The heart increases the flow by increasing the pressure, causing leakage from the blood vessels into the air sacs. It usually takes a few days to develop, and is exacerbated by overexertion. In bad cases, you can hear a gurgling of fluid in the lungs, and the victim brings up a bloody sputum. This is a serious condition which can kill in only a matter of hours. HAPE is best avoided by gradual ascent.
Treatment is immediate descent by several thousand feet and oxygen if available.
The body also responds to the lower oxygen levels by putting more red blood cells into circulation. Up to a point, this is a good thing. However, if it goes too far, the blood becomes thick and prone to clotting. Clots which get dislodged float around and can cause strokes, heart attacks, and pulmonary embolisms. The only treatment for thick blood is to be bled.