01 / 20

Cardiac Output &
Frank-Starling Law

Understand how the heart adjusts its pumping power based on blood volume, heart rate, and stroke volume to meet the body's demands.

Anatomy & Physiology

What Is Cardiac Output?

Cardiac Output (CO) is the total volume of blood the heart pumps per minute. It is the single most important measure of the heart's overall pumping performance and determines how well oxygen reaches every tissue in the body.

\(HR\)
Number of heartbeats per minute. Typical resting: ~72 bpm
\(SV\)
Volume of blood ejected per beat. Typical resting: ~70 mL
\(CO\)
Total output per minute. Typical resting: ~5 L/min

Why Does \(CO\) Matter?

Every cell in the body depends on a continuous supply of oxygen and nutrients delivered by the blood. If cardiac output drops, tissues become starved. If it rises appropriately, the body can meet increased demands during exercise, stress, or illness.

1
Exercise: \(CO\) can increase from ~5 L/min to over 25 L/min in trained athletes.
2
Heart failure: A failing heart cannot raise \(CO\) to meet tissue demand, causing fatigue and fluid buildup.
3
Hemorrhage: Blood loss reduces \(SV\), dropping \(CO\) and potentially causing shock.

The Three Factors of Stroke Volume (\(SV\))

Stroke volume is controlled by three interrelated factors. Understanding these is essential before exploring the Frank-Starling Law.

Preload
The degree of stretch on the ventricular wall before contraction. More venous return = more preload.
Afterload
The resistance the ventricle must overcome to eject blood (largely determined by arterial pressure).
Contractility
The inherent strength of contraction independent of preload. Increased by sympathetic stimulation.

The Frank-Starling Law

The Frank-Starling Law of the Heart states that the heart will eject a greater stroke volume when it is filled with more blood (i.e., when ventricular end-diastolic volume increases). In other words:

This intrinsic mechanism ensures that the left ventricle ejects the same volume that the right ventricle sends to the lungs, keeping the two sides of the heart in balance.

Visualizing the Frank-Starling Curve

End-Diastolic Volume (Preload) Stroke Volume Normal Heart Enhanced Contractility Heart Failure Plateau

The Sarcomere Mechanism

The Frank-Starling Law works at the cellular level. When cardiac muscle fibers stretch, the overlap between actin and myosin filaments becomes more optimal, increasing the number of cross-bridge connections.

Under-stretched Too much overlap Optimal Stretch Maximum cross-bridges Over-stretched Too little overlap Force Generated Low Maximum Reduced

Preload vs Afterload vs Contractility

Preload Afterload Contractility
Definition Ventricular stretch before contraction Resistance to ejection Intrinsic force of contraction
Main Determinant Venous return / EDV Arterial blood pressure Sympathetic nervous system
Effect on SV when increased \(SV\) increases \(SV\) decreases \(SV\) increases
Frank-Starling Role Primary driver Opposes ejection Shifts curve upward

How to Calculate Cardiac Output (\(CO\))

1
Identify HR: Find the heart rate in beats per minute (bpm).
2
Identify SV: Find the stroke volume in milliliters per beat (mL/beat). If not given directly, use \( SV = EDV - ESV \).
3
Multiply: \( CO = HR \times SV \). Express the result in mL/min or convert to L/min by dividing by 1000.
4
Verify: Normal resting \(CO\) is approximately \( 5{,}000 \) mL/min (~5 L/min). Values far outside this range suggest an error or a clinical condition.

Example 1: Resting \(CO\)

A healthy adult at rest has a heart rate of \(72 \text{ bpm}\) and a stroke volume of \(70 \text{ mL/beat}\). Calculate the cardiac output.

1
Given: \( HR = 72 \text{ bpm} \), \( SV = 70 \text{ mL/beat} \)
2
Apply formula: \( CO = HR \times SV = 72 \times 70 \)
3
Calculate: \( CO = 5{,}040 \text{ mL/min} = 5.04 \text{ L/min} \)

Example 2: Exercise \(CO\)

During intense exercise, the same person's heart rate rises to \(160 \text{ bpm}\) and stroke volume increases to \(110 \text{ mL/beat}\). What is the new cardiac output?

1
Given: \( HR = 160 \text{ bpm} \), \( SV = 110 \text{ mL/beat} \)
2
Apply formula: \( CO = 160 \times 110 \)
3
Calculate: \( CO = 17{,}600 \text{ mL/min} = 17.6 \text{ L/min} \)

Example 3: Finding \(SV\) from \(EDV\) and \(ESV\)

A patient has an \(EDV\) of \(130 \text{ mL}\), an \(ESV\) of \(60 \text{ mL}\), and a heart rate of \(80 \text{ bpm}\). Calculate SV and CO.

1
Find \(SV\): \( SV = EDV - ESV = 130 - 60 = 70 \text{ mL/beat} \)
2
Calculate \(CO\): \( CO = 80 \times 70 = 5{,}600 \text{ mL/min} \)
3
Convert: \( CO = 5.6 \text{ L/min} \)

Regulators of Cardiac Output (\(CO\))

\( CO = HR \times SV \)
Heart Rate (HR) Stroke Volume (SV) Sympathetic ↑ Parasympathetic ↓ Preload (EDV) Contractility Afterload Frank-Starling Law

Clinical Significance

Understanding the Frank-Starling Law is critical in clinical settings. Here is how it applies to common conditions:

Heart Failure
The curve flattens: even with increased preload, the failing ventricle cannot increase \(SV\). Fluid backs up into the lungs.
Hemorrhage
Blood loss reduces venous return, dropping preload and \(SV\). The body compensates with increased \(HR\) to maintain \(CO\).
IV Fluid Therapy
Giving IV fluids increases venous return and preload, leveraging Frank-Starling to boost \(SV\) and \(CO\) in hypovolemic patients.

Common Mistakes

✕ Confusing Preload with Afterload
Preload is about volume (filling before contraction). Afterload is about pressure (resistance during ejection). They affect \(SV\) in opposite directions.
✕ Assuming Unlimited Stretch
The Frank-Starling curve has a plateau. Overstretching the ventricle (as in severe volume overload) does not increase \(SV\) and can worsen function.
✕ Ignoring Units
\(SV\) is in mL/beat, \(HR\) in beats/min, and \(CO\) in mL/min or L/min. Forgetting to convert mL to L leads to answers off by 1000x.

Summary Table

Parameter Symbol Normal Value Formula
Heart Rate \(HR\) 60-100 bpm Measured
End-Diastolic Volume \(EDV\) ~120 mL Measured
End-Systolic Volume \(ESV\) ~50 mL Measured
Stroke Volume \(SV\) ~70 mL/beat \( EDV - ESV \)
Cardiac Output \(CO\) ~5 L/min \( HR \times SV \)
Ejection Fraction \(EF\) 55-70% \( \frac{SV}{EDV} \times 100 \)

Question 1 of 5

A patient has a heart rate of \(80 \text{ bpm}\) and a stroke volume of \(65 \text{ mL/beat}\). What is their cardiac output?

Question 2 of 5

According to the Frank-Starling Law, what happens when venous return increases?

Question 3 of 5

A patient's \(EDV\) is \(140 \text{ mL}\) and \(ESV\) is \(70 \text{ mL}\). What is their ejection fraction?

Question 4 of 5

Which factor is most directly responsible for shifting the Frank-Starling curve upward?

Question 5 of 5

During severe hemorrhage, cardiac output drops primarily because of a decrease in: