Retinal and cerebral hemodynamics redistribute to favor thermoregulation in response to passive environmental heating and heated exercise in humans.

Abstract

Core temperature (T_C) changes, alongside exercise, affect hemodynamic responses across different conduit and microvascular beds. This study investigated impacts of ecologically valid environmental heat and exercise exposures on cerebral, skin and retinal vascular responses by combining physiological assessments alongside computational fluid dynamics (CFD) modeling. Young, healthy participants (n = 12) were exposed to environmental passive heating (PH), and heated exercise (HE) (ergometer cycling), in climate-controlled conditions (50 mins, 40°C, 50% relative humidity) while maintaining upright posture. Blood flow responses in the common carotid (CCA), internal carotid (ICA) and central retinal (CRA) arteries were assessed using Duplex ultrasound, while forearm skin microvascular blood flow responses were measured using optical coherence tomography angiography. Three-dimensional retinal hemodynamics (flow and pressure) were calculated via CFD simulation, enabling assessment of wall shear stress (WSS). T_C rose following PH (+0.2°C, p = 0.004) and HE (+1.4°C, p < 0.001). PH increased skin microvascular blood flow (p < 0.001), whereas microvascular CRA flow decreased (p = 0.038), despite unchanged ICA flow. HE exacerbated these differences, with increased CCA flow (p = 0.007), unchanging ICA flow and decreased CRA flow (p < 0.001), and interactions between vascular (CCA vs. ICA p = 0.018; CCA vs. CRA p = 0.004) and microvascular (skin vs. retinal arteriolar p < 0.001) territories. Simulations revealed patterns of WSS and lumen pressure that uniformly decreased following HE. Under ecologically valid thermal challenge, different responses occur in distinct conduit and microvascular territories, with blood flow distribution favoring systemic thermoregulation, while flow may redistribute within the brain.