A pump for the body
A ventricle is a lower heart chamber that pushes blood out. We will follow the left ventricle, which sends blood to the body.
THE LAB / CORE MEDICINE / LESSON 3
Frank–Starling
& cardiac output.
Why a fuller heart pumps harder, and how filling, heart rate and contraction strength add up to cardiac output.
Educational, not for clinical use
First, blood returns
Between beats, the heart relaxes and fills. Venous return means blood flowing back to the heart through veins. More returning blood can make the next beat start with a fuller chamber.
Filling stretches the muscle
Preload means the stretch of heart muscle just before it contracts. The amount of blood in the filled chamber is a useful stand-in for that stretch.
More stretch, more force
Within its working range, extra stretch helps heart muscle develop more force. This is the Frank–Starling mechanism: more filling helps the heart send more blood out on the next beat.
Blood in one beat
Stroke volume means the blood pushed out by one ventricle in one beat. A fuller chamber usually produces a larger stroke volume when other conditions stay the same.
Follow the rising curve
Read left to right: more filling, more blood per beat. The curve levels off because extra filling cannot keep increasing output forever. This relationship is not a treatment target.
Strength is a separate input
Contractility means contraction strength at the same starting stretch. Stronger muscle pumps more blood from the same filling; weaker muscle pumps less. That shifts the whole curve.
Count the beats
Heart rate means how often the heart beats in a minute. More beats can send more blood forward, provided each beat still has enough time to fill.
The flow over a minute
Cardiac output means the blood one ventricle pumps in a minute. It depends on blood per beat and beats per minute. A very fast rate can reduce filling and shrink each beat.
See the whole picture
Filling, contraction strength and beat frequency work together. We hold resistance to pumping steady here; in the body it matters too. Revisit the pressure–volume loop to connect filling and emptying.
Intravenous fluids: more blood returns
Intravenous (IV) fluids enter a vein and can increase returning blood and preload. On this healthy-heart curve, more filling raises blood per beat. A failing heart may gain little output and develop congestion: excess fluid builds up in the lungs or other body tissues.
Diuretics: less filling
Diuretics help kidneys remove salt and water, reducing preload. Here, less filling means less blood per beat; in congestion, removing excess fluid can relieve symptoms. See the nephron and diuretics.
Positive inotropes: more strength
Positive inotropes are medicines that increase contraction strength. At the same filling, the ventricle pumps more blood per beat, moving to a higher curve.
Beta-blockers: slower and gentler
Beta-blockers reduce stress-signal effects, slowing heart rate and initially weakening contraction. At fixed filling, this model shows less blood per beat. Wider trace gaps show slower beats over the same time. This immediate effect differs from the benefits of long-term treatment in some heart failure.
Venodilators: room in the veins
Venodilators, including nitrates, relax veins so they hold more blood away from the heart. Less blood returns, reducing preload. This isolated effect moves left along the curve; other effects can change overall output.
Heart failure: filling is not enough
Heart failure means impaired pumping or raised filling pressures prevent the heart meeting the body’s needs normally. Here we hold filling constant and show weakened contraction reducing blood per beat. Other forms can have preserved contraction.
Step 1 of 16
Start with Next. Use arrow keys to move at your pace. Pictures are illustrative.
Reference, not a target.
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Markers are illustrative model outputs, not patient measurements. Single typical values are reference markers, not normal ranges. Directions isolate a mechanism; real responses vary.
Sources
One heart. Three moving parts.
Illustrative model — resistance to pumping is held fixed.
How this teaching model works
Stroke volume follows a saturating filling curve scaled by contraction strength, capped at 95% of end-diastolic volume so some blood always remains. The graph uses this same curve. Cardiac output is stroke volume × heart rate. Above the baseline rate, a smooth filling penalty grows more pronounced at high rates. This conceptual model omits reflexes, blood pressure, valve disease and long-term adaptation.