Flashcards in Lecture 8: Concentration and Dilution (Bolser) Deck (27):
the thicker the medulla the ________ you can make your urine
explain the conservation of water by the kidneys
produce urine more concentrated than plasma by countercurrent mechanisms in the loop of Henle
explain the countercurrent system
it requires two parallel and adjacent tubes separated by a membrane with a finite permeability. the flow in each tube is in the opposite direciton
explain the countercurrent multiplication in the loop of Henle
this system creates a concentration gradient between the lumen of the nephron and various places within the kidney to help drive reabsorption of water.
produces an osmotic gradient in the descending limb of loop of Henle and medullary interstitum.
NaCl is actively transported out of ascending limb (causing the filtrate in the ascending limb to become more dilute) where it then equilibriates with the descending limb fluids.
what is the permeability characteristics of the proximal tubule?
highly permeable to salts and water
what is the permeability characteristics of the descending limb?
permeable to water but only relatively permeable to ions
what is the permeability characteristics of the ascending limb?
impermeable to water, active transport for salts
what is the permeability characteristics of the distal tubule and collecting duct?
permeable to salts
what is the permeability characteristics of the distal tubule and collecting duct under conditions of high ADH?
permeable to water
what is the permeability characteristics of the distal tubule and collecting duct under conditions of low ADH?
impermeable to water
what is the permeability characteristics of a nephron under conditions of hypotonic urine?
fluid in the distal tubule and collecting duct remains hyposmotic due to the absence of ADH.
salts are reabsorbed, but water remains in the tubule creating a dilute urine = low osmalality but high volume
what does low ADH cause?
water impermeability and thus decreased water reabsorption
what is the permeability characteristics of a nephron under conditions of hypertonic urine?
fluid in the distal tubule and collecting duct equilibriates with the osmotic gradient of hte medullary interstitum and becomes concentrated in the presence of ADH. water passes out the tubule into the interstitum to create a concentrated urine.
where does most of the water reabsorption occur in conditions of hypertonic urine?
in the cortex because the tubular lumen is LESS concentrated than the interstitum of the cortex.
in situations of hypertonic urine, what happens in the medullary collecting duct?
water is taken out to concentrate filtrate to increase water reabsorption in the presence of ADH
in situations of hypertonic urine, what happens in the cortex collecting duct?
tubular fluid is diluted initially by taking salt out, causing water to flow out. this increases water reabsorption due to the presence of ADH.
what is the function of the vasa recta
it works like a thermoregulation countercurrent system.
They are anatomically opposed so that salt can move from one to the other and create a circulation of NaCl
when salt is pumped out of ascending limb, its collected in the descending vasa recta. this carries to venous system toward cortex while giving off salt to descending vasa recta which carries salt back into the medulla which is what gives the medulla its high osmolality.
there are twice as many vasa recta as descending vasa recta. what is the significance of this?
it gives the ascending vasa recta "capacitance" - the ability to handle extra water.
what are the 3 sets of countercurrent exchangers?
1. descending and ascending vasa recta
2. descending vasa recta and ascending limb of loop of henle
3. ascending vasa recta and collecting ducts
what is "washout"?
washout occurs if the vasa recta were arranged in a linear fashion such that the cappilaries passed straight through the medulla rather than looping as they actually do.
if this situation were to happen, high amounts of solutes would be "washed out" by capillary blood flow
what is the role of urea in the countercurrent system?
urea can be recycled from the collecting ducts into the loop of henle to help establish an osmotic gradient. this is done to help conserve water under situations of little energy availability.
what is urea transport into the medullary collecting ducts controlled by?
when is urea used as a concentration gradient?
when ADH is HIGH
where in the nephron does ADH function to increase permeability of urea?
the inner medullary collecting duct
where in the nephron does ADH function to increase the permeability of water?
the distal tubule and the collecting duct
how does urea leave the collecting ducts in conditions of high ADH?
when ADH is high, channels called urea porins are triggered to increase permeability of urea in inner medullary collecting ducts