Renal VII: Renal Concentration & Dilution of Urine Flashcards
What happens during antidiuresis?
Antidiuresis happens during conditions of dehydration and results in production of low volume, high concentration urine. With antidiuresis, there is a high concentration of ADH in the plasma, causing increased reabsoprtion of water and urea.
Is ADH present or absent in diuresis?
absent!
Is ADH present or absent in antidiuresis?
present (in high concentrations)!
Which part of the kidney has the highest osmolality?
inner medulla (~1200 mOsm/L)
What are the two major contributors to the corticomedullary osmotic gradient?
- NaCl (50% of gradient)
- Urea (other 50% of gradient)
What are the 3 mechanisms that regulate medullary hyperosmolality, and what role do each of the 3 mechanisms play in hyperosmolality?
- countercurrent multiplier: establishes hyperosmotic gradient
- urea cycle: strengthens gradient
- countercurrent exchanger: maintains gradient
Where in the nephron is urea permeable?
It is permeable only in the lower collecting duct, and only if ADH is present.
What happens in the descending vs. ascending vasa recta?
- descending: water moves out of capillary down osmotic gradient and salt moves in capillary down concentration gradient
- ascending: water moves into capillary down osmotic gradient and salt moves out of capillary down concentration gradient
What is responsible for urea moving into the ascending vasa recta capillaries?
water pulls urea into the capillaries (water leaves tubule through aquaporins and pulls urea with it, so both are reabsorbed)
What are the purposes of a high medullary urea concentration?
- protects vasa recta RBCs from crenation in a hyperosmotic environment
- sets up a gradient for urea to be excreted in low volume urine
- does NOT set up an osmotic gradient for the reabsorption of H2O
What happens to urea recycling vs. urea clearance when urinary flow rate exceeds 10 mL/min?
- no urea recycling can occur due to a high flow rate
- urea clearance plateaus, meaning that it can estimate GFR
How do urine and plasma osmolarities compare in negative free-water clearance?
urine osmolarity > plasma osmolarity (dark amber urine)
How do urine and plasma osmolarities compare in positive free-water clearance?
urine osmolarity < plasma osmolarity (pale yellow urine)
What is the range of possible urine concentrations?
50-1200 mOsm
What is the osmolality of the renal cortex?
isotonic with plasma (300 mOsm/L)
What is the formula for osmolar clearance?
Cosm (mL/min) = (Usom x urinary flow) / Posm
Describes the amount of pure (solute-free) water the kidney adds to the urine per unit of time, diluting it below the osmolality of blood.
positive free water clearance
Describes the amount of pure (solute-free) water the kidney subtracts from the urine per unit of time, concentrating the urine above the osmolality of blood.
negative free water clearance
Is the flow rate in the vasa recta high or low? How does this affect the deep medullary gradient?
The flow rate is low, allowing the deep medullary gradient to be maintained. The vasa recta would NOT be able to establish these gradients with high flow rates.
Why is urea picked up in the vasa recta deep in the inner medulla?
The inner medulla has a high hyperosmolarity, which could make the RBCs crenate do to being in a hypertonic environment. When urea enters, it brings water with it, thus creating a condition in which the RBCs are sitting in a hypotonic environment. This prevents them from crenating.
What are some interesting things to note about urea that reveals why it moves the way it does in the nephron?
-it is an uncharged solute
-it can easily diffuse across most membranes via simple diffusion
-there are no membrane pumps for it
(all similarities shared with water)
How much of our urea is excreted in the urine? What happens with the remainder of it?
50% excreted in urine and the rest is recycled
What is the point at which urea clearance estimates GFR?
When urine flow rate exceeds 10 mL/min, as no urea recycling can happen due to too fast of a flow rate
Describe a situation in which urea moves back into the collecting duct.
This occurs during water diuresis when there is no ADH present and therefore no aquaporins. Since water is not leaving, there as not as high of a concentration of urea building up in the lower CD. Thus, urea moves from the medullary interstitium and back into the CD to balance out its gradient.
