentration of urea change in the thin descending and ascending limb?
In descending limb, water reabsorption causes increased urea concentration.
Carrier-mediated diffusion aids in the secretion of urea in the ascending limb.
What happens to urea in the thick ascending limb to the medullary collecting duct? What about the late distal tubule and collecting duct?
Thick ascending and medullary CD impermeable to urea.
In the late distal tubule and collecting ducts urea concentration is elevated because water is reabsorbed.
What occurs with urea in the distal portion of the inner medullary collecting duct?
When ADH is present, urea permeability is high.
Urea is reabsorbed in this segment, leading to movement of urea into the medullary interstitial space.
(this is impermeable to urea when ADH is absent)
How do the vasa recta capillaries play a role in formation and concentration or dilution of urine?
They provide nutritive blood flow to renal tubular structures and serve and help return reabsorbed water and solute to the plasma.
