PDHPE

Exercise causes systolic blood pressure to increase.During exercise, cardiac output increases dramatically to allow the body to supply sufficient oxygen to the working muscles. In addition, the diameter of the blood vessels increases to allow more blood to be supplied to the working muscles. Under maximal conditions blood flow to the working muscle can increase by as much as 35 times. To cope with the opening of this large circulation there needs to be an increase in blood pressure. This increase is due mainly to a rise in the systolic blood pressure.

There is an everyday analogy that helps to explain why there is an increase in systolic blood pressure. Imagine that someone is showering in an upstairs bathroom when someone else in the house turns a tap on. This results in a loss of pressure in the shower, as two circulations are now open. The answer to this problem is to increase the capacity of the pumps. This is effectively what the heart does by increasing the force with which the heart contracts. This, combined with the increase in blood volume, requires a corresponding increase in systolic blood pressure.
Diastolic BP is a measure of peripheral resistance of the blood vessels. During aerobic exercise the peripheral blood vessels dilate to manage the increase in blood volume and help blood flow back to the heart. This reduces the peripheral resistance in the blood vessels. As a result there is no real change to diastolic pressure.