The sensitivity appears to be genetic. Once one is acclimatized at a given height they can still get ill at a higher elevation. In addition, once one descends these adjustments disappear at about the rate they came. Heart rate is increased. The heart must beat more quickly to meet the cells constant demand for oxygen.
Fluid shifts occur to lungs and brain.
Blood flow to the brain must increase to provide it with its required volume of oxygen. The pulmonary capillaries constrict in the lungs which increases the resistance to flow through the lungs and raises the pulmonary blood pressure. This decreases the partial pressure of carbon dioxide in the alveoli allowing the partial pressure of oxygen to increase. This mechanism starts as low as 4,850 ft. In addition, the decrease in the alveolar partial pressure of carbon dioxide causes the concentration of carbonic acid in the blood to decrease which makes the blood more alkaline.