Frank–Starling mechanism
The Frank–Starling mechanism is the heart's built-in rule that the more it is filled, the harder it contracts — stretch it and it pushes back stronger, like a longer pull on a bow shooting an arrow farther.
Physiologically, stretching cardiac muscle fibers by greater filling improves the overlap and sensitivity of the contractile machinery, so the next contraction generates more force and ejects more blood. This lets the heart automatically match its output to the amount of blood returning to it, beat by beat, with no nerve signal required.
This self-adjustment keeps the two sides of the heart balanced: if one ventricle suddenly receives more blood, it ejects more, preventing blood from backing up. It is why a healthy heart can handle a sudden change in venous return so smoothly.
The relationship holds only up to a point. Overstretching a chronically overloaded or failing heart no longer increases force and can worsen congestion — a failing ventricle operates on a flatter, lower Frank–Starling curve.
When you stand up and then lie down, venous return shifts; the Frank–Starling mechanism lets each ventricle instantly adjust its stroke volume to whatever is now flowing in.
The mechanism continuously matches output to venous return without conscious control.
The mechanism explains why increasing preload increases stroke volume. It is named for physiologists Otto Frank and Ernest Starling, who described it in the late 1800s and early 1900s.