Regarding the regulation of cardiac output, all of the following statements are correct EXCEPT:

Question

Accepted Answer

D. Decreased parasympathetic (vagal) tone increases heart rate and is a primary mechanism for increasing cardiac output during exercise. ## Analysis of Cardiac Output Regulation

**Key Point:** Cardiac output (CO) = Heart rate (HR) × Stroke volume (SV). During exercise, the primary mechanism for increasing CO is **increased sympathetic activity**, NOT decreased parasympathetic tone.

### Evaluation of Each Statement

| Statement | Accuracy | Mechanism |
|-----------|----------|----------|
| Sympathetic ↑ → HR ↑ + contractility ↑ → CO ↑ | **Correct** | Direct β₁-adrenergic effects on SA node and myocardium |
| Frank-Starling: preload ↑ → SV ↑ | **Correct** | Increased sarcomere stretch → optimal overlap of actin-myosin |
| Parasympathetic ↓ → HR ↑ (primary in exercise) | **INCORRECT** | Parasympathetic withdrawal is a *permissive* mechanism; sympathetic activation is the *primary* driver |
| Afterload ↑ → SV ↓ | **Correct** | Increased wall tension reduces ejection fraction |

**High-Yield:** During exercise, CO increases via **two mechanisms in parallel**:
1. **Sympathetic activation** (primary, active) — increases HR and contractility
2. **Parasympathetic withdrawal** (secondary, permissive) — removes vagal brake on HR

The distractor incorrectly implies parasympathetic withdrawal is the *primary* mechanism. In reality, sympathetic activation is the dominant driver; parasympathetic withdrawal merely allows the sympathetic effect to be expressed fully.

**Clinical Pearl:** In a denervated transplanted heart, exercise-induced CO increase is blunted because sympathetic reinnervation is delayed; parasympathetic withdrawal alone cannot account for the normal CO response.

**Mnemonic:** **SCAT** for CO regulation:
- **S**ympathetic = primary ↑ in exercise
- **C**ontractility = Frank-Starling + inotropic effects
- **A**fterload = inverse relationship with SV
- **T**one (parasympathetic) = permissive withdrawal, not primary

Answer

A. The Frank-Starling mechanism states that increased ventricular preload increases stroke volume up to an optimal point

Answer

B. Increased afterload decreases stroke volume by reducing the velocity of ventricular fiber shortening

Answer

C. Increased sympathetic activity increases heart rate and contractility, thereby increasing cardiac output

Regarding the regulation of cardiac output, all of the following statements are correct EXCEPT:

Explanation

Practice similar questions