Regulation of Water Balance

Question 1

What concentration difference can the thick ascending loop of henle maintain?

Answer 1

200 mOsmol/kg H2O btw tubular fluid and interstitium

Question 2

About what osmolality is the tubular fluid leaving the ascending LoH?

Answer 2

100 mOsm/L

hypotonic and less concentrated than the fluid entering the LoH

Question 3

What is the fluid entering the descending LoH like?

Answer 3

isotonic = 300 mOsm/L

Question 4

What limb of the LoH is the interstitial fluid in equilibrium with?

Answer 4

the descending limb

their osmolalities are equal

Question 5

What does the vasa recta do?

Answer 5

supplies blood to the medulla

highly permeable to solute and water = remove stuff that is removed from tubular fluid and put into the IF

Question 6

How is the vasa recta’s ability to maintain medullary interstitial gradient regulated?

Answer 6

flow dependent
increased blood flow –> decreases salt and solute transport by nephron segments in the medulla –> reduced ability to concentrate urine

Question 7

How does urea recycling work?

Answer 7

50% urea reabsorbed in PCT –> very little reabsorbed until the medullary collecting duct –> UT-A1 and UT-A3 transporters reabsorb urea into medullary interstitium –> some of the urea eventually diffuses back into thin LoH and goes again back to collecting duct

Question 8

Without ADH, what is the collecting duct like?

Answer 8

impermeable to water (will maintain low osmolality urine)

Question 9

What is the thin descending limb membrane like?

Answer 9

combo of aquaporins and no tight junctions –> water can flow freely w/out solutes

Question 10

What are the 2 nuclei in the pituitary that synthesize ADH?

Answer 10

supraoptic nucleus

paraventricular nucleus

Question 11

What part of the pituitary gland produces ADH?

Answer 11

posterior pit

Question 12

How is the pituitary stimulated to release ADH?

Answer 12

increased osmolarity or other factors –> supraoptic and paraventricular nuclei stimulated –> Ca entry –> stored ADH is released

Question 13

If osmoreceptors sense changed is osmolality, what does it signal the body to do?

Answer 13

first activated ADH pathway

thirst pathway activated later

Question 14

What cell type does ADH work on?

How does it affect these cells?

Answer 14

principal cells in late DCT and collecting duct

directs the insertion of aquaporin 2 channels –> H2O permeability is increased

Question 15

In general, what do principal cells do?

Answer 15

reabsorb Na+, Cl-, and secrete K+

acted on by ADH

Question 16

In general, what do intercalated cells do?

Answer 16

reabsorb K+ and secrete H+

Target of aldosterone

Question 17

How does aldosterone work?

Answer 17

In response to high plasma K levels or Angiotensin II:
increases ENaC channels, which pull in Na from the tubular lumen
Na is then transported into the IF via the Na/K ATPase pump = increased Na reabsorption and K secretion

Question 18

What side of a tubule cell is an aquaporin-2 channel inserted in response to ADH?

Answer 18

apical membrane

Question 19

What is the major difference btw the cortical and medullary collecting ducts?

Answer 19

cortical are always permeable to water

medullary H2O permeability is determined by ADH

Question 20

What is type I/central Diabetes Insipidus?

Answer 20

failure to produce or release ADH from the posterior pituitary (after head injuries or congenital)
see large volumes of dilute urine
if water intake is restricted –> severe dehydration
treatment = desmopressin

Question 21

What is type 2/ nephrogenic diabetes insipidus?

Answer 21

inability of the kidneys to respond to ADH
normal or elevated levels of ADH
result of failure of countercurrent or failure of distal and collecting tubules to respond to ADH
still see large volumes of dilute urine
loop diuretics, lithium, and tetracyclines can cause this

Question 22

What can treat type 2/ nephrogenic DI?

Answer 22

low sodium diet and treatment with thiazide diuretics can help hypernatremia

Question 23

What occurs in SIADH?

Answer 23

too much ADH –> hypotonic volume expansion –> cells swell
major cause of low sodium levels
low urine output
still have excessive thirst

Question 24

What is the most common electrolyte disterbance?

Answer 24

hyponatremia

present in 15-30% of hospitalized patients

Question 25

What are the most common causes of hyponatremia?

Answer 25

drugs, pain, nausea, decreased effective arterial volume, strenuous exercise
(NOT usually bc of consuming little solute or polydipsia)

Question 26

What defines hypernatremia?

Answer 26

deficit of free water relative to solute
inadequate free water intake
most ppl also have concurrent volume depletion
(see it in DI and osmotic diuresis)

Question 27

What defines polyuria?

What 4 mechanisms can cause it?

Answer 27

> 2.5 L/day
caused by:
1. increased intake of fluids
2. increased GFR as in hyperthyroidism, fever, hypermetabolic states
3. increased output of solutes (DM, hyperthyroidism, hyperpara, diuretics)
4. inability of the kidney to reabsorb water in DCT

Question 28

What is oliguria?

Answer 28

output below the minimum volume
300-500 ml/day
dehydration, blood loss, etc

Question 29

What is anuria?

Answer 29

virtual absence of urine production

< 50 mL/day

Question 30

What is water diuresis?

Answer 30

increased water excretion without corresponding increase in salt excretion

Question 31

What is solute diuresis?

Answer 31

increased water excretion concurrent w/ increased salt excretion

Question 32

What is obligatory urine volume?

Answer 32

minimal amount of solutes that a bedridden person must excrete in a day
about 700 mOsm
1000 mOsm/day for an active person

Question 33

What is maximal urine concentrating ability?

Answer 33

1200 mOsm/L

Question 34

What does it mean if free-water clearance is positive?

Answer 34

excess water is being excreted by the kidneys

Question 35

What does it mean if free-water clearance is negative?

Answer 35

excess solutes are being removed from the blood by the kidneys and water is being conserved
when urine osmolarity > plasma osmolarity

Question 36

What is the equation for free-water clearance?

Answer 36

CH2O = V-Cosm = V - (Uosm * V)/(Posm)

Question 37

What is responsible for the countercurrent exchange that sets up the medullary osmotic gradient?

Answer 37

the vasa recta