Can high altitude cause hypertension or aggravate it if it already exists? if so what causes this?

No, Internal Control: Bp: a result of cardiac output vs. Average resistance of the 20, 000 to 30, 000 end arteries (arterioles) which determine blood flow to each capillary bed, both co ; size of all arterioles (degree of constriction) are under split-second control from brain ; constantly being adjusted. BP is just a momentary result. Brain integrates many variables, including perceptions/feelings ; adjusts accordingly.