RENAL 07: CONCENTRATION AND DILUTION OF URINE Flashcards
Antidiuresis would be a condition of (high or low) water
Low water (ex. dehydration)
In a state of antidiuresis, what do we do to urea and water in the kidneys?
we reabsorb water and urea
What is the vasculature that helps the juxtamedullary nephrons accomplish the goal of concentrating urine
Vasa recta
Osmolarity of tubular fluid in the proximal tubule in antidiuresis (ADH present)
Isomotic
Osmolarity of tubular fluid in thin descending limb of loop of henle in antidiuresis (ADH present)
hyperosmotic
Osmolarity of tubular fluid in thick ascending limb of loop of henle in antidiuresis (ADH present)
hyposmotic
Osmolarity of tubular fluid in the distal convoluted tubule in antidiuresis (ADH present)
isosmotic
Osmolarity of tubular fluid in the collecting duct in antidiuresis (ADH present)
extremely hyperosmotic, up to 1,200mOsm
osmolarity of tubular fluid in proximal tubule in cases of diuresis (no ADH)
isosmotic
osmolarity of fluid in thin descending limb of loop of henle in diuresis (no ADH)
hyperosmotic
osmolarity of fluid in thick ascending limb of loop of henle in someone who is in diuresis (no ADH)
Hyposmotic
Osmolarity of fluid in distal convoluted tubule in someone who is in diuresis (no ADH)
hyposmotic
osmolarity of fluid in collecting duct in someone who is in diuresis (no adh)
Hyposmotic (as low as 50mOsm)
Avg bladder volume
600mL
What allows for concentration of urine (in broad terms, as far as kidney osmolrity goes)
As you et into deeper slices of the medulla it becomes higher osmolarity in interstitial fluid
What are the two (technically 3) major contributors to the gradient of osmolarity along the slice of kidney
NaCl (~50%)
urea (~50%)
3 mechanisms which regulate medullary hyperosmolarity
- Countercurrent multiplier
- Urea cycle
- Countercurrent exchanger
Purpose of countercurrent multiplier
Establishes hyperosmotic gradient
Purpose of urea cycle
Strengthens osmotic gradient
Purpose of countercurrent exchanger
maintains osmotic gradient
Thin descending loop of henle contribution to countercurrent multiplier
High water permeability, low salt permeability (passive, not active movement) therefore salt is not moving but water can move. Water can move out, salt stays behind, urine becomes more concentrated
Thin ascending loop of henle contribution to countercurrent multiplier
Low water permeability, high salt permeability - salt moves out, water stays behind. Little-to-no active salt movement (tends to be passive/paracellular)
Thick ascending loop of henle contribution to countercurrent multiplier
Active site of salt pumping (Na, Cl, and K). This is also water impermeable, so the fluid becomes hyposmotic. Thus it is named the diluting segment.
Distal tubule contribution to countercurrent multiplier
Increase in H2O permeability and salt transport (reabsorption)
Upper collecting duct contribution to countercrrent mulitplication
Active salt reabsorption and passive water reabsorption, under ADH control
Lower collecting duct contribution to countercurrent multiplication
Active salt reabsorption, passive water AND urea reabsorption, under ADH control
3 steps in countercurrent multiplication
- Shift tubular fluid
- pump salts
- Equilibrate osmolarity
(4: Repeat)
Urea is a product of muscle___
catabolism
What happens if urea is not excreted from the body?
Urea toxicity - can occur to renal failure patients awaiting dialysis or transplant
What is urea, physically speaking?
Small, uncharged, easily diffusable membrane (in most places of body, but in kidneys we need to keep in mind this won’t be the case everywhere).
Urea is permeable in what part of the collecting duct
Lower collecting duct
Under what important condition will urea move out of the collecting duct?
If it can follow water (therefore we need ADH present)
Does urea move through aquaporins
NO
Why does urea reuire water to be around to move
It’s moved passively, so you want water to establish a higher gradient for urea to move into (if water is diluting an area out, its easier for urea to go into it)
How is the urea gradient established?
Urine flows down the collecting duct, and water is exiting in the upper region. By lower region, he urea is highly concentrated and it will diffuse out into the medullary interstitium. Now, some will be picked up by vasa recta capillaries, but the remainder stays in medulla. Urea is concentrated iwthin the medulla nephron in order to pack the low volume urine with highly concentrated urea for excretion.
Can urea re-enter tubular fluid after leaving?
Yes, but only at the bottom of the loop of henle
What are the differences between urea recycling and countercurrent multiplication?
No pumping in urea cycling, all passive diffusion
There is a requirement of ADH (you must move water first to set up a gadient for urea to move
There are differential permeabilities of urea in different areas of the nephron
Aside from interstitium and the bottom of the loop of henle, can urea get in anywhere else?
The vasa recta
Why do red blood cells not crenate (shrivel up and die) in hyperosmotic (up to 1200 mOsm) conditions in the kidney?
Because urea will go into the RBC - remember this will not set up a gradient for reabsorbing water, because urea follows water, but not the other way around
Two purposes of urea recycling
- protects vasa recta RBCs against crenation in a hyperosmotic environment
- sets up a gradient for urea to be excreted in low - volume urine
How do the vasa recta establish the countercurrent exchanger (4)
capillaries are permeable to both salt and water; flow rate is slow so you don’t wash out everything and lose the gradient
- Vasa recta exits medulla with slightly more solutes than water
- Vasa recta flow is slow, so deep medullary gradient is maintained (you don’t wash it away)
- Water shunt - you keep excess water out of the deep medulla (imagine it just keeps passing along above the medulla without going down into it)
- Solute trapping - excess solutes are kept in lower medulla
How many days does it take for the osmotic gradient to be re-established if something happens to it
3-4 days
If osmolarity of urine/plasma osmolarity is greater than one, what kind of urine do we have?
Concentrated urine (ADH antidiuresis)
If osmolarity of urine/pasma <1, what kind of urine do we ahve?
Dilute urine (water diuresis)
