Baroreceptors are unique receptors that are located within the walls of blood vessels and are part of the autonomic nervous system. These receptors are a type of mechanoreceptor that measures the amount of pressure within blood vessels. When the pressure rises, the blood vessels expand, and this expansion triggers the firing of baroreceptors within the vessels. Through the usage of baroreceptors the body regulates blood pressure. This process of regulating blood pressure is known as the baroreceptor reflex or the baroreflex.
In 1852, Claude Bernard (1813—1878) discovered that the sympathetic portion of the autonomic nervous system innervated blood vessels in the skin of the rabbits he was studying. This discovery was the first on the road to identifying baroreceptors and their role in regulating blood pressure. Later in 1921, Heinrich Ewald Hering (1866—1948) identified baroreceptors in the carotid artery (located in the neck) and demonstrated that stimulating them resulted in hypotension (low blood pressure) and bradycardia (very slow heart rate). Corneille Heymans (1892—1968) expanded this work and managed to successfully outline the baroreflex, leading him to win the 1939 Nobel Prize in Physiology or Medicine.
Anatomy and Physiology
Baroreceptors are primarily located in the carotid arteries and the aorta. In these locations the receptors can monitor blood pressure going to both the brain and the body. This is essential for proper blood flow to occur throughout the body. Baroreceptors are tonically active receptors. This means that they are constantly firing. Due to this, any change in blood pressure will be reflected in the rate at which the receptors fire. If blood pressure increases, then the baroreceptors fire more quickly, resulting in a physiological response that lowers arterial blood pressure.
The baroreflex is a negative feedback loop, which functions to maintain a relatively constant blood pressure. The feedback loop works in a manner similar to a thermostat that is used to regulate temperature. The baroreflex process starts when baroreceptors detect a change in blood pressure by the change in size of the vessel walls. When blood pressure is high, the vessels will expand, and as a result the baroreceptors will fire at a faster rate. This information is transmitted to the brain through the glossopharyngeal nerve to the cardiovascular control center located in the medulla oblongata of the brainstem. Here the rate of the signals will be interpreted and a response will be generated. If the blood pressure increases, the cardiovascular control center will in turn increase parasympathetic activity and decrease sympathetic activity, resulting in a decrease in heart rate and a dilation of blood vessels. This combination of actions will result in a decrease in blood pressure. The baroreceptors respond to low blood pressure in the opposite manner. Heart rate will increase and the vessels will constrict, resulting in an increase in blood pressure.
Through this process baroreceptors maintain proper blood flow when the body’s position is altered rapidly, such as when a person sits up. Due to gravity, blood would naturally flow downward to the lower extremities. This lowers the amount of blood returning to the heart and in turn lowers the arterial blood pressure. The baroreceptor reflex is triggered, which then causes the heart rate to increase and vessels to constrict, which brings the blood pressure back to normal and allows blood flow to the brain to stay constant.
While the baroreflex monitors the arterial blood pressure, it is rarely attributed to long-term changes in blood pressure such as hypertension (high blood pressure). The reflex is associated with short-term changes in blood pressure. With hypertension there is a general depression of baroreflex sensitivity. Long-term changes in blood pressure are monitored primarily by the kidneys through control of blood volume. Despite this there are some cases in which baroreceptor reflex failure can result in hypertension (Heusser et al., 2005).
Baroreceptor insensitivity has been associated with various diseases and issues within the body. It has been shown that people suffering from chronic obstructive pulmonary disease (COPD) have reduced baroreceptor sensitivity. This results in a decreased response to short-term blood pressure changes as with exercise. This reduction of baroreceptor sensitivity is coupled with an increase in sympathetic activity (van Gestel & Steier, 2010).
In addition, baroreceptor insensitivity has been associated with sleep-related breathing problems (SRBP) in adolescents, which range from chronic snoring to sleep apnea. This insensitivity results in a rise in blood pressure, and researchers suggest it is likely a result of general autonomic dysfunction. Coverdale et al. (2012) have demonstrated that children with more severe SRBP had less sensitive baroreceptor reflexes. In addition to this they showed that the relationship is intensified by a higher body mass index (BMI).
See also: Autonomic Nervous System; Glossopharyngeal Nerve
Coverdale, Nicole S., Laura K. Fitzgibbon, Graham J. Reid, Terrance J. Wade, John Cairney, & Deborah D. O’Leary. (2012). Baroreflex sensitivity is associated with sleep-related breathing problems in adolescents. Journal of Pediatrics, 4, 610—614.
Heusser, Karsten, Jens Tank, Friedrich C. Luft, & Jens Jordan. (2005). Baroreflex failure. Hypertension, 45, 834—839.
Klabunde, Richard E. (2007). Arterial baroreceptor. Retrieved from http://cvphysiology.com/Blood%20Pressure/BP012.htm
van Gestel, Arnoldus J. R., & Joerg Steier. (2010). Autonomic dysfunction in patients with chronic obstructive pulmonary disease. Journal of Thoracic Disease, 4, 215—222.