Lecture 6: Control of Extracellular Fluid Flashcards
what are the hormones and messengers that regulate water balance
- ADH (vasopressin)
- RAAS
- Aldosterone
- ANP
what is ADH (vasopressin) inhibited by
ethanol, ANP, cortisol
what are the functions of ADH
- stimulates kidneys to reabsorb water
- vasoconstriction (raise arterial BP)
- effects in brain affect an individual’s behavior
what is the release stimulus for ADH
hypotonicity
if tonicity of plasma (normal around 300)
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)
ADH levels would increase
CD water permeability increases
water diffuses into hyperosmolar interstitium
what changes would happen to ADH in a case of polyuria associated w/ excessive intake of fluids
- 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
changes in tonicity are monitored by _______ located in the _________
monitored by osmoreceptors located in the hypothalamus
what does an increased activity of osmoreceptors stimulate
ADH synthesis and release is stimulated
what neurons produce ADH
supraoptic nucleus neurons in the hypothalamus
what releases ADH
Posterior pituitary
osmoreceptors are sensitive to what
the amount of positively charged ions, solute concentration
what is the effect of an increased solute concentration of interstitial fluid on osmoreceptors?
they lose water and shrink, resulting in a change in their firing rate —-> ADH synthesizing neurons are stimulated to release ADH
an increase in the number of aquaporins causes _______ in the collecting duct
causes an increase in water reabsorption in the CD
how is the body able to maintain normal plasma osmolarity?
by regulating water intake and urinary losses
ADH levels increase with ?
plasma sodium concentration
plasma vasopressin levels change in response to ________
plasma sodium
urine osmolarity changes in response to ______
vasopressin levels
changes in blood volume is monitored by what receptors
baroreceptors —> atrial volume receptor reflex
when ECF decreases in volume, what increases in response
ADH
when ECF increases in volume, what is produced in response
ANP (atrial natriuretic peptide)
- released by muscle cells in the atria
what factors cause the release of ANP
- increasing ECF volume or Na levels
what inhibits ANP
low Na levels
what is the function of ANP
ANP stimulates the kidneys to increase sodium excretion by
- vasodilating afferent arterioles & relaxing mesangial cells
- increasing capillary permeability
- inhibiting renin secretion
what does renin do?
catalyzes angiotensin I synthesis in the liver and angiotensin II synthesis in the lung
what is the effect of angiotensin II in the kidneys
- 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
what is the effect of angiotensin II outside of the kidneys?
- stimulates the release of aldosterone from the adrenal gland
- thirst, appetite
- NaCl transport in GI tract
- CO increased and vasoconstriction (MAP increased)
what is the role of aldosterone?
stimulates Na reabsorption in the DCT and CD
- more Na reabsorption = more water retention
- this helps regulate BP by maintaining / increasing blood volume
what stimulates aldosterone release
Angiotensin II and or elevated K+ levels (hyperkalemia)
how does aldosterone increase sodium reabsorption?
by inducing ENaC and the basolateral Na/K ATPase
what does angiotensin II inhibit
ROMK (the K+ secretion channel)
AII inhibits k+ secretion
what happens if both AII and Aldosterone is stimulated
Na+ reabsorption is stimulated w/out secreting K+
what are the 2 types of DI (Diabetes Insipidus)
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
what is urine specific gravity (USG)
measures the concentration of particles in urine (g/mL)
an indirect measure of urine osmolality
what is the normal value of USG for cats and dogs
CAT: 1.035-1.060 g/mL
DOG: 1.015-1.045 g/mL
what is hyposthenuria
USG is reduced, urine more diluted than plasma
- occurs from polydipsia, DI, tubular insensitivity to vasopressin, pyelonephritis or use of diuretics
what is isothenuria
- USG is the same as protein-free plasma
- neither concentrated nor diluted
- *the ability of kidneys to concentrate urine is compromised
what is hypersthenuria
USG is increased
- results from water deprivation or ADH administration
