So far, what we have covered is how osmolality and volume are regulated on a day to day basis - when there are minor changes in these two variables, calling for minor adjustments to be made.
However, when there is a sudden and severe enough drop in volume, the standard volume regulation system is much too slow to save the body from disaster.
In such circumstances, the osmolality system is diverted from its normal role and instead used as a backup to artificially increase the amount of water without increasing sodium along with it. It is almost as if the body says "hold on, this current system isn't working anymore, because volume is now the priority. Let's engage the override switch so we don't die from hypovolaemic shock". It does this by secreting ADH in much larger amounts than normal.
You can now see an extra button which toggles this override switch on and off. There is also a new line to mark the level below which this change is made, and although in real life there isn't such a clear threshold, it's used here for simplicity.
Make sure the override switch is on.
Abruptly decrease sodium as low as it will go, to simulate a sudden and large enough drop in volume.
Pay attention to the water level, and also the current osmolality.
A relatively quick increase in pure water occurs which helps a moderate amount in getting volume back to its normal level. However, this happens at the cost of a lower sodium concentration and lower osmolality. Up until now, osmolality has always quickly normalised to 100 or at least quickly reached somewhere quite close to 100. But in this state, osmolality stays much lower for a lot longer.
Moreover, this manoeuvre doesn't even fully fix the volume loss, and so the body enters a state in which it is both hypovolaemic and hyponatraemic, until both of these abnormalities are very gradually corrected. But what caused this situation in the first place was the hypovolaemic state which was severe enough (or perceived to be severe enough) that the body had to engage its backup system.
If you want, you can now turn the override switch off, drop sodium all the way down again and observe what happens. You should notice that a low osmolality is no longer a problem. However, volume ends up staying at severely low levels for a much longer time. This override mechanism is only one of many imperfect physiological (mal)adaptations which aim to save the body in the short term, at the cost of longer term health.
If needed, take some more time to explore this new mechanism before moving on.
Building up an algorithm
At this point, we've already covered one key cause of true hyponatraemia - namely a hypovolaemic state which is sensed to be severe enough that ADH is released to increase volume at the expense of a lower sodium concentration and lower osmolality.
Whether this ADH release is appropriate or inappropriate is an interesting question to consider. It is inappropriate in that it would not happen in a normal day-to-day situation because, as we have seen, the usual role of ADH release is to decrease osmolality when it is high. However, given that this is an adaptation that exists to attempt to increase volume from severely low levels, even if it does not produce the best outcome, we will say that in this case ADH is being appropriately released.
Thus, we can fill in the following in our hyponatraemia algorithm:
