The heart operates as a continuous pump, circulating blood throughout the body to deliver oxygen and nutrients. A specific measure used to gauge the heart’s performance is stroke volume (SV), which focuses on the output of a single beat. Monitoring SV provides direct insight into how effectively the heart is moving blood, which is a key indicator for assessing overall circulatory health.
What Stroke Volume Measures
Stroke volume (SV) represents the amount of blood the heart’s left ventricle ejects with each contraction. This measurement directly indicates the heart’s pumping capability. To calculate this volume, physicians use two main internal measurements: the End-Diastolic Volume (EDV) and the End-Systolic Volume (ESV).
The EDV is the volume of blood filling the ventricle just before it contracts. The ESV is the volume of blood remaining in the ventricle immediately after the contraction has finished, as not all blood is ejected during a beat. The stroke volume is the difference between these two measurements: SV = EDV – ESV.
The Average Value at Rest and During Activity
For a healthy adult, the average stroke volume at rest typically falls within a range of 70 to 100 milliliters (mL) per beat. While a single exact figure is not standard, a value around 70 mL is often cited. The volume varies depending on factors like body size, sex, and physical conditioning. This resting value is sufficient to meet the body’s minimal oxygen demands.
The heart’s output changes dramatically during intense exercise. During strenuous activity, stroke volume can increase significantly, sometimes by 50% or more, in untrained individuals. This increase boosts the delivery of oxygenated blood to the working muscles. Highly trained endurance athletes often achieve even higher volumes, demonstrating greater cardiac efficiency.
How the Body Regulates Stroke Volume
The body adjusts stroke volume through three interconnected physiological mechanisms: preload, afterload, and contractility.
Preload
Preload refers to the degree of stretch experienced by the ventricular muscle fibers just before they contract. An increase in the volume of blood returning to the heart, known as venous return, increases this stretch. This relationship is described by the Frank-Starling mechanism, which dictates that a greater stretch leads to a more forceful subsequent contraction, thereby increasing the stroke volume.
Afterload
Afterload is the resistance the heart must overcome to eject blood into the major arteries. This resistance is largely determined by the pressure within the systemic circulation. When afterload increases, such as with elevated blood pressure, the heart must work harder to push the blood out. A sustained increase in afterload generally causes the stroke volume to decrease because the ventricle cannot completely empty against the higher pressure.
Contractility
Contractility is the intrinsic strength of the heart muscle contraction, independent of preload. This force is often enhanced by signals from the nervous system and circulating hormones during times of stress or exercise. Increased contractility allows the heart to squeeze more forcefully and efficiently, pushing out a larger percentage of blood. By modulating these three factors, the body continually adjusts stroke volume to match changing metabolic needs.
Stroke Volume and Overall Heart Function
Stroke volume is a foundational component of Cardiac Output (CO), which is the total volume of blood pumped by the ventricle per minute. CO is calculated by multiplying the stroke volume by the heart rate: CO = SV x HR. This relationship shows how the heart adjusts its output by altering either the volume per beat or the number of beats.
Monitoring CO provides a comprehensive assessment of the circulatory system’s ability to deliver oxygen to the body’s tissues. Changes in stroke volume directly influence this total output, making it a key metric for evaluating the heart’s functional status. A consistently low stroke volume suggests reduced pumping efficiency and can signal underlying cardiac issues.