In the regulation of cardiac output, which autonomic mechanism acts to increase heart rate and contractility in response to a decrease in blood pressure?

Explanation

## Sympathetic Regulation of Cardiac Output **Key Point:** The sympathetic nervous system is the primary autonomic mechanism for rapid increases in cardiac output in response to decreased blood pressure or increased metabolic demand. ### Sympathetic Pathway and Effects 1. **Baroreceptor reflex activation** — decreased blood pressure detected by carotid sinus and aortic arch baroreceptors 2. **Increased sympathetic outflow** — via cardiac accelerator nerves 3. **β₁-adrenergic receptor activation** on: - **Sinoatrial (SA) node** → increased heart rate (positive chronotropic effect) - **Atrioventricular (AV) node** → increased conduction velocity (positive dromotropic effect) - **Ventricular myocardium** → increased contractility (positive inotropic effect) 4. **Result:** Increased cardiac output via both increased heart rate and increased stroke volume **High-Yield:** Sympathetic activation increases cardiac output through **three mechanisms**: ↑ HR, ↑ contractility, and ↑ conduction velocity. This is the body's rapid response to hypotension or stress. ### Molecular Mechanism ``` Norépinéphrine → β₁-adrenergic receptor → ↑ cAMP → ↑ Ca²⁺ influx → ↑ contractility & HR ``` **Clinical Pearl:** This is why β-blockers (e.g., metoprolol, atenolol) reduce cardiac output — they block the sympathetic effects on the heart and are used in hypertension and heart failure management. ### Comparison: Sympathetic vs. Parasympathetic | Parameter | Sympathetic | Parasympathetic | |-----------|-------------|------------------| | **Neurotransmitter** | Norepinephrine | Acetylcholine | | **Receptor** | β₁-adrenergic | M₂ muscarinic | | **Effect on HR** | ↑ (positive chronotropic) | ↓ (negative chronotropic) | | **Effect on contractility** | ↑ (positive inotropic) | ↓ (minimal direct effect) | | **Effect on AV conduction** | ↑ (positive dromotropic) | ↓ (negative dromotropic) | | **Response to** | Stress, hypotension, exercise | Rest, digestion, vagal maneuvers | **Mnemonic:** **SYMPATHETIC = SPEED & STRENGTH** — increases both heart rate (speed) and contractility (strength) to boost cardiac output. [cite:Harrison's Principles of Internal Medicine 21e Ch 297]