The cardiovascular system—consisting of the heart, blood vessels and blood—pumps oxygen-containing blood throughout the body to the cells. The nervous system, controlled by the brain, is responsible for sensing the internal and external environments and directing muscles and body organs, as well as for coordinating organ activities. These two systems work together to help the heart deliver blood to the body in accordance with cellular needs.
The vast majority of the time, human hearts don't need to beat very fast. Most individuals have resting pulses in the range of 60 to 80 beats per minute. The electrical signal that produces the heartbeat, however, is generated at around 110 pulses per minute. This, explains Dr. Gary Thibodeau in his book, "Anatomy and Physiology," is called the intrinsic rate of the heart. Since the heart doesn't actually need to beat this fast during most activities, the nervous system acts upon the heart to slow its pulse rate. This is accomplished by the vagus nerve, which runs from the brain to several organs, one of which is the heart. The resulting slowing of the heart rate is called vagal tone, and is one of the most important of the nervous system's effects on the cardiovascular system.
During periods of stress or activity, the heart must beat faster than its resting rate. While decreasing vagal tone increases the heart rate somewhat, very fast pulses require additional neural stimulation. In her book "Human Physiology," Dr. Lauralee Sherwood explains that stress and exercise activate the sympathetic nervous system, which prepares the body to fight or flee from attack. One of the effects of the sympathetic nerves innervating the heart is that they increase its rate of beating. Another effect is that the sympathetic nervous system makes the heart beat harder, forcing out a larger volume of blood with each beat, and forcing blood out with greater strength. These factors combine to produce increased blood flow to hard-working body tissues.
In addition to direct neural innervation of the heart, the nervous system can also affect the cardiovascular system through releasing chemical messengers, or neurotransmitters, into the bloodstream. One important neurotransmitter is epinephrine, sometimes called adrenaline. Dr. Thibodeau notes that this compound is released by nerves in a gland that sits on top of each kidney, and when released, travels through the bloodstream to target organs. One such target is the heart; when the neural product epinephrine reaches the heart, it increases pulse rate and contractility, increasing cardiac output and increasing the amount of blood that reaches the body tissues.