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baroreceptors are nerves which detect changes in the level of arterial blood pressure, in much the same way as a barometer detects changes in atmospheric pressure. These nerves are found in the walls of certain arteries and are stimulated by the stretch of the artery due to changes in the pressure of the blood. Stretch of the receptors gives rise to nerve impulses, which travel to the brain. They have a threshold — the minimum pressure required to cause some stimulation; an operating range, where changes in blood pressure cause proportionate changes in the frequency of the nerve impulses: and a saturation level — the pressure above which no increase in stimulation can occur. Not all baroreceptors have exactly the same operating ranges, so as blood pressure increases not only is the activity in individual nerves increased, but more receptors become active.

Baroreceptors do not just respond to the static level of blood pressure but they also respond to changes in pressure. These are their phasic properties. If blood pressure abruptly increases, baroreceptors initially give rise to a high frequency of nerve impulses, but then this declines to a lower, steady level. When pressure changes in a pulsatile way, as it does in arteries, baroreceptors discharge mainly or entirely during the rising phase of the pulse and are usually silent as pressure falls.

Much of our modern understanding of the ways in which baroreceptors function results from the work of the Belgian physiologist and Nobel Laureate Corneille Heymans and his collaborators. They used elaborate cross-circulation experiments in anaesthetized dogs to investigate in detail the reflex pathways by which the baroreceptors signalled changes in blood pressure.

Baroreceptors provide the input signal for a reflex which controls and stabilizes blood pressure. If pressure rises, baroreceptors are stimulated more and changes are brought about to reduce the pressure. Conversely, decreases in blood pressure are also limited by the baroreceptor reflex.

The best known and most extensively studied baroreceptors are those in the carotid sinuses, dilatations of the carotid arteries in the neck. The importance of the carotid baroreceptors is that they are ideally placed to ensure that the brain receives an adequate blood supply. The significance of this region was well known in antiquity, as compression of the carotid arteries could induce drowsiness — indeed, ‘carotid’ comes from the Greek word for deep sleep. Baroreceptors are also found in other major arteries, including the aortic arch and several of its branches, such as the coronary arteries which supply the heart muscle with blood.

In addition to arterial baroreceptors, low-pressure baroreceptors exist and these respond to the degree of stretch of veins and the cardiac atria. The best known of these are atrial receptors, which are nerves ending mainly at the junctions of the great veins with the atria. They are stimulated by stretch and can detect heart filling and possibly even blood volume. The reflex responses to their distension are an increase in heart rate, which reduces the filling at each beat, and an increase in urine flow, which reduces blood and extracellular fluid volume.

Roger Hainsworth, and E. M. Tansey

See also blood pressure; body fluids; sensory receptors.