González-Alonso et al. 2008, The cardiovascular challenge of exercising in the heat

Review (J Physiol 586(1):45–53). Exercising in the heat raises skin blood-flow demand for cooling on top of muscle demand, straining cardiac output. Progressive dehydration to ~4% body mass lowers systemic, muscle and skin blood flow, reduces stroke volume and raises heart rate (cardiovascular drift), and the strain is driven more by dehydration and hyperthermia than by heat exposure alone. Fatigue tends to occur at a core temperature slightly above 40 °C largely irrespective of starting temperature. Strong as a mechanistic review.