The force that keeps blood moving comes from the heart, but a complex system of nerve signals, hormones, and other elements regulate the blood flow to each organ by widening or constricting small muscular blood vessels called arterioles, much like a faucet controls the flow how much water. Blood pressure depends on a number of factors, including how much blood is flowing through the arteries, the rate of blood flow, and the resiliency of the arteries' walls. Blood pressure fluctuates from moment to moment and factors influencing it are the time of the day, it is lowest in the morning, and your degree of physical exertion or anxiety; blood pressure tends to go up with age!