Frank Starling Mechanism Definition

Frank Starling Mechanism Definition is the intrinsic property of cardiac muscle by which an increase in the diastolic stretch of myocardial fibers, produced by greater ventricular filling, results in a correspondingly greater force of contraction and stroke volume during the subsequent systole. This mechanism allows the heart to automatically match its output to venous return without requiring changes in heart rate or external neural and hormonal control, matching the volume ejected to the volume received on a beat-to-beat basis.

Cellular Basis

The Frank–Starling mechanism arises from the length-dependent properties of the sarcomere, the basic contractile unit of cardiac muscle.

Sarcomere Length and Force Generation

As myocardial fibers are stretched by increased diastolic filling, the overlap between actin and myosin filaments within the sarcomere is optimized up to a point, increasing the number of cross-bridges that can form and thereby increasing the force generated during contraction.

Calcium Sensitivity

Stretching cardiac muscle fibers also increases the sensitivity of the contractile proteins to calcium, further enhancing force generation independent of any change in the amount of calcium released during excitation-contraction coupling.

Relationship Between Preload and Stroke Volume

The Frank–Starling mechanism is most commonly represented as a relationship between a measure of ventricular filling and the resulting stroke volume or ventricular performance.

The Frank–Starling Curve

Plotting stroke volume, or a related measure such as cardiac output, against a measure of preload, such as end-diastolic volume or end-diastolic pressure, produces the Frank–Starling curve, which rises steeply at lower filling volumes and gradually flattens as filling approaches the physiological limit of the mechanism.

Shifts in the Curve

Changes in contractility or afterload shift the entire Frank–Starling curve upward or downward, meaning that for the same degree of filling, a more contractile or less loaded heart produces a greater stroke volume than a less contractile or more heavily loaded heart.

Physiological Significance

The Frank–Starling mechanism provides an immediate, intrinsic means of matching the output of the two ventricles to one another and to the body's momentary venous return.

Balancing the Two Ventricles

Because the right and left ventricles must eject equal volumes over time despite operating in series within a closed circulation, the Frank–Starling mechanism helps automatically equalize their outputs, since any transient increase in filling of one ventricle increases its output to match.

Response to Increased Venous Return

During conditions such as exercise, when venous return rises, the Frank–Starling mechanism allows stroke volume to increase immediately, before other regulatory mechanisms such as sympathetic activation take full effect.

Diagrammatic Summary

Clinical Relevance

Impairment of the Frank–Starling mechanism, as occurs in failing myocardium, results in a flattened curve in which increased filling produces little or no additional stroke volume, a hallmark physiological feature of systolic heart failure and an important consideration in guiding fluid management in critically ill patients.