Which of the following is the primary mechanism by which an increase in venous return leads to an increase in cardiac output according to the Frank-Starling law?

Explanation

## Frank-Starling Mechanism **Key Point:** The Frank-Starling law (also called the length-tension relationship) states that cardiac output increases with increased ventricular preload, up to an optimal fiber length. ### Mechanism of Action When venous return increases: 1. End-diastolic volume (EDV) increases 2. Ventricular myocardial fibers are stretched to a greater length 3. This optimal stretch positions actin and myosin filaments at their ideal overlap 4. Cross-bridge formation and force generation increase 5. Stroke volume and thus cardiac output increase **High-Yield:** This is an **intrinsic property of cardiac muscle** — it does NOT require neural input or hormonal changes. It operates at the sarcomere level through the length-tension relationship. ### Why This Matters Clinically **Clinical Pearl:** The Frank-Starling mechanism is the primary compensatory mechanism in acute volume loading (e.g., blood transfusion, fluid administration) and is responsible for the heart's ability to match venous return to cardiac output automatically. **Mnemonic:** **STRETCH = STRENGTH** — increased stretch of myocardial fibers → increased contractile strength → increased stroke volume. ### Distinction from Other Regulators | Regulator | Mechanism | Time Scale | |-----------|-----------|------------| | **Frank-Starling** | Intrinsic fiber stretch → optimal actin-myosin overlap | Immediate (beat-to-beat) | | Sympathetic activation | Increased contractility via β₁-adrenergic receptors | Seconds | | Heart rate changes | Increased frequency of contractions | Seconds | | Afterload reduction | Decreased resistance to ejection | Seconds to minutes | [cite:Guyton & Hall Textbook of Medical Physiology Ch 9]