Baroreceptors are mechanoreceptors in human arteries and veins that regulate blood pressure. They are connected to the medulla oblongata and register changes in blood pressure and heart rate. By keeping blood pressure constant, they perform important tasks in maintaining circulation.
What is a baroreceptor?
One of the most important sensory cells in the sense of touch are the mechanoreceptors. These receptors are the first instance of touch perception of external pressure stimuli. In addition to exteroceptive tasks, mechanoreceptors also perform interoceptive tasks and thus detect pressure stimuli within the human body. The pressoreceptors or baroreceptors are mechanoreceptors of interoception located in the wall of human blood vessels. They continuously collect information about the blood pressure in the arteries and veins. Depending on their localization, baroreceptors can be divided into arterial and venous receptors. Arterial baroreceptors are also called high pressure baroreceptors. They can be assigned to the receptor group proportional-differential receptor. Venous baroreceptors are called low-pressure baroreceptors. The sensory cells in the blood vessels are the main instance for mediating adjustments in cardiac output and total peripheral resistance. The regulation of blood volume also falls within their scope.
Anatomy and structure
Arterial baroreceptors are located at high density primarily in the aortic arch and carotid sinus. The density of pressoreceptors in the other body arteries is much lower compared with these structures. In the border region between the arterial baroreceptors, from a histological point of view, are intertwined nerve fibers that have an oval, lamellar terminal organ. These sensory cells are proportional-differential receptors and therefore register blood pressure changes as well as the value of mean blood pressure. Their discharge rate is not oriented to absolute values. When mean blood pressure changes permanently, the receptors adapt to the new baseline values. Because of their adaptive capacity, after a change in blood pressure, the receptors report the change but no longer send signals if the change in blood pressure persists.
Function and Tasks
In addition to the information mentioned above, ide sensory cells permanently collect information on the rate of change, blood pressure amplitude, and heart rate. They transmit this information as an action potential in proportion to the acting stimulus to the circulatory center of the medulla oblongata, where blood pressure undergoes regulation by means of negative feedback. Afferently, the baroreceptor nerves extend via nerve X or nerve IX to the brainstem, where they project to the nucleus tractus solitarii. The activity of the baroreceptors can be traced by means of baroreceptor reflex. This reflex corresponds to the baroreceptive response to changes in blood pressure. An increase in blood pressure activates the parasympathetic nervous system via the vagus nerve and simultaneously causes the tone of the sympathetic nervous system to drop. This produces a negative chronotropic effect on the heart and dilates the peripheral resistance vessels. On the other hand, when the blood pressure drops, an inhibition of the parasympathetic tone is initiated, the heart rate increases and the total peripheral resistance increases due to a contraction in the resistance vessels. Simultaneously with this reaction, there is an increase in venous return. In the veins of the body, instead of arterial venous baroreceptors are located. Their density is highest in the large body veins and in the right atrium of the heart. These sensory cells are not pressoreceptors but stretch receptors and regulate blood volume. The arterial baroreceptors, in particular, are vital because they keep arterial blood pressure constant and provide on-demand blood supply to the organs. For example, when blood pressure drops sharply after hypovolemic shock, aortic wall hardly dilates. The signal frequency from the pressoreceptors to the medulla oblongata decreases in this way, and the neurons of the medulla oblongata can send out regulatory signals to the heart muscle. The activity of all baroreceptors is permanent and thus fulfills mainly circulatory regulatory functions.
Diseases
The baroreflex is highly relevant medically and is associated primarily with circulatory diseases and blood pressure fluctuations.Every person’s circulatory system is exposed to high stresses day after day. 1000 milliliters of blood move from the legs into the abdominal cavity when merely standing up from a sitting or lying position. An intact baroreflex keeps blood pressure and heart rate constant with minor fluctuations despite these stresses when standing up and lying down. However, if there is damage to the nerves involved in the heart, blood vessels or kidneys, so-called autonomic failure occurs. This phenomenon is also called autonomic neuropathy. The blood pressure of those affected drops sharply when they stand up, and circulatory problems or even fainting occur. Long-standing diabetes, for example, can be responsible for such nerve damage. The baroreceptors themselves can also be affected by damage, for example in the context of severe poisoning. Patients with damaged baroreceptors or lesions of the nerve pathways to the brain are often affected by blood pressure fluctuations of extreme magnitude. Even the slightest exertion or excitement can drive their blood pressure up. The medical profession refers to this as baroreflex failure. A disturbance or failure of the baroreflex can lead to secondary diseases. Above all, defective baroreceptor functions have an effect on the course of chronic cardiovascular diseases, especially high blood pressure. The baroreflex can be examined invasively or non-invasively to prevent secondary diseases. When examining the reflex, the physician usually measures the changes in heart rate that can be provoked by a controlled change in blood pressure. Severe disturbances of the baroreceptor reflex can cause cardiovascular failure. In extreme cases, the consequence of this can be cardiac death.