The optimal design of vascular networks maximizes circulatory efficiency while minimizing power costs. We investigated the effects of acute changes in vascular tone on retinal arteriolar network geometry. Six hypertensive and six normotensive subjects each breathed air, 5% CO2 (with 12% O2), and 100% O2 for 5 min periods in random order. Retinal photographs were taken at the end of each test period. Bifurcation angles and arteriolar diameters were measured using operator-directed image analysis, and junction exponents were calculated. Arteriolar diameters narrowed on breathing O2. The magnitude of this change was significantly greater in normotensive than in hypertensive subjects. Angles narrowed in normotensive subjects, but not significantly in hypertensive subjects. Arteriolar diameters increased significantly on breathing CO2 in normotensive but not in hypertensive subjects, but there were no changes in angles. Despite changes in diameter, junction exponents did not change under any conditions. Vascular reactivity in the retinal arteriolar bed appears to be diminished in hypertensive subjects. The failure of junction exponents to change, despite alterations in diameter, suggests that arteriolar diameters at retinal bifurcations adhere to optimality principles when exposed to acute vasoactive stress. As vasoconstriction is associated with the narrowing of bifurcation angles, previous observations showing narrowed angles in hypertensive subjects could be explained by increased tone in the retinal arteriolar bed.

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