Just curious. What is the difference between "hitting the wall" and heat exhaustion or water intoxication. Is it muscular, mental, or an electrolite imbalance?

I drank 5-6 L coming up the Bright Angel from the river, a couple weeks ago. We drank and ate on a schedule and felt great when we got to the top. In fact, the wives picked us up in Flagstaff and we drove to Sedona and partied well into the night, with relatives.

On the other hand, a couple of people from the trip didn't fare so well, Both of them were very young adults and didn't pace them selves. One young lady became noticeably irritable, something she had never done throughout the week. It was quiet obvious to everyone that she was not hydrating or eating properly and her family made her rest, eat, and drink. We saw them later at the rim and she was fine.