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

Question

Accepted Answer

C. Sympathetic activation via β₁-adrenergic receptors on the sinoatrial and atrioventricular nodes and ventricular myocardium. ## 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]

Answer

A. Increased parasympathetic tone to the coronary vasculature

Answer

B. Parasympathetic activation via the vagus nerve

Answer

D. Decreased sympathetic outflow to the peripheral vasculature

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

Sympathetic Pathway and Effects

Molecular Mechanism

Comparison: Sympathetic vs. Parasympathetic

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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