Lecture 8: Concentration and Dilution (Bolser)

Question 1

the thicker the medulla the ________ you can make your urine

Answer 1

more concentrated

Question 2

explain the conservation of water by the kidneys

Answer 2

produce urine more concentrated than plasma by countercurrent mechanisms in the loop of Henle

Question 3

explain the countercurrent system

Answer 3

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

Question 4

explain the countercurrent multiplication in the loop of Henle

Answer 4

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.

Question 5

what is the permeability characteristics of the proximal tubule?

Answer 5

highly permeable to salts and water

Question 6

what is the permeability characteristics of the descending limb?

Answer 6

permeable to water but only relatively permeable to ions

Question 7

what is the permeability characteristics of the ascending limb?

Answer 7

impermeable to water, active transport for salts

Question 8

what is the permeability characteristics of the distal tubule and collecting duct?

Answer 8

permeable to salts

Question 9

what is the permeability characteristics of the distal tubule and collecting duct under conditions of high ADH?

Answer 9

permeable to water

Question 10

what is the permeability characteristics of the distal tubule and collecting duct under conditions of low ADH?

Answer 10

impermeable to water

Question 11

what is the permeability characteristics of a nephron under conditions of hypotonic urine?

Answer 11

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

Question 12

what does low ADH cause?

Answer 12

water impermeability and thus decreased water reabsorption

Question 13

what is the permeability characteristics of a nephron under conditions of hypertonic urine?

Answer 13

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.

Question 14

where does most of the water reabsorption occur in conditions of hypertonic urine?

Answer 14

in the cortex because the tubular lumen is LESS concentrated than the interstitum of the cortex.

Question 15

in situations of hypertonic urine, what happens in the medullary collecting duct?

Answer 15

water is taken out to concentrate filtrate to increase water reabsorption in the presence of ADH

Question 16

in situations of hypertonic urine, what happens in the cortex collecting duct?

Answer 16

tubular fluid is diluted initially by taking salt out, causing water to flow out. this increases water reabsorption due to the presence of ADH.

Question 17

what is the function of the vasa recta

Answer 17

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.

Question 18

there are twice as many vasa recta as descending vasa recta. what is the significance of this?

Answer 18

it gives the ascending vasa recta “capacitance” - the ability to handle extra water.

Question 19

what are the 3 sets of countercurrent exchangers?

Answer 19

1. descending and ascending vasa recta
2. descending vasa recta and ascending limb of loop of henle
3. ascending vasa recta and collecting ducts

Question 20

what is “washout”?

Answer 20

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

Question 21

what is the role of urea in the countercurrent system?

Answer 21

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.

Question 22

what is urea transport into the medullary collecting ducts controlled by?

Answer 22

ADH

Question 23

when is urea used as a concentration gradient?

Answer 23

when ADH is HIGH

Question 24

where in the nephron does ADH function to increase permeability of urea?

Answer 24

the inner medullary collecting duct

Question 25

where in the nephron does ADH function to increase the permeability of water?

Answer 25

the distal tubule and the collecting duct

Question 26

how does urea leave the collecting ducts in conditions of high ADH?

Answer 26

when ADH is high, channels called urea porins are triggered to increase permeability of urea in inner medullary collecting ducts

Question 27

which part of the urine regulates/controls the urea porins?

Answer 27

the inner medullary collecting duct by ADH