So excessive vomiting causes water loss, and metabolic alkalosis (as a result of loss of H+, and presumably some K+ as well)
What response does this cause in the kidneys. I'm reading different things.
One the one hand you don't want to further exacerbate hypokalemia, so Renin release should be inhibited.
But, if blood volume is low as a result of water lost to vomiting, then renin release and thus aldosterone should be stimulated?!
Confused
Thanks
You must understand the physiology. The body isn't teleological. "Im losing K, so I should prevent the loss of K!" its physiological. Each part of the body has a specific stimulus and does one thing in response to that stimulus.
The Macula Densa, the Juxtaglomerular Aparatus, does one thing. It measures fluid status. That's it. What confuses students is that is "measures fluid status by measuring the amount of sodium in the nephron." Take that out of your knowledge vocabulary. What its really saying is "the JG apparatus measures FLOW through the nephron."
If there is too much fluid IN THE VESSELS, the kidneys are well perfused, the glomerulus filters a lot of fluid into the nephron, lots of flow through the loop of henle makes the JG apparatus turn off. On the other hand, if there is not enough fluid IN THE VESSELS, the kidneys are underperfused, the glomerulus filters less fluid into the nephron, small flow through the loop of henle turns the JG apparatus on.
Stop. Make sure you understand that.
Increased flow turns it OFF decreased flow turns it ON
Ok. now what does the JG apparatus do? Its starts a chain of events, that, once initiated, goes through to completion.
JG Apparatus on --> Renin --> Angiotensinogen --> ANG II --> Aldo
- ANG II causes vasoconstriction
- Aldo causes the collecting ducts to express more Enac channels. Na is absorbed (and with it water) while K is lost.
Analyze that. When the
flow to the kidney
is low, JG apparatus is turned
on. When the flow to the kidney is low, the kidneys feel that the body is volume depleted. It doesn't matter what the status of the body is (overloaded from CHF, or euvolemic with renal artery stenosis), if the flow through the kidney is low, the kidney thinks the body is volume down. So, the response is to tighten up the blood vessels (ANGII) and to increase the vascular volume (aldo). The
cost of aldosterone activity is to lose K. The
benefit is to reabsorb water.
Also note that Aldo does something else.
Aldo excretes acid. You lose acid when you are volume down. Its just something aldo does. Its called a contraction alkalosis. As your fluid volume contracts (i.e. gets smaller, less flow), your aldo turns on to preserve that fluid volume. In doing so, it releases acid into the urine, retaining bicarb, making you alkalotic.
So the cost of being volume down, for any reason, burns, infection, or vomitting, is to decrease flow through the kidney. What does that do? Turns on JG apparatus, whcih turns on Renin, which turns on ANG II which turns on Aldo, which holds on to fluid, with the cost of losing K and Acid.
Now, what is the potassium sensor in the body? What is the physiologic structure that says "my K is low" or "my K is high?" you should be struggling. There isn't one. We are that sensor. We draw blood and take a look at K. If there is a derrangement in fluid status, we lose K to maintain fluid. If there is a derrangement in K, there is nothing to correct it. Instead, we, as physicians, must correct the K or the condition that has led to the bad K.
Got it?
