More Demand On The Heart
Ideally, during exercise we want the heart to pump out as much blood as possible in each beat so that the heart rate doesn’t skyrocket to sustain a given workload. What happens in the heat, however, is less than ideal.
With more blood at the periphery, there is less blood flowing to the heart. This decreases cardiac filling and stroke volume — the amount of blood that is pumped from the heart. To compensate, heart rate increases to sustain the workload. As a result, the relative intensity of exercise increases, more stress is placed on the heart and we max out sooner. In other words, an 8-minute pace may feel like a 6-minute pace because the heart is working that much harder.
Also, if blood volume decreases from high sweat rates (a loss of plasma) there is an increase in blood viscosity — a higher concentration of red blood cells — which puts more stress on the heart and vessels.