The human body's cardiovascular system keeps itself functioning at peak efficiency by maintaining homeostasis through proper blood pressure. Depending on the temperature relative to the body, blood vessels will either dilate or contract. If blood vessels contract, blood pressure will increase if the body maintains the same heart rate because of the amount of blood going through the circulatory system.
The internal temperature of the body, normally, is 98.6 degrees. During strenuous activity, the muscles exert energy and heat up. Although the body can perspire to cool itself off, this uses fluid and can cause dehydration. Therefore, the body has to find another way to lower its temperature. When blood vessels dilate because of heat, the blood is under less pressure, thus allowing it to cool off.
Blood pressure is a function of heart rate and width of blood vessels. According to madsci.org, a given volume of blood circulates through the body at around seven to eight times per minute (see References). That's one circulation about every 8.5 to 9 seconds. Every time the heart beats, it sends more blood through the blood vessels and increases the pressure.To keep blood pressure stable, the heart has to pump faster.