Blood pressure rises with exercise as a result of the heart pumping more blood than usual, faster than it usually does. It does this, according to WebMD, in order to increase cardiac output and provide more oxygen-rich blood to the tissues that need it during sustained exertion.
A healthy human heart pumps around 5 or 6 liters of blood every minute while resting. When the body becomes active, as during strenuous exercise for example, the muscles and organs of the body suddenly require far more oxygen than they did previously. In addition, the heightened activity inside the cells generates far more waste products than usual. All of this requires more blood to handle. This is why, according to WebMD, the total cardiac output of the heart can quadruple in seconds.
This rise in output is accomplished in two ways, and both have implications for blood pressure. First, the heart beats faster. This delivers more blood by the expedient of simply pumping more per minute than when it was at rest. The second method the heart has at its disposal, as WebMD reports, is to pump harder with each stroke to maximize volume delivered. Together, these measures can temporarily increase blood pressure.