In general oxygen deficiency comes from these factors.
1) lug function does not work well.
2) blood problem - hemoglobin function does not work well
3) hart problem - blood pumping does not work well
4) brain control problem-lug or respiratory system control problem
5) etc.
At the high altitude place, the pressure is very low.
So, the amount of oxygen in the blood is low.
Water is easily vaporized from lug to outside, so we must supply more water than usual ones.
If not, thick blood causes other problems.