Sodium Disorders Flashcards
define osmolality
particles per kg
define osmolarity
particles per litre
what is the normal osmolality in canine and feline plasma
dog: 290 to 310
mOsm/kg in dogs
cat: from290 to 330mOsm/kg in cats
what is the equation for osmolality?
osm =2Na +BUN/2.8 + glucose/18
2.8 and 18 are conversion factors to SI units
What does the specific gravity depend on?
number of particles present in the solution and also on their molecular weight
What does tonicity refer to?
Tonicity
refers to the ability of a solution to initiate water movement and is dependent on the presence of
impermeant solutes in the solution.
may be thought of as effective osmolality
What is the normal sodium for a dog and cat
Dog 140-155
Cat 149-162
Why may hyponatreamia occur
when the patient is unable to excrete ingested
water
or when urinary and insensible fluid losses have a
combined osmolality greater than that of ingested or parenterally administered fluids.
Why may hypernatraemia develop?
when water intake has been inadequate,
when the lost fluid is hypotonic to ECF,
or when an excessive amount of
sodium has been ingested or administered parenterally
renal handling of sodium
What is the main effect of aldosterone in relation to Sodium handling?
Increase the number of open luminal Na changes in the collecting duct
What are the two main control points for renal sodium balance?
glomerular filtration and tubular reabsorption
What is glomerulotubular balance
A mechanism in which the kidneys keep the fraction of filtered load resorbed constant regardless of GFR - this mains that filtered load will change appropriately with GFR
What stimulates aldosterone release?
stimulated by angiotensin
II, hyperkalemia, and adrenocorticotropic hormone
(ACTH). Its release is inhibited by dopamine and atrial
natriuretic peptide.
Which arteriole is usually more affected by catecholamine induced vasoconstriction? Afferent or efferent
Efferent
WHat vessel does angiotensin II preferentially constrict
Efferent
What are the effects of Atrial Natriuretic Peptide
Released in response to atrial stretch
dilation of the afferent
arterioles and constriction of the efferent arterioles
inhibits sodium reabsorption in collecting ducts
Inhibits renin release, and inhibits aldosterone secretion
Water handling
How does ADH work
Increases absorption of water in collecting duct - binds to V2 receptors –> cAMP –> results in Aquaporin 2 fusing to cell membrane and opening for water movement
What is the main stimuli for vasopressin release?
hypertonicity of plasma
changes in plasma osmolality as small
as 1% to 2% above normal lead to maximal vasopressin
release.
What is a secondary stimuli for vasopressin release?
decrease in blood volume of 5-10% lowers threshold for vasopressin release
Other stimuli for vasopressin release include nausea,
pain, and emotional anxiety.
What three conditions must be met for the kidneys to excrete a water load normally?
- Adequate delivery of tubular flow to distal tubules were NaCl is removed without water
- Ascending limb of Henle must function normally.
- in absence of vasopressin collecting duct is impermeable to water
How does the brain protect itself from hypotonicity?
Movement of CSF into the spinal canal
movement of potassium out of the cells
In 24-48hrs organic osmoles begin to be broken down to reduce cellular osmolality
Clinical Approach to Hypernatramia