Urine Concentration and Dilution

Question 1

How much fluid volume has been reabsorbed isoosmotically at the end of the proximal tubule?

Answer 1

2/3 of the fluid volume

Question 2

What is the range of urine osmolarity?

Answer 2

50 Osm/L to 1200 mOsm/L

Question 3

What is the obligatory urine output in a day in a 70kg person?

Answer 3

600 mOsm of solute perday

Question 4

factors that influence urine plasma osmolality

Answer 4

unique structural/functional arrangement of renal tubules and vasa recta capillaries

hormonal influences on the kidney (AVP, aldosterone, ANP)

Question 5

What happens when there are low concentrations of AVP in the plasma?

Answer 5

decreased absorption of water and urea

production of high volume, low concentration urine

Question 6

What happens when there is high concentrations of AVP in the plasma?

Answer 6

increased reabsorption of water and urea

production of low-volume, high-concentration urine

Question 7

Describe the osmolality of kidney slices.

Answer 7

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)

Question 8

What are the three major species that contribute to medullary hyperosmolality?

Answer 8

Na+ = 300 mOsm/L

Cl- = 300 mOsm/L

urea = 600 mOsm/L

Question 9

What are the mechanisms that regulate medullary hyperosmolality?

Answer 9

countercurrent multiplier establishes initial osmotic gradient

urea cycle strengthens osmotic gradient

countercurrent exchanger maintains osmotic gradient

Question 10

countercurrent multiplier

Answer 10

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

Question 11

How does the thin descending loop of Henle contribute to the countercurrent multiplier?

Answer 11

high water permeability, low salt permeability

water moves out of the tubule leaving salt behind

Question 12

How does the thin ascending loop of Henle contribute to the countercurrent multiplier?

Answer 12

low water permeability, high salt permeability

salt moves out of the tubule, leaving water behind

Question 13

How does the thick ascending loop of Henle contribute to the countercurrent multiplier?

Answer 13

site of most active salt pumping in the kidney (Na⁺,Cl^-)

water-permeable tubules become hyposmotic

Question 14

How does the distal tubule contribute to the countercurrent multiplier?

Answer 14

increased water permeability and salt transport (reabsorption)

Question 15

How does the upper collecting duct contribute to the countercurrent multiplier?

Answer 15

active salt reabsorption

passive water reabsorption under ADH influence

Question 16

How does the lower collecting duct contribute to the countercurrent multiplier?

Answer 16

active salt reabsorption

passive water and urea reabsorption under ADH

Question 17

purpose of the urea cycle in the kidneys

Answer 17

strengthens the medullary hyperosmotic gradient

Question 18

Describe the steps of the urea cycle.

Answer 18

1. urea is concentrated in the upper collecting duct - region where urea is impermeable
2. urea passively moves out of the lower collecting duct into the inner medulla - region where permeability is regulated by AVP
3. urea in the inner medulla is picked up by the ascending vasa recta
4. urea in the vasa recta diffuses out into the outer medulla
5. urea in the outer medulla diffuses into the descending thin loop of Henle
6. urea is recycled through the tubular system back to the lower collecting duct - this completes one urea cycle and starts another

Question 19

purpose of the countercurrent exchanger

Answer 19

maintains the hyperosmotic gradient

both H₂O, salt, and urea move passively across the vasa recta capillary walls in renal medulla

Question 20

What happens in the descending vasa recta in relation tot he countercurrent exchanger?

Answer 20

water movse out of the capillary down osmotic gradient

salt and urea move into the capillary down concentration gradient

Question 21

What happens in the ascending vasa recta in relation tot he countercurrent exchanger?

Answer 21

water moves into capillary down osmotic gradient

salt and urea out of capillary down concentration gradient

Question 22

What is the net effect of the countercurrent exchanger?

Answer 22

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

Question 23

How does the medullary gradient get washed out?

Answer 23

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

Question 24

osmotic diuresis

Answer 24

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)

Question 25

water diuresis

Answer 25

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)

Question 26

osmolar clearance equation

Answer 26

C_osm (mL/min) = U_osm x V/P_osm

if U_osm/P_osm > 1 -> concentrated urine (ADH antidiuresis)

if U_osm/P_osm < 1 -> dulute urine (water diuresis)

Question 27

free water clearance

Answer 27

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

C_H2O = V - C_osm

Question 28

What does a negative free-water clearance mean?

Answer 28

urine is hyperosmotic

Question 29

What does a positive negative free-water clearance mean?

Answer 29

urine is hyposmotic

Question 30

What does a zero free-water clearance mean?

Answer 30

urine is isosmotic