What Happens During Ventricular Systole?

During ventricular systole, the muscular walls of the heart’s two lower chambers contract forcefully, pressurizing the blood inside them and ejecting it into the body’s two great arteries. The entire process unfolds in roughly a third of a second at a resting heart rate, yet it involves a tightly coordinated sequence of electrical firing, molecular machinery, valve mechanics, and a wringing motion that squeezes blood upward and outward. What looks from the outside like a simple “pump squeeze” is, at closer range, an intricate chain of events that the heart repeats more than a hundred thousand times a day.

The Electrical Spark That Starts It All

Ventricular systole begins with an electrical event. A wave of depolarization spreads from the top of the heart’s conduction system down through the walls of both ventricles, triggering the muscle fibers to contract almost simultaneously. On an electrocardiogram, this electrical activation appears as the QRS complex, the sharp spike you see on a heart monitor.¹ The whole depolarization sweep takes less than about a tenth of a second in a healthy heart, ensuring that the left and right ventricles fire in near-perfect unison. If the electrical signal is delayed or takes an abnormal path, the ventricles contract unevenly, which can reduce the efficiency of the pump.

From Electrical Signal to Mechanical Squeeze

An electrical signal alone does not move blood. The bridge between the electrical impulse and actual muscle contraction is calcium. When the depolarization wave hits a heart muscle cell, a small amount of calcium enters through channels in the cell membrane. That initial trickle triggers a much larger release of calcium from internal stores within the cell called the sarcoplasmic reticulum.² This flood of calcium allows the contractile proteins inside the muscle cell to grab onto each other and shorten, generating force. The more calcium that floods the cell, the stronger the contraction. This entire cascade, from electrical signal to physical shortening, happens in milliseconds and repeats with every beat.

Isovolumetric Contraction

Once the ventricles begin squeezing, the first thing that happens is not ejection but a brief pressure buildup. At this instant, all four heart valves are closed: the inlet valves (mitral and tricuspid) have just shut because ventricular pressure exceeded atrial pressure, and the outlet valves (aortic and pulmonic) have not yet opened because the pressure in the ventricles has not yet exceeded the pressure in the arteries. The ventricles are essentially sealed chambers, contracting against a fixed volume of blood. This phase is called isovolumetric contraction because the volume of blood inside does not change; only the pressure rises.

This brief phase, lasting only about 50 milliseconds, is when the first heart sound occurs. The familiar “lub” you hear through a stethoscope is produced by the closure of the mitral and tricuspid valves and the vibrations that follow. It marks the onset of systole and tells a clinician that the ventricles have started contracting.

The Ejection Phase

Once ventricular pressure exceeds arterial pressure, the aortic and pulmonic valves pop open and blood surges out. This is the ejection phase, and it is where the real work of systole happens. Blood accelerates rapidly into the aorta and the pulmonary artery, though the two sides of the heart do not behave identically. In the aorta, peak blood flow velocity reaches about 92 centimeters per second, and the blood accelerates roughly two to three times faster than in the pulmonary artery, where peak velocity is around 63 centimeters per second. This difference exists even though the right ventricle sends the same volume of blood per beat as the left; it simply does so against a much lower resistance in the lung circulation.³

Ejection itself has two sub-phases. Early in ejection, the ventricle is contracting vigorously and blood flow accelerates to its peak. Then, as the muscle fibers begin to relax and the ventricle has expelled the bulk of its stroke volume, flow decelerates. In the aorta, the time from valve opening to peak velocity is about 98 milliseconds, and the total ejection time is roughly 294 milliseconds. On the pulmonary side, ejection lasts a bit longer, around 331 milliseconds, reflecting the lower-pressure, more compliant pulmonary vasculature.³

The Heart Does Not Just Squeeze, It Wrings

One of the more surprising aspects of ventricular systole is the heart’s twisting motion. The left ventricle does not simply contract inward like a fist closing. Instead, it wrings like a towel being squeezed. The base of the ventricle rotates clockwise while the apex rotates counterclockwise (when viewed from the tip of the heart looking up), creating a torsional movement that helps wring blood out efficiently.⁴ This twist is a direct consequence of how muscle fibers are arranged in spiral layers within the heart wall.

The twisting motion matters beyond just pumping efficiency. After systole ends, the ventricle rapidly untwists, and this recoil acts like releasing a wound-up spring, helping to suck blood back into the ventricle during the filling phase that follows. When heart disease damages the fiber architecture, torsion is one of the first things to deteriorate, sometimes before any obvious drop in overall pump strength. That makes torsion a sensitive early marker for clinicians trying to catch cardiac dysfunction before it becomes severe.⁴

How Systole Ends and the Second Heart Sound

Systole does not switch off all at once. As the ventricular muscle relaxes and pressure inside the chambers drops below arterial pressure, blood momentarily tries to flow backward. This backflow catches the aortic and pulmonic valve leaflets and snaps them shut. That closure produces the second heart sound, the “dub” of the “lub-dub” pair. The sound is not caused by the physical contact of the leaflets themselves but by the sudden deceleration of the column of blood hitting the closed valves, which sets the valve structures and surrounding blood vibrating.⁵ Recordings have shown that the audible sound actually appears 10 to 30 milliseconds after the valves physically close, confirming it is the vibration, not the contact, that you hear.⁵

The aortic valve typically closes slightly before the pulmonic valve. During normal breathing, inspiration increases the volume of blood returning to the right side of the heart, which slightly delays pulmonic valve closure and produces a subtle “splitting” of the second heart sound. Clinicians listen carefully for this split because changes in its timing or absence can signal problems with conduction or valve function.

What Happens at the Cellular Level When Systole Ends

For the ventricle to relax and refill, the calcium that drove the contraction must be cleared out of the cell’s interior. A protein pump called SERCA2a, embedded in the membrane of the sarcoplasmic reticulum, actively hauls calcium back into that internal store.⁶ As calcium levels in the cell drop, the contractile proteins release each other and the muscle cell lengthens. The speed of this calcium reuptake determines how quickly the heart relaxes and how effectively it fills for the next beat.⁷

This is why heart failure is not always about weak contraction. In many patients, especially older adults, the heart squeezes adequately but relaxes poorly. When SERCA2a activity is reduced, calcium lingers in the cell, the muscle stays stiff, and the ventricle cannot fill properly. This form of heart failure, sometimes called “heart failure with preserved ejection fraction,” can be just as debilitating as the kind where the squeeze itself is too weak.

How the Heart Adjusts Its Output Beat by Beat

The heart does not pump the same volume every time. One of the most fundamental ways it adjusts is through the Frank-Starling mechanism: when more blood flows into the ventricle during filling, the muscle fibers stretch further, and the subsequent contraction is stronger, ejecting a larger stroke volume. This ensures the heart automatically matches output to demand without needing a signal from the nervous system. Research using computational models has confirmed that this response depends on a property of heart muscle called length-dependent activation, where a stretched fiber generates more force. Without this property, the heart cannot properly increase ejection in response to increased filling.⁸

The heart also responds to changes in the resistance it pumps against. If arterial pressure suddenly rises (say, you grab a heavy suitcase and brace), the ventricle initially ejects less blood per beat. Over the next few beats, the residual blood stretches the chamber, triggering the Frank-Starling mechanism to boost the next contraction and partially restore stroke volume. This interplay between filling and afterload creates a two-phase response to sudden pressure changes: an initial dip followed by compensation.⁹

Ejection Fraction and What Doctors Actually Measure

The most commonly used measure of systolic performance is ejection fraction: the percentage of blood inside the ventricle at the end of filling that gets ejected during systole. A large meta-analysis of population-based studies found that the average left ventricular ejection fraction in healthy adults is about 63%, with women averaging slightly higher (around 64%) than men (around 62%). The normal range extends from roughly 52% at the low end to about 74% at the high end, and fewer than one in a hundred healthy people have an ejection fraction below 50%.¹⁰

Ejection fraction is useful but imperfect. It depends on how much blood fills the ventricle beforehand and how much resistance the arteries present, so it can shift substantially just because of changes in hydration, blood pressure, or body position. It is also only moderately reproducible from one measurement to the next, meaning a five-point swing between two echocardiograms may reflect measurement variability rather than a true change in heart function.¹¹ That is why cardiologists increasingly look at additional metrics, including the twisting and strain patterns discussed earlier, to get a fuller picture of how well systole is actually performing.

What Systolic Murmurs Tell You

When a doctor places a stethoscope on your chest and hears a whooshing sound between the first and second heart sounds, that is a systolic murmur. Not all systolic murmurs signal disease; many are “innocent” or “flow” murmurs, especially in younger people, caused by vigorous blood flow through normal structures. But murmurs can also indicate valve problems that directly disrupt systole.

A study of adults with systolic murmurs found that the location and quality of the murmur are strong clues to the underlying cause. A murmur heard loudest at the base of the heart radiating toward the neck, especially when accompanied by a slow-rising pulse in the carotid artery and an absent second heart sound, strongly suggests aortic valve disease. A broad murmur heard best at the apex of the heart points toward mitral regurgitation, where blood leaks backward through the mitral valve during systole. And a murmur along the left lower sternal border raises the likelihood of tricuspid regurgitation.¹² These bedside observations, combined with the timing of the murmur within systole, give clinicians a surprisingly detailed picture of what is going wrong mechanically before any imaging is ordered.

How Exercise Changes the Story

At rest, the heart has plenty of time to fill and eject. During exercise, the heart rate climbs and each cardiac cycle gets shorter, which means systole must finish faster. The heart manages this partly by contracting more forcefully and partly by relaxing more quickly, so both ejection and filling can fit into a tighter window. Sympathetic nervous stimulation, driven by adrenaline, increases the amount of calcium released into each heart cell and speeds up the SERCA2a pump that removes it, accelerating both contraction and relaxation.

There is also a direct relationship between heart rate and contractile strength known as the force-frequency effect. Within a certain range, beating faster itself makes each contraction stronger. In animal studies, progressively increasing the heart rate through pacing produced a positive boost in contractile force up to a point, after which further rate increases caused force to plateau or decline.¹³ This built-in rate-boosting mechanism works alongside the Frank-Starling mechanism and autonomic nerve signals to scale cardiac output from about five liters per minute at rest to over 20 liters per minute during intense exercise in a fit individual.

Imaging Systole Beyond the Basics

Traditional echocardiography gives you a moving picture of the heart and lets you measure chamber sizes and estimate ejection fraction. But newer techniques go further. Speckle-tracking echocardiography, for instance, follows the motion of tiny acoustic markers within the heart muscle frame by frame throughout the cardiac cycle. This allows clinicians to quantify how much the muscle shortens in different directions, longitudinally along the length of the ventricle, radially inward, and circumferentially around the chamber, as well as the rotation and torsion discussed earlier.¹⁴

Why does this matter? Because a ventricle can have a normal ejection fraction yet show abnormal strain patterns. The muscle might be compensating in one direction to make up for weakness in another, masking the problem from a simple ejection fraction measurement. Strain imaging can catch this early, which is particularly valuable in patients receiving chemotherapy drugs that are toxic to the heart or in people with conditions like hypertension that gradually stiffen the muscle without obviously reducing overall pump strength. In these scenarios, a drop in longitudinal strain can appear months or years before the ejection fraction falls.

Why Some Hearts Have Undivided Ventricles

Mammals and birds have fully divided ventricles: the left side handles oxygenated blood heading to the body, and the right side handles deoxygenated blood heading to the lungs. Reptiles, by contrast, often have partially divided or undivided ventricles, which allows oxygenated and deoxygenated blood to mix during systole. For a long time this was viewed as a deficiency, but researchers now understand that this mixing, called cardiac shunting, likely evolved as an adaptation. It allows reptiles to redirect blood flow away from the lungs during breath-holding dives or periods of low oxygen demand, conserving energy in ways that fully separated ventricles cannot.¹⁵ In mammals and birds, where metabolic demands are consistently high, such shunting would be harmful, but in ectothermic animals with variable metabolic rates, it provides a genuine physiological advantage. The mammalian design of ventricular systole, with its high-pressure left ventricle and low-pressure right ventricle working completely independently, is optimized for the constant high oxygen delivery that warm-blooded metabolism demands.

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