Water Homeostasis

Question 1

when renal free water balance is zero, serum sodium concentration is ?

Answer 1

NORMAL (136-145 mmol/L)

Question 2

when renal free water balance is positive, serum sodium concentration is ?

Answer 2

LOW = HYPONATREMIA (<136 mmol/L)

Question 3

when renal free water balance is negative, serum sodium concentration is ?

Answer 3

HIGH = HYPERNATREMIA

Question 4

osmolarity - defined

Answer 4

number of solute particles per 1 L of solvent

Question 5

osmolality - defined

Answer 5

number of solute particles in 1 kg of solvent

Question 6

water balance - defined

Answer 6

*regulation of the osmolality of body fluids
*purpose of regulating water balance is to maintain constant osmolality in your cells and body fluids
*purpose is to prevent movement of water between fluid compartments
*regulated by regulating renal water excretion

Question 7

renal water excretion is determined by

Answer 7

1. amount of solute in urine
2. osmolality of urine

Question 8

urine water excretion = ? (equation)

Answer 8

urine solute excretion / osmolality of urine

Question 9

regulation of the osmolality of body fluids - overview

Answer 9

osmoreceptors in the hypothalamus act as sensor → modulate antidiuretic hormone AND triggers thirst mechanisms

Question 10

major factors that regulate H2O handling

Answer 10

PATHWAY:
1. osmoreceptors
2. ADH
3. aquaporin water channels

DRIVING FORCE = hyperosmolar medulla

Question 11

pathway leading to increased water excretion

Answer 11

decreased serum osmolality detected by osmoreceptors → suppression of ADH → water excretion

Question 12

pathway leading to water retention

Answer 12

increased serum osmolality detected by osmoreceptors → activation of ADH → insertion of aquaporin channels + hyperosmolar medulla → water reabsorption

Question 13

ADH & aquaporin (AQP2) channels - overview

Answer 13

*ADH binds V2 receptors in the principal cells (in cortical collecting duct) → increased cAMP → insertion of AQP2 channels on apical membrane that are permeable only to water → increased water reabsorption

note - aquaretics (vaptans) block this V2 receptor (prevent ADH from binding V2 → excrete additional water)

Question 14

medullary concentration gradient

Answer 14

*concentration difference of solutes (NaCl, urea) within the kidney’s medulla
*creating a progressively increasing osmotic pressure from the outer → inner layer of the medulla
*allows for the concentration of urine by facilitating water reabsorption from the collecting duct

Question 15

mechanisms contributing to making the medullary interstitium hypertonic

Answer 15

1. reabsorption of NaCl without water
   -occurs in thick ascending loop of Henle via active transport via NKCC
   -no H2O absorption
   -diluting segment
2. urea entry into the inner medullary interstitium
   -via passive diffusion at the collecting duct
   -urea re-enters tubular fluid at the loop
3. vasa recta

Question 16

reabsorption of NaCl without water → hypertonic medullary interstitium

Answer 16

-occurs in thick ascending loop of Henle via active transport via NKCC
-no H2O absorption
-diluting segment

Question 17

urea entry into the inner medullary interstitium → hypertonic medullary interstitium

Answer 17

*when ADH is present, additional urea is reabsorbed into the medullary interstitium → increased medullary concentration gradient → promotes water reabsorption in the collecting duct
*via passive diffusion
*urea re-enters tubular fluid at the loop

Question 18

vasa recta contributes to → hypertonic medullary interstitium

Answer 18

*vasa recta helps to maintain the concentration gradient
*counter current mechanism utilizes energy to develop an osmotic concentration gradient along the medullary interstitium
*the layout of the vasa recta around its own Loops of Henle and the counter current flow that allows for the maintenance of the osmotic gradient

Question 19

diluting vs. concentrating urine

Answer 19

*if volume overloaded, kidneys dilute urine to excrete increased H2O in excess of solute to dump the excess volume
*if volume depleted, kidneys concentrate urine to excrete less H2O in relation to solute to conserve H2O
*this is achieved by not only the presence/absence of ADH but also through development/maintenance of the hypertonic medullary interstitium

Question 20

ADH secretion is regulated by

Answer 20

1. plasma osmolality
2. arterial volume

note - plasma osmolality is the primary trigger (arterial volume only contributes if there are large changes > 15%)

Question 21

plasma osmolality & regulation of ADH secretion

Answer 21

*increased plasma osmolality → sensed by osmoreceptors → release of ADH (from posterior pituitary) to maintain osmolality → increased free water reabsorption

*decreased plasma osmolality → suppressed ADH release → decreased free water reabsorption (increased water excretion)

Question 22

arterial volume & regulation of ADH secretion

Answer 22

*decreased arterial volume → decreased arterial stretch → increased ADH → increased water reabsorption → increased blood volume

*increased arterial volume → increased arterial stretch → decreased ADH → decreased water reabsorption (increased water secretion)

Question 23

what is the MAIN trigger for ADH secretion

Answer 23

increased plasma osmolality

Question 24

impaired free water excretion in diabetes insipidus

Answer 24

*a condition characterized by inability to concentrate urine & conserve water
*central: ADH not produced (hypothalamic or pituitary issue)
*nephrogenic: kidneys do not respond to ADH
*result: HYPERNATREMIA (increased serum sodium due to lack of water to dilute it)

Question 25

impaired water excretion in syndrome of inappropriate ADH secretion (SIADH)

Answer 25

*sustained increase in ADH secretion that is NOT due to changes in plasma osmolality or changes in arterial volume
*sustained increase in ADH → increased water reabsorption → HYPONATREMIA (due to increased water volume AND increased sodium excretion due to plasma volume expansion)

Question 26

effects of decreased Na+ and water intake on kidneys (SIMPLE)

Answer 26

*kidneys should REABSORB SODIUM AND WATER (i.e. decreased excretion of sodium and water)

Question 27

effects of increased Na+ and water intake on kidneys (SIMPLE)

Answer 27

*kidneys should EXCRETE MORE SODIUM AND WATER (i.e. decreased reabsorption of sodium and water)