Why blood pressure increases at high altitudes (>8000 ft)?

I am a very healthy person with and average blood pressure of 120/75 at 200 ft elevation where I am currently living (Portland, Oregon). I was born up in the andes of Peru at about 10,000 ft elevation, lived there my first 15 years and then moved to lima (coast level ~300 ft). I used to go back to my home town once a year. I moved to USA back in 1988 and lived in Portland since there. Four yeras eago when I went back to the andes, I did experience my blood pressure went up to 180/100. Talked to my doctor but he could not tell me the reason for the change. Does anybody outhere can explain me why this could happen? by the way when I am back in Portland my blood pressure is back to normal again. Thanks.