PHYS20008 Lecture Notes - Lecture 26: Vasopressin, Osmotic Concentration, Osmoreceptor
12 Jun 2018
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Lecture 26
PHYS20008 - HUMAN PHYSIOLOGY
LECTURE 26
URINE CONCENTRATION & DILUTION
WATER BALANCE
•Balance = input and output. Input = usually food, but we can make our own H2O. Output =
mostly urine, it’s the main one we can control.
•We can regulate Na balance using ECF volume.
•A negative water balance (dehydration) causes more concentration (increased osmolarity).
DETECTING CHANGES IN OSMOLALITY
•Osmoreceptor cells
•Located in the anterior hypothalamus
•Respond to changes in cell size: changes in stretch so that as we become dehydrated and lose
H2O volume (from ECF and hence ICF), the cells are going to shrink.
•Negative H2O balance will normally result in: (as we have been breathing, sweating, filling your
bladder all night) thirst and concentrated urine.
•Things needed to concentrate urine include: vasopressin. Aldosterone + angiotensin 2 = Na
balance. Vasopressin = H2O regulation.
VASOPRESSIN & OSMOLARITY (diagram 1)
•Increased osmolarity = dehydration. If it goes up, we want to keep water. Vasopressin
(antidiuretic hormone) does this. So as osmolarity increases so does vasopressin.
•The set point is a point at which if we are at negative H2O balance we can release more
vasopressin but if we are at positive H2O balance we want to
release more H2O; increase diuresis.
•+ H2O balance = less vasopressin = more dilute urine.
VASOPRESSIN & ECF VOLUME (diagram 2)
•Under normal circumstances, ECF doesn’t care about
vasopressin volume. With modest changes in ECF fluid volume
not much happens. With an extreme loss (Eg. a hemorrhage) of
ECF fluid volume, vasopressin will step in and stop us from
excreting urine. Osmolarity, not volume, regulates vasopressin.
•If you donate 500mL blood, that won’t change osmolarity at
all cause you’re pulling out blood not water and thus
shouldn’t impact vasopressin.
Document Summary
Input = usually food, but we can make our own h2o. Output = mostly urine, it"s the main one we can control: we can regulate na balance using ecf volume, a negative water balance (dehydration) causes more concentration (increased osmolarity). Detecting changes in osmolality: osmoreceptor cells, located in the anterior hypothalamus, respond to changes in cell size: changes in stretch so that as we become dehydrated and lose. Vasopressin & osmolarity (diagram 1: increased osmolarity = dehydration. If it goes up, we want to keep water. Vasopressin & ecf volume (diagram 2: under normal circumstances, ecf doesn"t care about vasopressin volume. With modest changes in ecf fluid volume not much happens. With an extreme loss (eg. a hemorrhage) of. Ecf fluid volume, vasopressin will step in and stop us from excreting urine. Osmolarity, not volume, regulates vasopressin: if you donate 500ml blood, that won"t change osmolarity at all cause you"re pulling out blood not water and thus shouldn"t impact vasopressin.
