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Frank Starling curve preload contractility cardiac output physiology

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This is the Frank-Starling curve of the heart.

What the axes mean

  • X-axis: Ventricular EDV / preload
    This means how much blood fills

Quick answer

This is the Frank-Starling curve. It shows how the heart’s pumping changes when more blood enters it.

Axes

  • X-axis: Ventricular EDV / preload
    How much blood is inside the ventricle just before it contracts.
    Think: how full the heart is.
  • Y-axis: Stroke volume / cardiac output
    How much blood the heart pumps out with each beat.
    Think: how strongly and effectively it pumps.

Main rule

More blood enters the heart → heart muscle stretches more → it pumps out more blood.
Like stretching a rubber band slightly before releasing it: a suitable stretch gives a stronger recoil.
This is why, during exercise, extra blood returns to the heart and the heart pumps more blood to the muscles.

Reading the curves

1. Black curve: Normal heart at rest

A normal heart pumps more as filling increases.
  • At rest, it needs relatively little output.
  • With more filling, output rises.
  • At higher filling, the curve flattens because the heart cannot keep increasing output forever.

2. Blue curve: Normal heart during exercise

During exercise, sympathetic stimulation and adrenaline make the heart contract more strongly.
So, even at the same preload, it pumps much more blood than at rest.
That is why this curve lies higher and to the left.

3. Lower purple/brown curve: Heart failure

In heart failure, the heart muscle is weak and contractility is reduced.
So, for the same amount of filling, it pumps out less blood.
The body may retain salt and water, increasing preload, but this only helps a little. Too much extra filling mainly causes congestion, such as lung fluid and leg swelling.

4. Yellow curve: Heart failure + positive inotrope

A positive inotrope is a drug or stimulus that makes the heart contract more strongly.
In a patient with heart failure, it moves the curve upward:
  • Better contraction
  • Higher stroke volume
  • More cardiac output for the same preload
But it usually does not return the curve completely to normal.

Easy one-line summary

  • Move right on the same curve: more filling, so more pumping.
  • Move up to another curve: stronger heart muscle contraction.
  • Move down: weaker contraction, as in heart failure.
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