Contributed by Dr. Devor
You will have an increased heart rate at the same pace on a hot and humid day. This is because as much as 20% of your blood flow is diverted to vessels under your skin so that the heat the blood is carrying can be lost to the environment, then the cooled blood is returned to the core. So if as much as 20% of your blood flow is devoted to keeping your body cool, that is 20% that is not able to be sent to the working skeletal muscles during running. If you stay at the same pace, your heart will have to supply enough blood flow to cool your body (the skin blood flow), and will also still need the necessary amount to your working skeletal muscles. Thus the number of heart beats per minute will rise at the same running pace.