We have recently demonstrated that heterogeneous cerebral blood flow (CBF) response in each cerebral artery may contribute to the maintenance of circulatory homeostasis in the brain. However, to what extent weightlessness associated with fluid shift from peripheral to central circulations influences distribution of CBF in each cerebral artery remains unknown. We hypothesised that dry immersion-induced fluid shift (weightlessness condition) would cause heterogeneous CBF response in each cerebral artery. During and after a 3-day dry immersion, internal carotid (ICA), external carotid (ECA) and vertebral arteries (VA) blood flows were measured by Doppler ultrasonography using an 8-MHz linear transducer. Although the 3-day dry immersion and the 2-day recovery period did not change blood flow in each cerebral artery, both ICA and VA conductance decreased during dry immersion on day 2 and day 3 (ICA, 2.95 and 3.23 ml min−1 mmHg−1; VA, 1.10 and 1.05 ml min−1 mmHg−1) from the baseline (ICA, 3.47 ml min−1 mmHg−1, P = 0.027; VA, 1.23 ml min−1 mmHg−1, P = 0.004). In addition, Pearson correlation analysis demonstrated that the 3-day dry immersion-induced a decrease in cardiac output (P = 0.004) was associated with changes in ICA (P = 0.046) and VA blood flow (P = 0.021), but not ECA blood flow (P = 0.466). These findings suggest that short exposures to weightlessness via a cephalad redistribution of fluid volume and blood flow in the human body influenced cerebral vasculature in each cerebral artery but did not cause heterogeneous CBF response in each cerebral artery.
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