The heart is perhaps the most durable organ of the body and, along with the brain, the most important. A woman’s heart, which weighs about eight ounces, beats 100,000 times a day (2.5 billion times in a lifetime), pumping blood with the force roughly required to squeeze a tennis ball. That’s enough blood for an 80-year life to fill 100 Olympic-sized swimming pools.
To maintain 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 0.5 percent of its mass. On a weight basis, the heart has the largest energy requirements of any human organ.
“The heart blows the brain,” says Gary Lopaschuk, a professor at the Center for Cardiovascular Research at the University of Alberta. “The heart is probably the most aerobic organ you have.”
The heart’s sole purpose is to contract, sending blood through 60,000 miles of blood vessels each day. It is so dedicated to this mission that it will continue to beat even after the brain dies, as is the case with “corpses with beating hearts” – bodies kept alive with oxygen and nutrients so that organs can be harvested for transplants.
“You can take the heart out of the body and it will continue to 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 key source of energy. Its energy reserves rest on a delicate balance: without new ATP, the reserves are depleted in 2 to 3 seconds and the heart muscle can no longer contract, causing a heart attack.
To protect against deficiency, the heart works by receiving constant infusions of oxygenated blood from the nearby lungs. To further protect the the heart needs nutrients for energy production, including fatty acids, carbohydrates, proteins and ketones.
“The heart is omnivorous,” says Lopaschuk. “It will use almost any fuel.”
Fatty acids account for up to 90 percent of ATP production because they provide a reliable source of energy, but the heart has a backup plan. During malnutrition, it uses glucose reserves and recycles lactate molecules to get the energy it needs.
What does the heart do?
With this energy, the heart follows its own metronome independent of the brain, a basic tempo varying from 60 to 100 beats per minute, depending on the individual. Like other mammals, humans possess a special cluster of heart cells designed to produce a regular electrical signal (a ‘pacemaker’ cell cluster) and transmit it to the heart. Following special pathsthe 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 like neurons and send signal after signal.
A 2014 paper describes an experiment in which researchers cultured heart cells from human stem cells and watched them “self-assemble into a thin, spontaneously beating” piece of heart tissue. This proto-organ beats at about 70 beats per minute, within normal limits, for about a month.
Even when the heart does not work, it often occurs with few or no severe side effects. Nearly 45 percent of all heart attacks produce mild “silent” effects that sufferers mistake it for another condition, such as exhaustion.
Other accidents can go quietly: Sometimes pacemaker cells can temporarily stop and take a benign “sinus pause” before continuing as before.
Or they can adopt unusual rhythma condition that may not require treatment.
Read more: What happens when hearts attack
In 2012, an unprecedented rhythm took over the heart of a patient at St. Vincent Hospital in Worcester, Massachusetts, pushing him 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 heart’s wiring. Yet for 20 seconds, a 57-year-old quadriplegic produced an electrocardiogram that resembled the tips of a fine-toothed comb.
The man was initially checked into the hospital after suffering from chest pains and stayed for several nights until his condition improved. But then the pain started again and doctors found arrhythmias, or irregular rhythms, in the atrial and ventricular regions of his heart.
Read more: How a heart can stop after a big blow
A sudden turn
After a particularly severe chest pain, the man lost consciousness and his heart rate began to increase every 100 milliseconds, as fast as the average human eye can blink. After about 20 seconds the speed dropped from 600 to 300 and suddenly dropped to normal.
After two days of an undetectable heartbeat, the man left the hospital, leaving doctors puzzled as to how a human heart beats at a rate of about 10 times every second. First, the main electrical relay in the heart, the atrioventricular junction, can fire only every 0.2 seconds, limiting heart rate with about 300 beats per minute.
What had allowed the arrhythmias to spiral out of control? Doctors hypothesized that the man must have had abnormal electrical connections in his heart, along with some kind of autonomic nervous disorder related to his quadriplegia.
Extreme heart attacks
Cases of heart rates over 300 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, it jumped to 303 bpm in a day and a half.
In 2018, a A 10-year-old English boy registered 301 beats and came to life, the result of a special pathway in his heart, doctors said.
From the case of 600 beats per minute, doctors learned nothing more because the man refused to undergo an electrophysiological examination to test the electrical work of his heart. Still, they knew: his precautions had carried him through an unprecedented 20 seconds before returning to normal speed.
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