Systole: How the Heart Contracts and Pumps Blood

Systole is the phase of the heartbeat when the heart muscle contracts and pushes blood out into the body’s arteries. It accounts for roughly one-third of each cardiac cycle at a resting heart rate, and the force it generates is what you feel as your pulse and what a blood-pressure cuff captures as the top number in your reading. While the concept sounds simple, systole involves a tightly coordinated chain of electrical, chemical, and mechanical events that starts inside individual heart-muscle cells and ends with blood surging through every organ you have.

What Triggers the Contraction

Every heartbeat begins with an electrical impulse that spreads across the heart muscle. When that signal reaches an individual heart-muscle cell, it opens channels in the cell membrane that let a small amount of calcium flow in from outside. That initial trickle of calcium acts like a trigger: it causes a much larger release of calcium from internal storage compartments within the cell called the sarcoplasmic reticulum, a process sometimes described as calcium-induced calcium release.¹ The flood of calcium that follows is what allows the contractile proteins inside each cell to slide past one another and shorten, and when billions of cells do this in near unison, the chamber walls squeeze inward and pressure inside the ventricle climbs rapidly.

The strength of that squeeze can be dialed up or down. When your body releases adrenaline or activates the sympathetic nervous system during stress or exercise, beta-adrenergic receptors on the heart-muscle cells ramp up both the amount and the speed of calcium cycling. The result is a stronger contraction with a faster return to relaxation, meaning the heart can pump harder and still refill quickly enough to keep up with a faster rate.² This built-in reserve is why a healthy heart can triple or even quadruple its output during heavy exercise compared to rest.

The Phases Inside a Single Systole

Systole is not one smooth squeeze. It unfolds in distinct phases, each lasting a fraction of a second. The first is isovolumetric contraction: the muscle is contracting and pressure inside the ventricle is rising, but all the valves are closed, so no blood is moving yet. Think of it as the brief moment when you push against a locked door before it finally gives way. Once the pressure inside the ventricle exceeds the pressure in the artery on the other side of the outflow valve, that valve pops open and blood is ejected.³

Ejection itself has a rapid phase, when most of the blood leaves the ventricle in the first half or so of systole, followed by a slower tail as contraction winds down and inertia carries the last bit of blood forward. After that, the outflow valve snaps shut, producing the second heart sound you hear through a stethoscope, and the heart enters diastole, its filling and relaxation phase. Although these phases are typically presented in textbooks as rigid boundaries, the transitions are smoother in a living heart, and researchers have debated for decades exactly where one phase ends and the next begins.

How the Two Ventricles Work Together

When most people picture systole, they think of the left ventricle, the thick-walled chamber that sends blood to the entire body. But the right ventricle contracts at the same time, pushing blood through the lungs. These two pumps are not independent. They share the interventricular septum, the muscular wall between them, and that shared wall turns out to be a major contributor to right-sided pumping. In an animal model where researchers could isolate the contributions of each wall, the left ventricle and septum together accounted for roughly two-thirds of the pressure generated inside the right ventricle, with the right ventricle’s own free wall responsible for only about a third.⁴

This interdependence has real consequences. When the left ventricle weakens, the right ventricle loses some of the mechanical assist it normally gets through the septum. And when a mechanical pump is used to unload a failing left ventricle, ramping up the device’s speed can paradoxically hurt right ventricular function by altering the way the septum moves and the pressures it transmits.⁵ That kind of trade-off is something clinicians have to balance carefully in patients with advanced heart failure who are on mechanical support.

Systolic Blood Pressure and Your Arteries

The systolic number on a blood-pressure reading is the peak pressure your arteries experience each time the left ventricle ejects blood. In a young person with flexible arteries, the walls stretch to absorb that pulse of blood, and systolic pressure stays moderate. As people age, arterial walls stiffen and the aorta’s diameter and compliance change. This means the same volume of blood is being pushed into a less yielding tube, and peak pressure rises.⁶

Isolated systolic hypertension, where the top number is elevated but the bottom number stays normal or even falls, is the most common form of high blood pressure in older adults. Calcium deposits in the aortic wall accelerate stiffening and are an independent predictor of this pattern.⁷ Interestingly, among people who already have systolic hypertension, those with the widest gap between top and bottom numbers tend to have the stiffest and narrowest aortas, rather than simply more aggressive wave reflections bouncing pressure back toward the heart.⁸ That distinction matters because the underlying mechanism influences which treatments are most likely to help.

What a Systolic Murmur Means

If a doctor hears an extra whooshing sound between the first and second heart sounds, that is a systolic murmur. It means blood is flowing turbulently during the contraction phase, usually because a valve is either too narrow (stenosis) or leaking backward (regurgitation). Not all systolic murmurs are dangerous. Many are “innocent” or “flow” murmurs caused by normal blood flowing a bit faster than usual, especially in children, athletes, pregnant people, or anyone with a high-output state like fever or anemia.

When the murmur does point to a structural problem, its characteristics help narrow down which valve is involved. A murmur pattern that is loudest at the base of the heart and radiates upward, combined with a delayed carotid pulse and an absent or faint second heart sound, strongly suggests aortic valve disease. A broad murmur centered at the apex of the heart, on the other hand, points toward significant mitral valve regurgitation.⁹ These physical-exam clues are not perfect, but they help clinicians decide who needs an echocardiogram and how urgently.

Measuring How Well the Heart Squeezes

Ejection fraction has been the workhorse measurement of systolic function for decades. It represents the percentage of blood in the left ventricle that gets pushed out with each beat, and a normal value is generally somewhere in the mid-50s to low 60s percent range. When ejection fraction drops to 40% or below, clinicians classify the patient as having heart failure with reduced ejection fraction, a condition marked by progressive enlargement and remodeling of the left ventricle.¹⁰

Ejection fraction has a well-known limitation: it can look normal even when the heart muscle is already deteriorating. A newer approach called speckle-tracking strain echocardiography tracks tiny movements of the heart wall during contraction and can detect subtle losses in function before ejection fraction drops. A recent scientific statement from the American Heart Association noted that left ventricular global longitudinal strain offers stronger diagnostic and prognostic value across a range of heart conditions compared with ejection fraction, and it can pick up problems at an earlier, subclinical stage.¹¹ Another way researchers evaluate systolic performance is through pressure-volume loops, where they plot the pressure inside the ventricle against its volume throughout each heartbeat. The slope of the line connecting the end-systolic points across multiple beats gives a measure of the heart’s intrinsic contractile strength that is less dependent on loading conditions than ejection fraction.¹²

How Aging Reshapes Systole

Aging changes both sides of the equation: the arteries stiffen and the heart stiffens right along with them. In a large study tracking this coupling, researchers found that as arterial stiffness increased with age, the ventricle’s own end-systolic stiffness rose in tandem, keeping the ratio between the two roughly constant across age groups.¹³ In other words, the heart compensates for stiffer arteries by becoming stiffer itself. The cost, however, is that the whole system becomes more sensitive to small changes in blood volume. A little dehydration, a shift in posture, or a missed dose of blood-pressure medication can cause larger swings in systolic pressure in an older person than in a younger one.

This stiffening is not entirely a sign of a stronger pump. A community-based study that followed people over time found that the increase in ventricular stiffness correlated with worsening diastolic stiffness and was more reflective of passive structural changes, like increased collagen deposition, than of enhanced contractile ability. The increase was especially pronounced in women.¹⁴ The long-term consequence is that the aging heart handles its resting workload acceptably but has less reserve to draw on during stress, which helps explain why older adults are more prone to exercise intolerance and to heart failure with preserved ejection fraction, a condition where the pump looks strong on paper but fills poorly.¹⁵

Exercise and the Failing Heart

In a healthy person, exercise boosts systolic performance through all the mechanisms already discussed: more adrenaline, faster calcium cycling, a stronger squeeze. But in someone whose left ventricle is already weakened, the response to exercise looks very different. In patients with heart failure and a reduced ejection fraction, exercise caused ejection fraction to actually decline rather than rise, and pressures inside the right ventricle and pulmonary circulation climbed sharply.¹⁶ The failing heart cannot recruit the reserve it needs, and the added demand exposes the gap between what the body asks for and what the pump can deliver.

Whether exercise training over weeks or months can reverse some of that limitation is a separate question, and the evidence is mixed. A study of men with reduced left ventricular function found that even high-intensity training did not change resting stroke volume, exercise hemodynamics, or left ventricular size and ejection fraction as measured by MRI.¹⁷ This does not mean exercise is useless for people with heart failure; the benefits of cardiac rehabilitation include improved muscle efficiency, better quality of life, and reduced hospitalizations. But the improvements seem to come largely from the periphery, from how well the muscles extract and use oxygen, rather than from making the heart itself pump harder.

Drugs That Target the Squeeze Itself

Most drugs used in heart failure work by reducing the load on the heart (lowering blood pressure, removing excess fluid) rather than directly strengthening the contraction. Older inotropes like dobutamine and milrinone do boost contractile force, but they also speed the heart rate, raise oxygen demand, and have been linked to worse long-term outcomes when used chronically. A newer approach tries to make each contraction more efficient without those side effects.

Omecamtiv mecarbil is a drug designed to activate cardiac myosin, the molecular motor responsible for contraction, directly. In clinical testing, it increased the time the heart spends in systolic ejection by up to 80 milliseconds and raised stroke volume by up to about 10 milliliters, while reducing the volume of blood left behind in the ventricle after each beat.¹⁸ The idea is appealing: make the existing contraction go further rather than demanding more of already-stressed cells. However, the picture is not entirely rosy. Experimental work has shown that the drug increases the heart’s oxygen consumption and may impair cardiac efficiency, in part because it activates myosin even during the resting phase of the cycle when the muscle should be quiet.¹⁹ The tension between a longer, more productive contraction and a higher energy bill is exactly the kind of trade-off that defines inotropic therapy and makes it so tricky to get right.

The Atrial Contribution You Rarely Hear About

When people discuss systole, the conversation almost always centers on the ventricles. But the atria have their own contraction phase, sometimes called atrial systole or the “atrial kick,” that happens at the tail end of ventricular filling, just before the ventricles fire. In a normal rhythm, atrial contraction tops off the ventricles with an extra 15 to 25 percent of their total filling volume, giving the ventricles a running start. The importance of this contribution becomes obvious when it disappears. During arrhythmias like atrial fibrillation, where the atria quiver chaotically instead of contracting in sequence, not only is that extra filling lost, but blood can actually be pushed backward into the pulmonary veins, a phenomenon described as a “negative atrial kick” that further undermines overall cardiac output.²⁰

This is part of why atrial fibrillation can cause such dramatic symptoms in some people, especially those whose ventricles are already stiff from age or disease and rely heavily on that atrial top-off to fill adequately. The loss of coordinated atrial systole turns what was a finely tuned sequence into a disorganized, less efficient pumping cycle.

Systole Before Your First Breath

The fetal heart operates under completely different conditions than the adult heart. Because the lungs are not yet in use, the right ventricle does not need to pump blood through a low-resistance lung circuit; instead, most right-sided output crosses through the ductus arteriosus and joins the systemic circulation. As a result, the fetal right ventricle is the dominant chamber, actually larger relative to the left ventricle than it will ever be again. Paired echocardiographic measurements taken before and after birth show that the ratio of right to left ventricular size at end-diastole drops from about 1.2 in the fetus to roughly 0.8 in the newborn, and both chambers change shape substantially within days of delivery.²¹ Once the lungs inflate and pulmonary resistance falls, the left ventricle takes over as the high-pressure pump and begins the thickening process that will define it for the rest of life. The speed of this transformation, happening over the first hours and days after birth, is one of the more remarkable feats of cardiovascular adaptation in human physiology.