Baroreceptors, also called pressoreceptors, are sensory nerve endings in human blood vessels that detect blood pressure levels and report abnormal blood pressure to the central nervous system, which responds by regulating the resistance of the blood vessels and the rate and strength of the heart's contractions. This process is known as the baroreflex. Baroreceptors work by detecting stretching in the blood vessel walls.
There are two major types of baroreceptors, arterial or high pressure and low pressure. The former is found only in the aortic arch leading from the heart and in the the carotid sinuses at the origin of the carotid arteries, the major arteries supplying the brain. As their name implies, high pressure baroreceptors are active at the area of the circulatory system where blood pressure is at its highest.
Arterial baroreceptors respond very quickly to changes in blood pressure, firing more rapidly as blood pressure increases, resulting in a lowered heart rate and lowered blood vessel resistance. When they stop firing, the central nervous system responds by increasing heart rate and blood vessel resistance. They respond only to short term changes, so if blood pressure remains high over a few days, they will reset themselves to the new "normal" blood pressure in the body, resulting in hypertension.
Low pressure baroreceptors regulate blood volume in the body. They are located in the walls of major veins and in the right atrium of the heart, which receives deoxygenated blood from the body. In addition to effecting the circulatory system, low pressure baroreceptors can also have an effect on the kidneys, causing the retention of salt and water if blood volume is too low. If they are stimulated through the stretching of vein walls, the kidneys respond by secreting excess salt and water through urination. Through these means, they help maintain homeostasis, or biological stability.