How many times does a heart beat in a day? What about in a lifetime?

More than any other organ in the body, the heart inspires the imagination. Throughout human History, people around the world have written, spoken and sung about the heart in thousands of languages, describing it as the seat of love, kindness and courage.

But the heart's primary function is to keep us alive. This fist-size muscular organ drives the circulatory system, pumping oxygenated blood throughout the body. It may speed up or slow down depending on our emotions or physical exertion, or due to injury or disease. But in general, a Healthy heart thumps steadily and reliably.

So how many heartbeats does that add up to in a single day, or over the course of a human life span?

There's a lot of variation to how fast your heart beats in a day; whether you're sitting at a desk, walking to the store, or running on a treadmill, your heart responds to different energy requirements by beating faster or slower. Heartbeat intervals even slow down and speed up by about 100 milliseconds every time you inhale and exhale, said Dr. Partho Sengupta, chief of cardiovascular medicine for Robert Wood Johnson Medical School at Rutgers University in New Jersey.

"The most interesting aspect of the heart is its ability to modulate its rate and its function depending upon the metabolic needs of the body," he told Live Science. "The physiology is continuously adapting — it almost has its own brain to sense the body's needs."

Estimating a lifetime of heartbeats begins with timing your ticker minute by minute, and then seeing if your average resting heart rate falls within a certain range. Heart rate is measured in beats per minute (BPM). The range for a healthy adult is 60 to 100 BPM while at rest, though the rate for most adults is between 55 and 85 BPM, according to Harvard Medical School. By comparison, the average resting heart rate for a newborn is 70 to 190 beats per minute, per the University of California San Francisco Benioff Children's Hospitals, to meet the energy demands of a faster metabolism.

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