Regulation of extracellular Na & H20 Flashcards
What is the normal range of sodium concentration in the ECF?
135-142 mM
T or F?
Losses of ECF sodium cause greater changes in sodium concentration than they do on ECF volume
False.
Losses of ECF sodium actually cause greater changes in ECF volume than they do on sodium concentration. This is because sodium is the major osmotic substance in the ECF
You eat a Big Mac and fries for lunch, and you ingest 10g of sodium. Before you ate, your ECF Na concentration was 10g and your ECF volume was 2 L. What would you expect your ECF volume to be now?
(these #s are not accurate, they’re just to test a concept)
Your ECF volume should be 8L now.
Since cellular volume is 2x the volume of the ECF compartment, gains/losses of Na in the ECF yield 2x the change in ECF volume than in ECF Na concentration.
(Your ECF Na doubled, so your ECF volume should quadruple)
You are fasting so your ECF Na concentration decreases below normal limits. What impact does this have on ECF volume? How does the body respond?
The decreased Na leads to decreased osmolality, meaning less water moves into the ECF and your MAP drops.
As a result, the kidneys release renin which results in more aldosterone secretion which causes greater Na (and consequently water) reabsorption. You still need to eat more Na, but this mechanism helps.
If your Na is too high the complete opposite happens (higher MAP, less aldosterone, less Na reabsorption)
What is the hormone responsible for only water reabsorption (not Na reabsorption)?
Where is it produced, and from what is it released?
What physiologic conditions lead to its release?
The hormone is ADH.
Produced in the hypothalamus, released from the posterior pituitary.
Responds to both decreased ECF volume and increased ECF osmolarity (sweating is a good example of something that causes both of these since sweat is so hypotonic)
Where is a low ECF volume primarily sensed?
When baroreceptors sense low ECF what do they stimulate?
ECF volume primarily sensed in the atria. This stimulates the baroreceptor reflex which stimulates the hypothalamus to produce ADH and increase water reabsorption in the kidney.
Which pathway, volume or osmolarity, is more dominant in normal day to day water regulation?
The osmolarity regulation is dominant and is prioritized by the body in normal day to day regulation.
A patient comes in under extremely hypovolemic conditions, having pooped and puked out over 4 liters of fluid. You’re stressed out and wondering what you should do to treat them when your attending turns to you and asks, “which mechanism is dominating right now, the volume or osmolarity regulation mechanism?”
You remember this flashcard and reply:
The volume mechanism (ADH response to low ECF volume) is dominating right now sir.
Even though pumping them with fluids will lower their ECF osmolarity (which normally would suppress ADH secretion), since they are still so hypovolemic the volume response trumps the osmolarity reponse until volume levels are safer.
Think of the volume response as the passive aggressive shy kid who only freaks out once he’s pushed past his breaking point. After a little while he calms down and goes back to the corner, but he freaks out when he needs to.
Describe the actions of ANP in response to a hypervolemic state, including the 3 targets that it acts on.
Pro ANP is released from the atrium (atrial natriuretic factor) and responds to hypertension, which is sensed by atrial stretching. Pro ANP is cleaved to Active ANP which acts on 3 targets:
1) Decrease ADH and water reabsorption
2) Inhibits renin release and decreases aldosterone
3) Causes dilation of afferent and efferent arteriole, inceases GFR, more excretion of water and sodium.
