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?
A. Decreased afterload on the left ventricle
B. Increased sympathetic nervous system activity
C. Increased heart rate mediated by baroreceptor reflex
D. Increased stretch of ventricular myocardial fibers leading to optimal overlap of actin and myosin filaments
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-YieldNEET PG
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.
