The effect upon the retinal circulation of exposure to an altitude of 17,500 feet has been studied in acclimatized individuals and in those exposed for a short period. Retinal blood flow is increased 89% over control values within two hours at altitude and is increased 128% after four days at altitude. It is increased 105% over control values in the acclimatized individuals. Retinal vessel size is maximal after four days at altitude and the vessels are smaller in the acclimatized subjects. All subjects are hypoxic and hypocapnic at 17,500 feet. These changes in the retinal circulation may reflect the changes which occur during acclimatization or a shift in the proportion of the neural tissue supplied by the retinal and choroidal circulations.
Frayser R, Houston CS, Gray GW, Bryan AC, Rennie ID. The Response of the Retinal Circulation to Altitude. Arch Intern Med. 1971;127(4):708-711. doi:10.1001/archinte.1971.00310160186014