When the heart muscle contracts, it forces oxygenated blood through all of the major vessels in the body, and out into the capillaries where the oxygen is delivered to the tissues. Blood pressure is the force of circulating blood against the walls of the arteries. The pressure of blood in the arteries correlates directly to the amount of blood pumped by the heart and the amount of resistance in the peripheral blood vessels, according to Merck Manuals Medical Library. Blood pressure is normally held within a narrow range to provide an optimal amount of blood to the tissues without damaging the walls of the vessels. Although many factors influence blood pressure, there are two primary ways the body maintains blood pressure control.
The Central Nervous System
The sympathetic nervous system provides a quick response to a drop in blood pressure. Areas of the carotid artery and the aorta contain pressure-sensitive receptors that detect tiny changes in blood pressure. When pressure falls, these receptors send signals to the brain stem. Hormones are then released from the adrenal glands, causing the heart to beat faster and to pump blood more forcefully. The diameter of the peripheral blood vessels narrows, pushing more blood through a tighter space. These events lead to a rise in blood pressure. Conversely, when blood pressure rises, the heart muscle is stretched. This results in the release of a hormone from the heart that brings blood pressure back down.
Long-term blood pressure control is provided by the kidneys due to its effects on blood volume, explains the American Heart Association. The kidneys, like the aorta and carotids, contain receptors that respond to pressure changes in the arteries. If blood pressure falls too low, an enzyme called renin is released from the kidneys. This enzyme drives the production of a potent constrictor of blood vessels called angiotensin II, creating a rise in blood pressure.
Angiotensin II also causes the body to retain sodium which leads to an increase in blood volume, further contributing to an elevation in blood pressure. On the other hand, when blood pressure rises too far, the kidneys excrete more sodium and water, bringing pressure back into the normal range.