Regulation of Plasma Osmolarity Flashcards
what is the normal range of plasma osmolarity? what are the major solute determinants?
280-300 mOsm/L
Na and Cl
what type of water transportation does not occur?
active
what are the major sources of water? what are the major methods of excretion?
fluids, food and metabolism (less)
urine, feces, skin, sweat and exhaled air
what parts of water loss are regulated? why does this occur?
only urine output
very small osmolarity differences can compromise CNS function
what does the ratio of urine to plasma osmolarity do?
normalizes the concentrating/diluting ability of the kidney to changes of plasma osmolarity
when U/P osmolarity equals 1, what does that mean? when it is greater than one? when it is less than one?
1- urine is isotonic to ECF
>1- urine is concentrated (retaining water in excess of solute)
<1- urine is dilute (excreting water in excess of solute)
what does “free water” refer to?
water in excess of solute that is either taken from or left in the tubular fluid (not obligated to remain with solutes in tubular fluid)
what hormone reflects the status of ECF and plasma osmolarity? what does it do?
ADH
increase in osmolarity increases ADH- retention of water in the kidney
what is the equation for osmolar excretion? how does it change with high and restricted water intake?
U osmolarity x volume
does not change although osmolarity and volume will
what is the average osmolar excretion per day? what is the typical osmolarity of urine?
600 mOsm/day
400 mOsm/L
how much can the kidney reduce Uosm to? what is the maximum urine volume?
30 mOsm/L
20L/day
how high can the kidney increase Uosm? what is the minimum urine volume?
1200 mOsm/L
.5 L/day
what is the equation for free water clearance?
C H20= V- Cosm (osmotic clearance)
Cosm= (Uosm V)/Posm
what is C osm if U/P osm equals 1? C H2O?
C osm=V
C H2O=0
kidney does not add or remove free water from plasma
what is C osm if U/P osm is greater than 1? C H2O?
C osm>V
C H2O<0
kidney retains free water to add to plasma
what is C osm if U/P osm is less than 1? C H2O?
C osm0
kidney eliminates free water taken from the plasma
what does ADH signal in respect to free water clearance?
tells kidney to decrease free water clearance to make hypertonic urine
what limits the magnitude of free water clearance?
the limits of the concentrating and diluting ability of the kidney as well as magnitude of solute consumed
what will occur to free water clearance with a diet low in solute?
increase negative FWC and decrease positive
more free water is available for reabsorption because there is less solute in the tubular fluid
what will occur to free water balance with a diet high in solute?
decrease negative FWC and increase positive
amount of water osmotically obligated to remain in the tubular fluid is increased so less free water is available
how can the volume of free water clearance be measured from the volume of urine?
subtract urine volume from what isosmotic urine would be (2 L)
if solute consumption is 600 mOsm/day what is the largest volume of urine that will be excreted? the smallest? (solute balance)
largest-20 L (600/30) most dilute urine
smallest-0.5 L (600/1200) most concentrated urine
if solute consumption is 1200 mOsm/day what is the largest volume of urine that will be excreted? the smallest? (solute balance)
largest-40 L (1200/30) most dilute urine
smallest-1 L (1200/1200) most concentrated urine
what causes the excretion of dilute urine?
solute resorption in the ascending limbs (water impermeable) dilutes the fluid and then it is excreted.
what causes the excretion of concentrated urine?
solute reabsorption in the thick ascending limb generates a hypertonic environment. tubular fluid in the collecting duct is equilibrated with this environment because it becomes water permeable
what is the long term correction for high plasma osmolarity?
satisfying thirst
what does urine osmolarity depend on?
whether or not absorption occurs in the collecting duct
what is antidiuresis? diuresis?
anti- restricted water intake
D- excess water intake
what are the osmolarities of the proximal tubule, descending limb of the loop of henle, ascending limb and collecting duct compared to plasma?
PT- isosmotic
DL- hyperosmotic
AL- hypoosmotic
CD- hyper or hypo depending up on ADH presence
describe antidiuretic hormone.
9 AA peptide made in paraventricular and supraoptic nuclei of hypothalamus. secreted by posterior pituitary
what triggers the release of ADH?
changes in plasma osmolarity are detected by osmoreceptors in hypothalamic neurons. send AP to posterior pituitary inducing ADH release
what is the half life of ADH? what is the significance of this?
18 min
minimizes an over correction because once the plasma returns to normal, the pituitary stops releasing it
how does ADH increase water permeability in the outer and inner medullary collecting tubules and ducts?
vesicles containing aquaporin fuse with apical membrane allowing H2O to flow transcellularly out of the tubule and into the interstitium
what do high levels of ADH do elsewhere in the body?
cause vasoconstriction to maintain blood pressure and flow when volume is severely reduced (hemorrhage)
what segments of the nephron are highly permeable to water? which are not?
permeable- PT tDLH
impermeable- tALH TAL DCT (distal convoluted tubule)
what receptors does ADH bind to and in what cells of the collecting tubule/duct?
V2 receptors in the principal cells from the initial collecting tubule until the end of the duct
how does receptor binding cause vesicle fusion in ADH signaling?
g linked receptor stimulates AC->cAMP-> PKA
PKA phosphorylates proteins and results in synthesis of new aquaporin and fusion of vesicles
in the collecting tubule, where are aquaporins constitutively located? where are they added with ADH?
always exist on principal cells’ basolateral membrane. added to apical membrane
describe an aquaporin.
4 monomers with a hollow center and many transmembrane regions. mediates diffusion of water only
with posterior pituitary removal, what response is changed? which is kept intact?
response to dilution of plasma is kept intact while response to increased osmolarity of plasma is gone
what is the threshold for posterior pituitary secretion of ADH with euvolemia? what does this mean for normal physiological conditions?
280 mOsm/L
there is usually a little bit of ADH in the blood
with euvolemia, what increase in plasma osmolarity will cause detectable increase in ADH levels?
only 1%
other than plasma osmolarity, what is the secretion of ADH reliant upon?
dependent upon volume status
hypovolemia results in more ADH secretion for same plasma osmolarity and hypervolemia results in less ADH secretion for the same osmolarity
where are CNS osmoreceptors? how do they sense plasma osmolarity?
the organum vasculosum of the lamina terminalis and the subforical organ
there is a gap in the BBB and changes in plasma cause changes of stretch in the neuronal cell membranes (sense cell swelling and shrinkage)
in the CNS osmoreceptors, what happens with increased osmolarity?
it opens mechanosensitive cation channels causing depolarization and increase frequency of action potentials to the hypothalamus (decrease closes them)
how do ADH secreting neurons respond to changes in circulating volume?
decreased volume sensed by low pressure receptors in the left atrium. vagal input to brain induces release of ADH. renin angiotensin system also causes release
