The minds of OUWB continue to provide thoughtful questions.
My roommate and I have encountered a question regarding the content on Sodium/Water Balance and also its application to SIADH. We have been using some outside resources to supplement the learning in class, and I feel that they have been somewhat contradictory in these 2 scenarios. The following are the scenarios that I am trying to think through
1) Patient eats a high salt meal, increasing total body Na+, resulting in an increase in ADH release (via increased plasma osmolarity) and eventually reaching baseline Na+ concentration and osmolarity at a higher ECV. Now, the increase in ECV would result in a down regulation of Sympathetic NS and RAAS; however, what I am hearing is that this down regulation would just return the kidney to Na+ in = Na+ out and would not actually return the individual to the original ECV. So, my question is how does this person get back to original ECV? What I am reading is that the person will continue to operate at this higher ECV until sodium restriction takes place. However, I am wondering how decreased RAAS (decrease aldosterone – decrease Na+ reabsorption – increase sodium excretion) wouldn’t do this, and also if pressure natriuresis wouldn’t do this also? Basically, why don’t these mechanisms do the work automatically, and why do you have to sodium restrict?
You have it right. That is the currently accepted understanding of sodium metabolism. It is not quite complete, because, though some subjects increase their blood pressure with increased sodium intake, not all patients increase their blood pressure. As to why the renin-angiotensin aldosterone system does not down regulate itself sufficiently to fully correct the volume overload situation, it is not well understood. The sodium regulating systems in the body strive to match sodium absorption with sodium excretion. With an increase in sodium intake there will be a modest expansion of the extracellular compartment until the sodium excretion is upregulated to match sodium intake. We can see evidence of the increase total body sodium with an increase in body weight associated with increased sodium intake.
2) In SIADH – high levels of ADH cause increased water reabsorption but euvolemic hyponatremia. Fitting in with my previous questions in the earlier scenario, how does the patient maintain euvolemic status? If increased water reabsorption occurs and the ECV is increased, the same down regulation of Sympathetic NS and RAAS would occur. Now, the outside resources in this case state that a decreased RAAS would actually cause increased sodium excretion that would allow for increased water excretion that would maintain euvolemic status. This makes sense because then the hyponatremia that results is not only an effect of the dilution from increased water reabsorption, but also from the increased excretion of Na+. But, this goes directly against the whole logic of needing to sodium restrict in the earlier case (i.e. RAAS can’t do the work to return the individual in scenario 1 back to a normal ECV).
So again you are well versed in what is happening in SIADH. SIADH is largely euvolemic and largely is a situation where patients are in sodium balance, i.e. sodium = sodium out. However if you do meticulous metabolic balance studies you will find that patients do gain weight during SIADH. There is excess water and this does serve to expand the patient’s extracellular volume. This also will suppress the renin-angiotensin-aldosterone-system so that patients will get a modest increase in urine sodium excretion. But I don’t quite understand how you think this is any different than the first scenario. There is a modest increase in sodium excretion but in the presence of continued unremitting ADH activity the patient continues to deal with the modest increase in volume. So like the first scenario, the modulation of the RAAS is unable to fully restore euvolemia.
For more on SIADH and volume status see this post.