## Sympathetic Nervous System Effects on Blood Pressure **Key Point:** Sympathetic activation increases blood pressure through three simultaneous mechanisms: increased heart rate (positive chronotropic effect), increased myocardial contractility (positive inotropic effect via β₁-adrenergic receptors), and increased peripheral vascular resistance (via α₁-adrenergic-mediated vasoconstriction). ### Mechanism of Action The sympathetic nervous system acts on adrenergic receptors distributed throughout the cardiovascular system: | Receptor Type | Location | Effect on BP | | --- | --- | --- | | β₁-adrenergic | Heart (SA node, ventricles) | ↑ HR, ↑ contractility → ↑ CO | | α₁-adrenergic | Arterioles, veins | ↑ vasoconstriction → ↑ SVR | | β₂-adrenergic | Skeletal muscle vessels | Vasodilation (minor effect) | ### Formula for Blood Pressure $$BP = CO \times SVR$$ where CO = Heart Rate × Stroke Volume Sympatheticactivation increases both CO (via ↑ HR and ↑ contractility) and SVR (via ↑ vasoconstriction), resulting in a synergistic rise in blood pressure. **High-Yield:** The integrated sympathetic response is coordinated by the vasomotor center in the medulla oblongata in response to baroreceptor feedback, chemoreceptor input, and higher brain centers. **Clinical Pearl:** This is why acute stress (fight-or-flight response) causes immediate blood pressure elevation — sympathetic discharge increases all three components simultaneously.
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