The baroreceptor reflex is a classic example of a negative feedback loop aimed at controlling mean arterial pressure (MAP). When blood pressure rises, the arterial baroreceptors in the carotid sinuses and aortic arch are stretched, increasing their rate of action potential firing. These signals travel via the glossopharyngeal nerve (carotid baroreceptors) and the vagus nerve (aortic baroreceptors) to the nucleus tractus solitarius (NTS) in the medulla oblongata. The NTS processes these signals and stimulates the parasympathetic nervous system while inhibiting the sympathetic nervous system.
The result of this autonomic adjustment is a reduction in heart rate (negative chronotropy), decreased cardiac contractility (negative inotropy), and vasodilation of peripheral blood vessels. Together, these changes lower blood pressure by reducing cardiac output and total peripheral resistance. Conversely, when blood pressure drops, the baroreceptor firing rate decreases, leading to a reduction in parasympathetic activity and an increase in sympathetic outflow. This causes vasoconstriction, an increase in heart rate, and enhanced cardiac contractility, all of which raise blood pressure back to normal levels.
The baroreceptor reflex operates on a very short timescale, responding within seconds to changes in blood pressure. This reflex is critical during postural changes (e.g., standing up quickly) to prevent orthostatic hypotension, where blood pressure may suddenly drop, causing dizziness or fainting. Additionally, baroreceptors play a key role during periods of stress, such as during exercise or emotional responses, by modulating the cardiovascular response to maintain adequate perfusion to vital organs.
| Step | Blood pressure rises | Blood pressure falls |
|---|---|---|
| Vessel wall | Carotid sinus and aortic arch walls stretch | Vessel walls relax, with less stretch |
| Baroreceptor firing | Rate of action potential firing increases | Firing rate decreases |
| Sympathetic outflow | Inhibited | Increased |
| Parasympathetic activity | Stimulated | Reduced |
| Heart rate | Falls — negative chronotropy, bradycardia | Rises — tachycardia |
| Contractility | Decreased — negative inotropy | Enhanced |
| Vascular tone | Vasodilation of peripheral vessels | Vasoconstriction |
| Net result | Cardiac output and total peripheral resistance fall, lowering blood pressure | Cardiac output and resistance rise, returning blood pressure to normal |
