A study on endurance-trained cyclists revealed that dehydration significantly impairs cardiovascular function during exercise, especially when combined with hyperthermia. While hyperthermia or dehydration alone reduced stroke volume by 7–8%, the combination led to a 20% reduction in stroke volume and a 13% drop in cardiac output, causing greater cardiovascular strain. Restoring blood volume reversed the effects of dehydration alone, underscoring the critical role of hydration in maintaining cardiovascular stability during intense exercise in hot conditions.