Lecture 6: Control of Extracellular Fluid

Question 1

what are the hormones and messengers that regulate water balance

Answer 1

• ADH (vasopressin)
• RAAS
• Aldosterone
• ANP

Question 2

what is ADH (vasopressin) inhibited by

Answer 2

ethanol, ANP, cortisol

Question 3

what are the functions of ADH

Answer 3

• stimulates kidneys to reabsorb water
• vasoconstriction (raise arterial BP)
• effects in brain affect an individual’s behavior

Question 4

what is the release stimulus for ADH

Answer 4

hypotonicity
if tonicity of plasma (normal around 300)

Question 5

what changes would happen to ADH levels if there is extracellular volume contraction or isoosmotic volume depletion in the collecting ducts (caused by V+, D+, hemorrhage, heart failure)

Answer 5

ADH levels would increase
CD water permeability increases
water diffuses into hyperosmolar interstitium

Question 6

what changes would happen to ADH in a case of polyuria associated w/ excessive intake of fluids

Answer 6

• decreased plasma osmolality
• less ADH secreted, so fewer aquaporins (AQP2) inserted into the CD
• arterial blood volume increases
• a more dilute hypotonic urine is produced in order to maintain volume homeostasis

Question 7

changes in tonicity are monitored by _______ located in the _________

Answer 7

monitored by osmoreceptors located in the hypothalamus

Question 8

what does an increased activity of osmoreceptors stimulate

Answer 8

ADH synthesis and release is stimulated

Question 9

what neurons produce ADH

Answer 9

supraoptic nucleus neurons in the hypothalamus

Question 10

what releases ADH

Answer 10

Posterior pituitary

Question 11

osmoreceptors are sensitive to what

Answer 11

the amount of positively charged ions, solute concentration

Question 12

what is the effect of an increased solute concentration of interstitial fluid on osmoreceptors?

Answer 12

they lose water and shrink, resulting in a change in their firing rate —-> ADH synthesizing neurons are stimulated to release ADH

Question 13

an increase in the number of aquaporins causes _______ in the collecting duct

Answer 13

causes an increase in water reabsorption in the CD

Question 14

how is the body able to maintain normal plasma osmolarity?

Answer 14

by regulating water intake and urinary losses

Question 15

ADH levels increase with ?

Answer 15

plasma sodium concentration

Question 16

plasma vasopressin levels change in response to ________

Answer 16

plasma sodium

Question 17

urine osmolarity changes in response to ______

Answer 17

vasopressin levels

Question 18

changes in blood volume is monitored by what receptors

Answer 18

baroreceptors —> atrial volume receptor reflex

Question 19

when ECF decreases in volume, what increases in response

Answer 19

ADH

Question 20

when ECF increases in volume, what is produced in response

Answer 20

ANP (atrial natriuretic peptide)
- released by muscle cells in the atria

Question 21

what factors cause the release of ANP

Answer 21

• increasing ECF volume or Na levels

Question 22

what inhibits ANP

Answer 22

low Na levels

Question 23

what is the function of ANP

Answer 23

ANP stimulates the kidneys to increase sodium excretion by
- vasodilating afferent arterioles & relaxing mesangial cells
- increasing capillary permeability
- inhibiting renin secretion

Question 24

what does renin do?

Answer 24

catalyzes angiotensin I synthesis in the liver and angiotensin II synthesis in the lung

Question 25

what is the effect of angiotensin II in the kidneys

Answer 25

• vasoconstriction of efferent arteriole
• increased sodium reabsorption in the PT, TAL, DCT, CD
• stimulates NHE, NCC, ENaC to promote more sodium reabsorption = more water retention

Question 26

what is the effect of angiotensin II outside of the kidneys?

Answer 26

• stimulates the release of aldosterone from the adrenal gland
• thirst, appetite
• NaCl transport in GI tract
• CO increased and vasoconstriction (MAP increased)

Question 27

what is the role of aldosterone?

Answer 27

stimulates Na reabsorption in the DCT and CD
- more Na reabsorption = more water retention
- this helps regulate BP by maintaining / increasing blood volume

Question 28

what stimulates aldosterone release

Answer 28

Angiotensin II and or elevated K+ levels (hyperkalemia)

Question 29

how does aldosterone increase sodium reabsorption?

Answer 29

by inducing ENaC and the basolateral Na/K ATPase

Question 30

what does angiotensin II inhibit

Answer 30

ROMK (the K+ secretion channel)

AII inhibits k+ secretion

Question 31

what happens if both AII and Aldosterone is stimulated

Answer 31

Na+ reabsorption is stimulated w/out secreting K+

Question 32

what are the 2 types of DI (Diabetes Insipidus)

Answer 32

Central DI - caused by insufficient ADH release (due to damage to hypothalamus or posterior pituitary)

Nephrogenic DI - occurs when tubules don’t respond to vasopressin

Question 33

what is urine specific gravity (USG)

Answer 33

measures the concentration of particles in urine (g/mL)

an indirect measure of urine osmolality

Question 34

what is the normal value of USG for cats and dogs

Answer 34

CAT: 1.035-1.060 g/mL
DOG: 1.015-1.045 g/mL

Question 35

what is hyposthenuria

Answer 35

USG is reduced, urine more diluted than plasma
- occurs from polydipsia, DI, tubular insensitivity to vasopressin, pyelonephritis or use of diuretics

Question 36

what is isothenuria

Answer 36

• USG is the same as protein-free plasma
• neither concentrated nor diluted
• *the ability of kidneys to concentrate urine is compromised

Question 37

what is hypersthenuria

Answer 37

USG is increased
- results from water deprivation or ADH administration