## Baroreceptor Reflex Mechanism **Key Point:** The baroreceptor reflex is the primary acute blood pressure regulation mechanism. When arterial pressure rises, baroreceptors in the carotid sinus and aortic arch increase their firing rate, triggering a parasympathetic response and withdrawal of sympathetic tone. ### Neural Pathway 1. Increased arterial pressure → baroreceptor stretch 2. Increased afferent signalling via CN IX (carotid sinus) and CN X (aortic arch) 3. Activation of nucleus tractus solitarius in the medulla 4. Increased parasympathetic (vagal) outflow → decreased heart rate and contractility 5. Decreased sympathetic outflow → vasodilation and reduced peripheral resistance 6. Net result: blood pressure falls ### Dual Response | Component | Effect | Result | |-----------|--------|--------| | Parasympathetic ↑ | ↓ HR, ↓ contractility | ↓ Cardiac output | | Sympathetic ↓ | Vasodilation | ↓ Peripheral resistance | | Combined | Both reduce BP | Rapid pressure normalization | **High-Yield:** The baroreceptor reflex operates within seconds and is the fastest blood pressure regulation mechanism. It is effective for acute fluctuations but not for chronic hypertension (baroreceptor adaptation occurs over days). **Clinical Pearl:** Loss of baroreceptor function (as in carotid sinus hypersensitivity or after carotid surgery) results in orthostatic hypotension and blood pressure lability.
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