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Frank-Starling Mechanism of the Heart



  • Adjusts cardiac output based on venous return.

    • Increase in venous return ⟶ Cardiac output increases

    • Decrease in venous return ⟶ Cardiac output decreases

  • Internal to heart i.e. operates without any external control on the heart.


Length-Tension Relationship

  • Contraction of muscle depends on interaction between actin and myosin filaments.

  • The force of contraction depends on how many such interactions can occur.

  • The number of interactions in tern depends on the initial length of the muscle.

At Short Lengths

Opposing acting filaments overlap each other

↓

Spatial orientation between actin and myosin is distorted

↓

Less chance of actin-myosin interaction

↓

Less force of contraction.


As Length Increases

  • As the initial length of muscle increases, two things happen that increase force of contraction.


1. Increase in length

↓

Actin and myosin come in more optimal position for interaction

↓

Chance of interaction increase

↓

Force of contraction increases.



2. Increase in muscle length (Stretching)

↓

Muscle diameter decreases

↓

At molecular level: The side-by-side distance between actin and myosin decreases

↓

Chance of interaction increases

↓

Force of contraction increases.


At Higher Lengths

Actin filaments are pulled beyond myosin filaments

↓

Interaction decreases

↓

Less or no force is generated


In short, starting from the shortest length:

  • Initially with increasing length: the force of contraction increases. This occurs up to a limit.

  • After a limit: With increasing initial length, the force decreases.


Frank-Starling Mechanism of the Heart

  • Normally the heart never crosses the limit, so with increasing initial length, the force of contraction increases.


Increase in venous return

↓

↓

Increase in initial length of ventricular musculature

↓

Increase in force of contraction as explained in length-tension relationship

↓

Increased cardiac output


  • Thus, increased venous return ⟶ increases cardiac output.


Importance

Equalizes output from right & left heart

  • Maintains proper distribution of blood in pulmonary and systemic circulation.

  • For example:

If right heart pumps more blood

↓

more blood reaches pulmonary circulation

↓

Increased venous return to left heart

↓

Frank-Starling mechanism

↓

Increased output by left heart

↓

Extra blood is cleared from pulmonary circulation

  • Without this adjustment by the left heart, the blood would accumulate in pulmonary circulation.


When more cardiac output is needed

  • Some control systems to increase cardiac output do not act directly on the heart. They just increase venous return to the heart. The heart then automatically increases cardiac output due to the Frank-Starling mechanism.

  • For example:

Exercise

↓

1. Increased muscle pump activity.

2. Sympathetic activation causes vasoconstriction.

↓

Increased venous return

↓

Frank-Starling mechanism

↓

Increased cardiac output

↓

Meets the demand of muscles for exercise

 

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