From my own mountaineering experience, I cannot say with certainty that I urinate more at high altitude. But there are plenty of claims, some probably based on evidence, that this indeed happens.
Can You Climb Teide Without a Permit?
El Teide (or Mount Teide, 3718 m) is an incredibly popular mountain but there is a limit for the number of people that can climb it per day. Permits are free but you have to obtain it many months in advance.
How to Start Mountain Climbing
This page provides some general but essential instructions and tips for those who are looking for an answer on question how to start mountain climbing. I myself started mountaineering almost 4 decades ago by climbing Triglav (2864 m) in what is now Slovenia, so I feel I could say something about this activity.