Regulation of Body Fluid Osmolarity Flashcards
(44 cards)
What is a good indicator of total body water? How is it calculated?
Plasma osmolarity
= 2x plasma [Na] + [glucose]/18 + [BUN]/2.8
What senses changes in body osmolarity? Where is this located?
What stimulates it?
Osmoreceptors
- located on supraoptic and paraventricular nuclei of the hypothalamus
- stimulated by an increase in body fluid osmolarity. (i.e. cell shrinkage)
How do the sensors of osmolarity regulate?
- Activation at the cell body of supraoptic nerves transmits signal to the nerve ending that is located in the posterior pituitary gland.
- The final cellular signal in the nerve ending is increase in intracellular calcium.
- Elevated intracellular calcium stimulates membrane fusion of ADH containing vesicles resulting in exocytosis of ADH into ECF.
Where does ADH work?
Collecting duct epithelial cells
-specifically on receptors of the basolateral membrane
How does ADH work?
increases water reabsorption
- by increasing water permeability in the CD causing reduced urinary excretion of water
Besides secretion of ADH, how else do the osmoreceptors attempt to regulate increased osmolarity?
Stimulates thirst centers in the hypothalamus
Other name for ADH?
- Vasopressin
- Arginine-vasopressin
Describe the intracellular activity once ADH binds to its receptor
- Activates the adenylate cyclase that converts ATP into cAMP
- cAMP activates protein kinase A which phosphorylates vesicles containing aquaporin-2 in their membranes
- AQP-2s fuse to plasma membrane on the luminal side
=> increases water permeability of the cells.
What is the name of the receptor that ADH binds to?
V2
- coupled to adenylate cyclase
T or F: ADH activity is rapid, taking less than 10 minutes
T
Where is ADH degraded after its use?
In the proximal tubule and liver
ADH levels during dehydration?
HIGH
ADH levels when hydrated
Low
Normal range of AVP/ADH level?
.5 - 1.8 pM
ADH level at 270 mOsM?
about .5 pM
Describe the trend of plasma [ADH] as a function of osmolarity
- increases linearly w/ increasing osmolarity untill it reaches a maximum of about 18pM
- plasma [ADH] levels off at an osmolarity of 290 pM and greater
When is the thirst response stimulated?
Only at high levels of osmolarity
- so small changes in osmolarity is mainly corrected by ADH
What is the effect of ECF volume on ADH?
inversely proportional
What has a more dominant effect on ADH levels,
ECF volume or osmolarity?
Under condition of severe loss of ECFV its effect overrides osmolarity’s
In other words, it does not matter what the osmolarity is at low ECFV, the AVP level rises high.
Describe how ECF volume’s effects on ADH are “disconnected” from those of osmolarity?
- plasma ADH is increased proportionally with 1-7% increase in plasma osmolarity
- Plasma ADH level increased only when the ECFV reduced by greater than 10%
How does Diarrhea and Vommiting result in hyponatremia
Both cause large drop in isotonic fluid from the body
- dec ECFV => ADH secretion => increased water reabsorption
-inc water w/ low levels of Na => hyponatremia
Clinical signs of hyponatremia?
- Lethargy
- hyporeflexia
- mental confusion
Tx of hyponatremia?
Isotonic saline
infuse slowly!
Conditions associated with hyponatremia to consider in DDx?
- Heart Failure
- where low BP stimulate secretion of hypovolemic hormones - Liver Failure
- reduced Plasma Volume due to fluid redistribution into ISF driven by low plasma oncotic pressure. Reduced plasma volume stimulates hypovolemic hormone secretion
