Imagine the following situation. You have a person who initially eats 10 mEq/day of salt. He then, at t=0 begins to eat 150 mEq/day of Na, and will continue to do so, because you're forcing them to, forever. My question is about how the kidney responds to such a situation. There's two options.
- Option #1: The Kidney ensures sodium balance (sodium intake = sodium excretion). In this situation, the kidney regulatory mechanisms ramp up Na excretion until it equals 150 mEq/day. But since this takes a couple of days, there is a bit of time in which intake > excretion and so total body Na rises. Because of osmoregulation, more water is drunk to balance the increased total body Na, and so the person's weight rises.
- Option #2: The Kidney brings total body sodium back to normal. In this situation, the kidney regulatory mechanisms ramp up Na excretion, but it briefly ramps up excretion to more than 150 mEq/day (excretion > intake) to excrete out the increase in total body sodium that built-up. In this case, the weight gain is temporary.
See a graph describing the two different responses.
The situation is inspired by Chapter 40 of Boron & Boulapep's Physiology textbook. The reason for the question is that Boron states that Option #1 is correct, whereas my teacher in class said differently ¯_(ツ)_/¯ .
