All of the following are mechanisms by which the baroreceptor reflex increases blood pressure EXCEPT:

Explanation

## Baroreceptor Reflex Mechanism The baroreceptor reflex is a rapid, short-term mechanism for blood pressure regulation. When blood pressure rises, baroreceptors in the carotid sinus and aortic arch are stimulated and send signals via the glossopharyngeal (IX) and vagus (X) nerves to the nucleus tractus solitarius in the medulla. ### Normal Baroreceptor Response to Increased BP **Key Point:** The baroreceptor reflex operates via two complementary pathways: 1. **Increased parasympathetic (vagal) outflow** → decreased heart rate and contractility 2. **Decreased sympathetic outflow** → decreased heart rate, contractility, and vasoconstriction This combination lowers blood pressure back toward normal. ### Response to Decreased BP Conversely, when blood pressure falls, baroreceptor firing decreases, leading to: 1. **Decreased parasympathetic outflow** → increased heart rate 2. **Increased sympathetic outflow** → increased heart rate, contractility, and vasoconstriction ### Why Option 4 Is Incorrect **High-Yield:** Parasympathetic (vagal) innervation does NOT reach systemic blood vessels in a meaningful way. The parasympathetic nervous system: - Innervates the heart (decreases HR and contractility) - Innervates GI, GU, and ocular structures - Does NOT provide vasodilatory innervation to systemic arteries and arterioles Systemic vasodilation during the baroreceptor reflex occurs due to **withdrawal of sympathetic tone**, not active parasympathetic vasodilation. ### Summary Table: Baroreceptor Reflex Components | Mechanism | Effect on BP | Mediated By | | --- | --- | --- | | ↑ Sympathetic to heart | ↑ HR, ↑ contractility | Sympathetic (norepinephrine) | | ↑ Sympathetic to vessels | ↑ Vasoconstriction | Sympathetic (norepinephrine) | | ↑ Parasympathetic to heart | ↓ HR, ↓ contractility | Parasympathetic (acetylcholine) | | Parasympathetic to vessels | None (absent) | — | **Clinical Pearl:** This is why anticholinergic drugs (atropine) block the cardiac parasympathetic effects but do NOT cause vasodilation — there are no parasympathetic receptors on systemic blood vessels to block.