I mean, I’ve heard that you can typically only survive about three days without water, but what exactly causes your body to fail when you dehydrate too much?
I guess one point is lack of salts (if you sweat a lot) but I’m specifically wondering about lack of water (although a closer explanation about how lack of salts will kill you is also appreciated)
That’s a very incorrect statement. NaCl, KCl, and MgCl2 are essential for our bodies to function appropriately. If you are working out for an hour or mowing the lawn on a hot day, you are mostly correct. You can replace the lost salt though a balanced diet. If you have sustained sweating, such as when you run a marathon or play a match of football, it can be deadly to not replace these salts.
In the vast majority of cases, people don’t need salt when they exercise. You just described a fringe case that virtually no one experiences. You high school football coach shouldn’t be giving you salt. That could be dangerous. They should be giving you water.
The coach won’t be giving them salt directly, but will always provide a sports drink which contains salts.
Ever watch any sport and a player falls over with a cramp? They have drunk too much water and not enough electrolytes.
Here’s a study from one hospital in Oslo, Norway. Norway is not considered a hot region of the world. Between 2010 and 2015 they treated 31,000 patients in the ER from a population of 135,000 for electrolyte imbalance. That’s roughly 18 patients a day. This is not some sort of fringe case.