Concentrate or dilute urine Flashcards
How does osmolality change through the kidneys?
It reduces
ECF is 300mOsm/L entering the kidney, reducing to 100 at DCT and further reduced to 50 by collecting duct as there is additional reabsorption of nacl and impermeability to water
The failure to reabsorb water and continued reabsorption of ions leads to a large volume of dilute urine
Making concentrated urine
Allows conservation of water when intake limited
Can make urine up to a max of 1200-1400mOsm/L which is 3-4x that of plasma
For this to happen there must be a high level of ADH that allows DCT and collecting ducts to become permeable to water
Also needs gradient to pull this water out so there needs to be a high osmolarity of renal medullary interstitial fluid
Factors that contribute to build up of osmolarity in medulla
Passive absorption of ions across epithelia of thin ascending limb of loop of Henle
Active transport of ions from collecting duct
Facilitate diffusion of urea from medullary portion of collection ducts into medullary interstitium
Diffusion of only small amounts of water from medullary tubules into medullary interstitium and far less than reabsorption of ions occurs there - sets up an osmotic imbalance and gradient
Facilitate diffusion of urea from medullary portion of collection ducts into medullary interstitium
Diffusion of only small amounts of water from medullary tubules into medullary interstitium and far less than reabsorption of ions that occurs there - sets up an osmotic imbalance and gradient
Counter current mechanisms
Step 1 - fluid enters the loop of Henle from PCT at 300mosm/l (same as plasma)
Step 2 - AT of ions from thick ascending limb establishes a 200 gradient between tubular fluid and interstitial fluid. 400 occurs in medullary interstitium compared to 200 in ascending tubule fluid
Step 3 - Tubuar fluid in descending limb now equibrilates with interstitial fluid as water moves out of descending limb into medullary interstitial fluid. Continued transport of ions but not water in ascending limb maintains the gradient
Step 4 - Flow of fluid into loop of Henle from PCT moves fluid in the limbs on. The hyperosmotic fluid in descending limb moves on into ascending limb
Step 5 - additional ions are pumped out of fluid from ascending limb until a 200 gradient is again established between the ascending limb tubule fluid and medullary interstitium. This time the interstitial osmolality rises to 500mosm/l and ascending tubule falls to 300
Step 6 - again a movement of water out of descending limb of loop of Henle to reach osmotic equilibrium with medullary interstitial fluid. This increases the osmolality of tubule fluid in descending limb up to 500mOsm/l which moves on into ascending limb for processes of sodium and other ions movement to continue
Steps are repated to give osmolality in deepest part of medulla of 1200-1400mosm/l
Where is water reabsorbed in presence of ADH?
In cortical section of collecting ducts
Rapidly transported out of kidney by cortical peritubular capillaries
Absorption in cortex rather than medulla helps preserve the osmotic gradient in medulla
Contribution of urea to concentrating urine
Contributes to osmolarity of medullary interstitial fluid when the kidney needs to form maximally concentrated urine in dehydration
Normally 40-50% of urea reabsorbed in PCT but conc of urea still increases as reabsorption is less than that of water
In descending section of loop of henle the urea conc continues to rise due to further water reabsorption into medullary interstitium and movement of urea from medullary interstitial fluid back into the tubule
Thick ascending limb, DCT and cortical collecting ducts all relatively impermeable to urea so no movement
However presence of ADH further increases urea conc in tubule
As tubular fluid moves into medullary collecting ducts even more water is absorbed and urea even more conc. This high conc of urea causes it to diffuse out of medullary collecting ducts into medullary interstit fluid by specific urea transporters, one of which activated by ADH
A moderate amount of this urea can move back into ttubule at loop of henle so can recirculate through tubule distal to nephron several times - this can contribute to conc of urea in distal tubular fluids in times of dehydration. In full hydration where water is to be lost then tubular flow is greater and recirculation of urea is less marked.
Blood flow through the vasa recta
Medullary blood flow is low and about 5% of total renal blood flow
This is enough to supply the metabolic needs of tissues but does not result in loss of solute from medullary interstitium
Vasa recta serves as a counter current mechanism to prevent the washout of solutes from the medullary interstitium
Blood becomes more hypertonic as it descends into medullary interstitium and then becomes less hypertonic as it ascends back towards the cortical regions
Solutes and water move to mirror concs in medulla
What factors increase ADH?
Increase plasma osmolality Dec blood volume Dec blood pressure Nausea Hypoxia Drugs - morphine, nicotine, cyclophosphamide
What factors decrease ADH?
Decreased plasma osmolality
Inc blood volume
Inc BP
Drugs - alcohol, clonidine, haloperidol
