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Cardiac Output

Volume of blood pumped per minute.

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Core idea

Overview

Cardiac Output represents the total volume of blood pumped by the heart per unit of time, typically measured in liters per minute. It serves as a primary indicator of the circulatory system's ability to meet the metabolic demands of the body's tissues.

When to use: This equation is used to assess cardiovascular health and hemodynamic stability during clinical exams or exercise physiology testing. It assumes the heart is beating at a consistent rate and that the volume of blood ejected per contraction is representative of the period being measured.

Why it matters: Calculating cardiac output is vital for diagnosing conditions like heart failure or circulatory shock and for monitoring patient responses to intravenous fluids or vasoactive medications. It illustrates how the heart adjusts its performance through changes in rhythm or contractility to sustain systemic blood pressure.

Symbols

Variables

CO = Cardiac Output, HR = Heart Rate, SV = Stroke Volume

CO
Cardiac Output
ml/min
HR
Heart Rate
bpm
SV
Stroke Volume
ml

Walkthrough

Derivation

Formula: Cardiac Output

Cardiac output is the volume of blood pumped by the heart per minute, usually quoted for the left ventricle.

  • Heart rate and stroke volume are measured under steady conditions.
  • Left and right ventricles pump approximately the same volume over time.
1

Identify Heart Rate and Stroke Volume:

Measure heart rate (beats per minute) and stroke volume (volume ejected per beat).

2

Calculate Cardiac Output:

Multiply HR by SV to obtain CO, typically in L/min.

Result

Source: AQA A-Level Biology — Mass Transport

Free formulas

Rearrangements

Solve for HR

Make HR the subject

Heart Rate can be found by dividing Cardiac Output by Stroke Volume.

Difficulty: 2/5

Solve for SV

Make SV the subject

Stroke Volume can be found by dividing Cardiac Output by Heart Rate.

Difficulty: 2/5

The static page shows the finished rearrangements. The app keeps the full worked algebra walkthrough.

Visual intuition

Graph

The graph displays a straight line starting from the origin where Cardiac Output increases at a constant rate as Heart Rate increases, reflecting a directly proportional relationship where the slope represents Stroke Volume. For a biology student, this linear shape demonstrates that at low heart rates the volume of blood pumped per minute is minimal, while high heart rates result in a significantly larger volume of blood being moved through the body. The most important feature of this curve is its linearity, which means that doubling the Heart Rate will exactly double the Cardiac Output provided the Stroke Volume remains constant. Since Heart Rate must be positive, the domain is restricted to values greater than zero.

Graph type: linear

Why it behaves this way

Intuition

Imagine the heart as a rhythmic pump, where the frequency of its contractions (heart rate) combined with the volume expelled per contraction (stroke volume)

CO
The total volume of blood pumped by the heart's left ventricle into the systemic circulation per minute.
This is the overall 'flow rate' of blood leaving the heart. A higher cardiac output means more blood is circulating to meet the body's demands.
HR
The number of times the heart beats per minute.
This represents how frequently the heart contracts. A faster heart rate means the heart is pumping more often.
SV
The volume of blood ejected by the left ventricle with each beat.
This is how much blood is pushed out with each individual pump. A larger stroke volume means each beat is more effective at moving blood.

Free study cues

Insight

Canonical usage

Ensuring dimensional consistency where the product of heart rate (inverse time) and stroke volume (volume) yields cardiac output (volume per time).

Common confusion

A common mistake is using inconsistent time units (e.g., seconds for one variable and minutes for another) or failing to understand how the 'beats' unit in HR and SV cancels out to yield a volume/time unit for cardiac

Unit systems

COL/min - Cardiac output is typically measured in liters per minute (L/min) or milliliters per minute (mL/min).
HRbeats/min - Heart rate is the number of beats per minute (bpm). For dimensional analysis, 'beats' is often treated as a dimensionless count.
SVmL/beat - Stroke volume is the volume of blood ejected per beat, commonly in milliliters per beat (mL/beat). When HR is in beats/min, SV must be in volume/beat for CO to be in volume/min.

Ballpark figures

  • Quantity:
  • Quantity:
  • Quantity:

One free problem

Practice Problem

Practice Problem 1

An athlete has a resting heart rate of 55 beats per minute and a stroke volume of 90 mL per beat. Calculate the cardiac output in mL per minute.

Heart Rate55 bpm
Stroke Volume90 ml

Solve for: CO

Hint: Multiply the heart rate by the stroke volume to find the total volume pumped in one minute.

Practice Problem 2

A patient’s cardiac output is measured at 6000 mL per minute, and their stroke volume is recorded as 75 mL per beat. Determine the patient's heart rate in beats per minute.

Cardiac Output6000 ml/min
Stroke Volume75 ml

Solve for: HR

Hint: Rearrange the formula to isolate heart rate by dividing cardiac output by stroke volume.

Practice Problem 3

During a stress test, a participant reaches a cardiac output of 18,000 mL per minute with a heart rate of 150 beats per minute. What is the participant's stroke volume in mL?

Cardiac Output18000 ml/min
Heart Rate150 bpm

Solve for: SV

Hint: Isolate stroke volume by dividing the total cardiac output by the heart rate.

The full worked solution stays in the interactive walkthrough.

Where it shows up

Real-World Context

When estimating cardiac output during exercise testing in a lab, Cardiac Output is used to calculate the CO value from Heart Rate and Stroke Volume. The result matters because it helps turn a changing quantity into a total amount such as area, distance, volume, work, or cost.

Study smarter

Tips

  • Ensure heart rate and stroke volume units are consistent, typically beats per minute and milliliters per beat.
  • To convert the result from mL/min to L/min, divide the final Cardiac Output by 1000.
  • Understand that factors like preload, afterload, and contractility primarily influence the Stroke Volume variable.

Avoid these traps

Common Mistakes

  • Mixing bpm with seconds.
  • Forgetting to convert mL to L.

Common questions

Frequently Asked Questions

Cardiac output is the volume of blood pumped by the heart per minute, usually quoted for the left ventricle.

This equation is used to assess cardiovascular health and hemodynamic stability during clinical exams or exercise physiology testing. It assumes the heart is beating at a consistent rate and that the volume of blood ejected per contraction is representative of the period being measured.

Calculating cardiac output is vital for diagnosing conditions like heart failure or circulatory shock and for monitoring patient responses to intravenous fluids or vasoactive medications. It illustrates how the heart adjusts its performance through changes in rhythm or contractility to sustain systemic blood pressure.

Mixing bpm with seconds. Forgetting to convert mL to L.

When estimating cardiac output during exercise testing in a lab, Cardiac Output is used to calculate the CO value from Heart Rate and Stroke Volume. The result matters because it helps turn a changing quantity into a total amount such as area, distance, volume, work, or cost.

Ensure heart rate and stroke volume units are consistent, typically beats per minute and milliliters per beat. To convert the result from mL/min to L/min, divide the final Cardiac Output by 1000. Understand that factors like preload, afterload, and contractility primarily influence the Stroke Volume variable.

References

Sources

  1. Guyton and Hall Textbook of Medical Physiology
  2. Wikipedia: Cardiac output
  3. Britannica: Cardiac output
  4. Britannica, The Editors of Encyclopaedia. 'Cardiac output'. Encyclopedia Britannica, 26 Jan. 2024.
  5. Wikipedia, 'Cardiac output' (as of 15 May 2024).
  6. Hall, John E., and Michael E. Hall. Guyton and Hall Textbook of Medical Physiology. 14th ed. Elsevier, 2021.
  7. Guyton and Hall Textbook of Medical Physiology, 14th Edition
  8. Human Physiology: From Cells to Systems by Sherwood, 9th Edition