Urine Concentration and Dilution Flashcards
How much fluid volume has been reabsorbed isoosmotically at the end of the proximal tubule?
2/3 of the fluid volume
What is the range of urine osmolarity?
50 Osm/L to 1200 mOsm/L
What is the obligatory urine output in a day in a 70kg person?
600 mOsm of solute perday
factors that influence urine plasma osmolality
unique structural/functional arrangement of renal tubules and vasa recta capillaries
hormonal influences on the kidney (AVP, aldosterone, ANP)
What happens when there are low concentrations of AVP in the plasma?
decreased absorption of water and urea
production of high volume, low concentration urine
What happens when there is high concentrations of AVP in the plasma?
increased reabsorption of water and urea
production of low-volume, high-concentration urine
Describe the osmolality of kidney slices.
renal cortex = isotonic with plasma (300 mOsm/kg H2O)
outer medulla = mild hyperosmolality (300-480 mOsm/kg H2O)
inner medulla = strong hyperoxmolality (480-1200 mOsm/kg H2O)
What are the three major species that contribute to medullary hyperosmolality?
Na+ = 300 mOsm/L
Cl- = 300 mOsm/L
urea = 600 mOsm/L
What are the mechanisms that regulate medullary hyperosmolality?
countercurrent multiplier establishes initial osmotic gradient
urea cycle strengthens osmotic gradient
countercurrent exchanger maintains osmotic gradient
countercurrent multiplier
responsible for generating the hyperosmotic graadient
countercurrent flow in the two limbs of Henle’s loop results in differential fluid and solute movements, generating vertical osmotic gradients
How does the thin descending loop of Henle contribute to the countercurrent multiplier?
high water permeability, low salt permeability
water moves out of the tubule leaving salt behind
How does the thin ascending loop of Henle contribute to the countercurrent multiplier?
low water permeability, high salt permeability
salt moves out of the tubule, leaving water behind
How does the thick ascending loop of Henle contribute to the countercurrent multiplier?
site of most active salt pumping in the kidney (Na+,Cl-)
water-permeable tubules become hyposmotic
How does the distal tubule contribute to the countercurrent multiplier?
increased water permeability and salt transport (reabsorption)
How does the upper collecting duct contribute to the countercurrent multiplier?
active salt reabsorption
passive water reabsorption under ADH influence
How does the lower collecting duct contribute to the countercurrent multiplier?
active salt reabsorption
passive water and urea reabsorption under ADH
purpose of the urea cycle in the kidneys
strengthens the medullary hyperosmotic gradient
Describe the steps of the urea cycle.
- urea is concentrated in the upper collecting duct - region where urea is impermeable
- urea passively moves out of the lower collecting duct into the inner medulla - region where permeability is regulated by AVP
- urea in the inner medulla is picked up by the ascending vasa recta
- urea in the vasa recta diffuses out into the outer medulla
- urea in the outer medulla diffuses into the descending thin loop of Henle
- urea is recycled through the tubular system back to the lower collecting duct - this completes one urea cycle and starts another
purpose of the countercurrent exchanger
maintains the hyperosmotic gradient
both H2O, salt, and urea move passively across the vasa recta capillary walls in renal medulla
What happens in the descending vasa recta in relation tot he countercurrent exchanger?
water movse out of the capillary down osmotic gradient
salt and urea move into the capillary down concentration gradient
What happens in the ascending vasa recta in relation tot he countercurrent exchanger?
water moves into capillary down osmotic gradient
salt and urea out of capillary down concentration gradient
What is the net effect of the countercurrent exchanger?
vasa recta exits medulla with slightly more solutes than water
vasa recta flow is very slow, so medullary gradient is secured
water shunt - excess water is kept out of deep medulla
solute trapping - excess solutes are kept in the lower medulla
How does the medullary gradient get washed out?
medullary osmotic gradient depends upon Na, Cl, and urea movements
any factors that increase vasa recta blood flow wash out the medullary gradient and increase urine flow
vasa recta flow increases - medullary osmolality decreases and urine volume increases
when the medullary gradient has been greatly diluted, it takes several days to be reestablished
osmotic diuresis
due to a block in tubular reabsorption, excess of normally reabsorbed solute or presence of nonreabsorbed solute
can occur when there is excess tubular fluid solute and decreased fluid reabsorption in any portion of the nephron
non-hormonal - does not depend on presence or absence of any hormone
urine flow can approach GFR (75 mL/min)
urine osmolarity will aproach that of glomerular filtrate (isoosmotic)
water diuresis
due to a change (decrease) in water permeability of the collecting duct
hormonal - depends on low levels of AVP
maximum urine flow will be 15-25 mL/min because most fluid reabsorption int he rest of the nephron is normal
urine osmolarity will always be less than that of glomerular filtrate (hypoosmotic)
osmolar clearance equation
Cosm (mL/min) = Uosm x V/Posm
if Uosm/Posm > 1 -> concentrated urine (ADH antidiuresis)
if Uosm/Posm < 1 -> dulute urine (water diuresis)
free water clearance
expresses the amount of pure water that must be either added to or subtracted from urine to yield the observed urine osmolar concentration to be equal to plasma
provides a quantitative measure of the rate at which the kidney is excreting or reabsorbing water and, therefore, how it is affecting body fluid osmolarity
CH2O = V - Cosm
What does a negative free-water clearance mean?
urine is hyperosmotic
What does a positive negative free-water clearance mean?
urine is hyposmotic
What does a zero free-water clearance mean?
urine is isosmotic
