Cardiac output is the amount of blood the heart pumps each minute, while stroke volume is the amount pumped with each heartbeat. Cardiac output equals heart rate multiplied by stroke volume, so both measurements help explain how the heart supplies blood to the body.
These measurements connect the structure of the heart to its function, a central topic in human anatomy. They also explain why the heart pumps more blood during exercise and how changes in circulation can affect the delivery of oxygen and nutrients.
What Is Cardiac Output?
Cardiac output (CO) measures the volume of blood pumped by one ventricle of the heart in one minute. It is usually expressed in liters per minute (L/min). A typical resting cardiac output for a healthy adult is approximately 5 L/min, although the actual amount varies with body size, activity, and other factors.
The formula is straightforward: Cardiac Output = Heart Rate × Stroke Volume. Heart rate is measured in beats per minute, and stroke volume is measured in milliliters per beat. Multiplying them gives milliliters per minute, which can be divided by 1,000 to obtain liters per minute.
Cardiac output helps determine how much oxygenated blood reaches the systemic circulation. The relationship between oxygen delivery and tissue demand explains why blood flow must adjust as cells become more active. The movement of that blood through arteries also depends on pressure and resistance, concepts explored in hemodynamics in large vessels.
What Is Stroke Volume?
Stroke volume (SV) is the volume of blood ejected by one ventricle during a single contraction. It is measured in milliliters per beat (mL/beat), with typical resting values in adults often ranging from approximately 60 to 100 mL.
Stroke volume depends on how much blood fills the ventricle before contraction and how much remains afterward. It can be calculated as Stroke Volume = End-Diastolic Volume − End-Systolic Volume. End-diastolic volume is the blood present after filling, while end-systolic volume is the blood remaining after contraction.
For example, if a ventricle contains 120 mL before contraction and 50 mL afterward, its stroke volume is 70 mL. The ventricle does not normally eject all the blood it contains with every beat.
How to Calculate Cardiac Output
Consider a person resting with a heart rate of 70 beats per minute and a stroke volume of 70 mL per beat.
- Multiply heart rate by stroke volume: 70 × 70 = 4,900 mL/min.
- Convert milliliters to liters: 4,900 ÷ 1,000 = 4.9 L/min.
- The calculated cardiac output is 4.9 liters per minute.
If heart rate increases to 100 beats per minute while stroke volume remains at 70 mL, cardiac output becomes 7 L/min. This example illustrates the mathematical relationship, although stroke volume can also change when heart rate changes.
Factors That Affect Stroke Volume
Three major factors influence how much blood a ventricle ejects with each contraction: preload, contractility, and afterload.
Preload: How Much the Heart Fills
Preload describes the stretching of ventricular muscle before contraction, which is related to the amount of blood returning to and filling the heart. Within normal physiological limits, greater filling produces a stronger contraction through the Frank-Starling mechanism, increasing stroke volume.
Contractility: How Strongly the Heart Contracts
Contractility describes the heart muscle’s ability to generate force at a given preload and afterload. Sympathetic nervous system activity and hormones such as epinephrine can increase contractility, allowing the ventricle to eject more blood. Reduced contractility can decrease stroke volume.
Afterload: Resistance to Ejection
Afterload is the load the ventricle must overcome to eject blood. For the left ventricle, it is influenced by arterial pressure and other factors. Increased afterload can reduce stroke volume if the heart cannot compensate, because more force is required to push blood into the arteries.
How Heart Rate Influences Cardiac Output
Heart rate determines how frequently the ventricles eject blood. Increasing heart rate generally raises cardiac output when stroke volume is maintained, but extremely rapid heart rates can shorten ventricular filling time and reduce stroke volume.
The coordinated sequence of filling and contraction is explained in how the heart pumps blood. Electrical signals and autonomic nervous system activity adjust the pace of these contractions, as described in heart rate control and autonomic regulation.
A slower resting heart rate does not automatically indicate inadequate cardiac output. For example, endurance-trained individuals may maintain sufficient resting blood flow with fewer beats because their stroke volume is greater.
Changes During Exercise
During exercise, working muscles require more oxygen. The sympathetic nervous system increases heart rate and contractility, while greater venous return can help increase stroke volume. Together, these responses raise cardiac output.
Resting cardiac output is commonly around 5 L/min, but it can rise to approximately 20–25 L/min during intense exercise in healthy young adults, with higher values possible in trained endurance athletes. These changes are part of the broader circulatory adaptations during exercise.
When Cardiac Output Changes Abnormally
Cardiac output varies naturally with physical activity and the body’s metabolic needs. However, inadequate output may reduce blood supply to tissues, potentially contributing to fatigue, dizziness, or impaired organ function. Heart failure and certain serious circulatory conditions can cause reduced cardiac output.
Unusually high cardiac output can occur in conditions such as severe anemia or hyperthyroidism. Whether an output is appropriate depends on the person’s circumstances, not simply whether the measured value is above or below an average.
Frequently Asked Questions
What is the main difference between cardiac output and stroke volume?
Stroke volume measures blood ejected per heartbeat, while cardiac output measures blood pumped per minute. Cardiac output combines stroke volume with heart rate.
Can cardiac output increase without an increase in heart rate?
Yes. If stroke volume increases enough while heart rate stays constant, cardiac output increases. Greater ventricular filling or stronger contraction can contribute to this change.
Is a higher stroke volume always better?
No. A higher stroke volume can be a normal adaptation to endurance training, but it must be interpreted alongside heart size, function, blood pressure, and overall health. A single measurement does not establish whether the heart is functioning normally.
Cardiac output describes the heart’s total pumping volume over time, while stroke volume reveals how much blood is ejected with each contraction. Understanding both measurements makes it easier to explain changes in circulation during rest, exercise, and certain medical conditions.
This article is for general education and is not medical advice. Talk to a qualified health professional about any medical concern.
Sources: Guyton and Hall Textbook of Medical Physiology; Boron and Boulpaep Medical Physiology; American Heart Association.
