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The increased pulmonary artery pressure or right ventricular hypertrophy at high altitude has been reported to vary widely among species and individuals of the same species even when they are exposed to the same altitude. Reeves et al. reported species differences in the increase in the pulmonary artery pressure resulting from chronic exposure to high altitude; although pulmonary hypertension induced by exposure to high altitudes was remarkable in the cow and horse, it was minimal in the llama, dog, sheep and rabbit. It was also shown that there are two types of cow, i.e., the susceptive type, which shows marked increases in pulmonary artery pressure when exposed to high altitude and the resistant type, which is less responsive to changes in altitude. Genetic factors have been suggested to play an important role in terms of sensitivity to exposure to high altitude. Similar differences in the responsiveness to hypoxia have also been noticed in humans; some individuals develop marked pulmonary hypertension but others do not. [ABSTRACT FROM AUTHOR]