The influence of aging on graded peripheral venous return and complex blood flow features through human veins.

Aging is associated with reduced venous compliance and altered muscle pump function, though how these features impact venous return during volume-stressed conditions remains poorly characterized. We examined lower-limb venous flow dynamics and muscle pump efficiency between healthy younger (YA) and older adults (OA) under two volume-stressed conditions. Fourteen YA (7F, 26 ± 4 years) and fourteen OA (9F, 76 ± 6 years) underwent assessments at head-down tilt (HDT, -3°) and head-up tilt (HUT, + 20°) positions. Participants performed isometric plantarflexion contractions at 8 kg and 50% maximum voluntary contraction (MVC) (with YA achieving 16.4 ± 5.4 kg and OA 11.2 ± 4.1 kg at 50% MVC). Femoral vein diameters and blood velocities were recorded via conventional Doppler ultrasound and vector flow imaging (VFI) for quantifying flow dynamics. Muscle pump efficiency was evaluated by strain gauge plethysmography to assess shank volume changes. Venous diameter increased with HUT in both groups, with YA showing a greater increase (+ 41.3 ± 22.7% vs. + 23.0 ± 10.9%, P = 0.01). Blood flow and ejected blood volume significantly increased from HDT to HUT in both groups (P < 0.01). Muscle pump efficiency improved with HUT; OA had a greater relative increase in volume change during the 50% MVC effort. VFI revealed increased venous flow complexity with HUT and a stronger inverse relationship between peak vorticity and volume pumped in OA. Despite reduced venous compliance, OA maintained effective muscle pump function under volume stress, suggesting differing hemodynamic strategies between age groups. These findings provide insights into the interplay between venous compliance, flow dynamics, and muscle pump efficiency in aging.