Two more OUWB questions

Hi Dr. Topf,

I had a question regarding the Macula Densa.  When reviewing your powerpoint on volume control, you have a slide that said there is only one osmoreceptor (Hypothalamus) because osmolarity across the body is the same at all times.  I’ve had some confusion regarding the Macula Densa, but from what I understand it is also an osmoreceptor (sensing Na+ in the tubule), which would make sense because the tubules are the only part of the body where osmolarity is different.

I thought that the Macula Densa would affect GFR and stimulate the release of renin from the Juxtaglomerular cells, but that would seem to affect volume (RAAS System maintains volume), so my question is why does the macula densa (which senses Na+) controlling volume and not Osmolarity?

Good question.

So the macula densa is a major part of a process called tubuloglomerulo feedback

As the name implies this is important for balancing GFR with tubular reabsorption.

If you had excess GFR and limited tubular reabsorption, people could literally pee them selves to death in minutes

Think about the math, you have 3 liters of plasma and filter 125 ml of it every minute. so it would only take 24 minutes to completely filter all of the plasma. if you are not constantly reabsorbing 99% of that very rapidly you could become volume depleted and suffer from cardiovascular collapse.

Tubular glomerular feedback prevents that. At the end of the thick ascending limb of the loop of henle, there are chloride receptors as part of the juxtaglomerular apparatus. If there is too much filtration and not enough reabsorption, the excess chloride will bind these receptors and cause a release of intra-renal signals that decrease GFR by adjusting the dilation of the afferent and efferent arterioles.

So yes there are receptors that bind chloride and you can think of them responding to the various concentrations of chloride (like an osmoreceptor) but they are not involved in volume regulation or osmoregulation, but rather the safe running of the kidney to prevent a person from accidentally peeing themselves to death.

Hope this helps

Hello Dr. Topf,

I hope you are doing well. I had a few questions in regard to your last lecture at OUWB. I was wondering if you could explain the pathophysiology behind euvolemic hyponatremia caused by hypothyroidism and adrenal insufficiency. Also, in the case of SIADH, whay wouldn’t the person have hypervolemia if there is a constant reabsorption of water? Is there a pathophysiologic explaination for this as well? I could not find any answers online.


So the key here is to remember that volume is determined by total body sodium and that SIADH is generally Na in = Na out. So they are in sodium balance and will not be volume overloaded.

You are right that these patients will have excess water, but much of this water disappears into the intracellular compartment and the excess volume can not be picked up clinically (by exam or by conventional blood and radiology tests). Yes there is excess water.

We try to reserve terms like hypervolemia for excess total body sodium, and this is not found in SIADH.

Hope that is helpful.