The heart is perhaps the body’s most resilient organ and, along with the brain, its most essential. The female heart, which weighs about eight ounces, beats 100,000 times a day (2.5 billion times in a lifetime), pumping blood with roughly the force needed to squeeze a tennis ball. That’s enough blood in an 80-year lifetime to fill 100 Olympic-sized swimming pools.

To sustain this breakneck pace, the heart feeds greedily on its own blood supply, consuming about 5 percent of the body’s oxygen while making up about .5 percent of its mass. On a per-weight basis, the heart has the greatest energy requirements of any human organ.

“The heart blows the brain away,” says Gary Lopaschuk, a professor at the Cardiovascular Research Centre at the University of Alberta. “The heart is probably the most aerobic organ you have.”

Heart Cadavers

The heart’s sole purpose is to contract, sending blood down 60,000 miles of blood vessels every day. It’s so dedicated to this mission that it will continue to beat even after the brain has died, as is the case with “beating-heart cadavers” – bodies kept alive with oxygen and nutrients so the organs can be harvested for transplants.

“You can take the heart out of the body, and it will still beat if you give it the right fuel,” says Lopaschuk.

How the Heart Works

For the most part, the heart relies on oxygen to produce the all-important adenosine triphosphate (ATP), a crucial energy source. Its energy reserves rest on a delicate balance: without new ATP, reserves run dry in 2 to 3 seconds, and the heart muscle can no longer contract, causing a heart attack.

To protect itself against a shortfall, the heart works by receiving steady infusions of oxygenated blood from the nearby lungs. To further safeguard itself, the heart needs nutrients to produce energy, including fatty acids, carbohydrates, proteins and ketones.

“The heart is an omnivore,” says Lopaschuk. “It will use pretty much any fuel.”

Fatty acids account for up to 90 percent of ATP production as they provide a reliable source of energy, but the heart has a backup plan. During times of malnutrition, it taps into glucose reserves and recycles lactate molecules to come up with the energy it needs.

What Does the Heart Do?

With that energy, the heart follows its own metronome independent from the brain, a base tempo ranging from 60 to 100 beats per minute, depending on the individual. Like other mammals, humans possess a special cluster of heart cells built to produce a regular electrical signal (a cluster of “pacemaker” cells) and relay it throughout the heart. Following special pathways, the signal contracts the upper atria and then the lower ventricles to pump blood out of the heart.

And barring death or other complications, the process never stops. Pacemaker cells repeatedly absorb sodium ions, causing them to fire as neurons do and send out signal after signal.

A 2014 paper describes an experiment in which researchers cultivated cardiac cells from human stem cells and watched as they “self-assembled into a thin, spontaneously beating” piece of cardiac tissue. This proto-organ beat at about 70 beats per minute, well within the normal range, for about a month.

Temporary Deviations

Even when the heart malfunctions, it often carries on with few or no severe side effects. Nearly 45 percent of all heart attacks produce mild “silent” effects that sufferers mistake for another condition, such as exhaustion.

Other mishaps may pass quietly: On occasion, pacemaker cells may stop temporarily and take a benign “sinus pause” before continuing on as before.

Or, they may adopt an unusual rhythm, a condition that may not require treatment.

Read More: What Happens When Hearts Attack

A Heart Beat Mystery

In 2012, an unprecedented rhythm took hold of a patient’s heart at Saint Vincent Hospital in Worcester, Mass., pushing it to 600 beats per minute, about 10 times the usual rate. Until then, experts had long considered 300 beats to be the maximum normal heart rate, given the wiring of the heart. Yet for 20 seconds, a 57-year-old quadriplegic had produced an electro-cardiogram that resembled the peaks of a fine-toothed comb.

The man had initially checked into the hospital after suffering from chest pains and stayed a couple of nights as his condition improved. But then the pains started up again, and doctors found arrhythmias, or irregular rhythms, in the atrial and ventricular areas of his heart.

Read More: How a Heart Can Stop After a Forceful Impact

A Sudden Turn

Following a particularly severe chest pain, the man lost consciousness, and his heart reading began to spike about every 100 milliseconds, as fast as the average human eye can blink. After about 20 seconds, the rate declined from 600 to 300 and suddenly dropped to a normal level.

After two days of an unremarkable heartbeat, the man left the hospital and left the doctors to puzzle over how a human heart had beat at a rate of about 10 times every second. For one, a major electrical relay in the heart, the atrioventricular junction, can only fire about every 0.2 seconds, capping heart rate at about 300 beats per minute.

What had allowed the arrhythmias to spiral out of control? Doctors proposed the man must have possessed unusual electrical connections in his heart, along with some kind of autonomic nervous disturbance related to his quadriplegia.

Extreme Heart Beats

Cases of 300-plus heart rates are rare but not unheard of.

In the 1940s, a woman’s heart rate rose to 310 beats per minute and stayed there for about 12 hours without killing her. On another occasion, she soared to 303 beats per minute for a day and a half.

In 2018, a 10-year-old English boy registered 301 beats and lived, the result of a special pathway in his heart, doctors said.

Of the 600 beats-per-minute case, doctors learned nothing more as the man refused to undergo an electrophysiological study, to test the electrical workings of his heart. Still, they knew: its safeguards had carried it through 20 unprecedented seconds before returning to a normal rate.

Read More: What the Discovery of an Extra Artery Means for Human Evolution