Frank-Starling Law of the Heart
This fundamental principle states that the stroke volume of the heart increases in response to an increase in the volume of blood filling the heart (the end-diastolic volume) when all other factors remain constant.
1. The Core Mechanism
- Preload: Increased venous return stretches the ventricular myocardium.
- Sarcomere Length: Increased stretch moves sarcomeres toward their optimal length for force generation, increasing cross-bridge formation.
- Outcome: Increased force of contraction, which increases stroke volume and cardiac output.
2. Curve Shifts
| Condition | Shift of Curve | Physiological Result |
|---|---|---|
| Increased Contractility | Upward and to the left | Higher stroke volume at the same filling pressure. |
| Decreased Contractility (Heart Failure) | Downward and to the right | Lower stroke volume at the same filling pressure. |
Physiology Board Hint: For boards, distinguish between preload (moves you *along* the curve) and contractility (moves the *entire curve*). If a patient is given an inotrope (like digoxin or dobutamine), they move to a new, higher Starling curve.