Is there a more ubiquitous symbol on Earth than the heart?

The character spans all countries, religions, and philosophies. It can represent love, courage, conscience, emotion, the seat of life, or the seat of the soul.

And, of course, the 20th century brought us the commercialized popularity of Valentine’s Day.

Valentine's Day Card circa 1909

Like most symbols, this pictogram stems from a physical object. If you’re a vertebrate and not a coral, jellyfish, insect, or pre-Emerald-City Tin Man, you have a beating heart. Even Kim Jong-un has one!

The physical heart of an animal is a muscular organ that pumps blood through that being’s circulatory system. It’s often compared to the engine of a body, pumping oxygen to cells. The actuality is a bit more complex, as the heart provides blood-rich oxygen and other nutrients to the body while also sending metabolic waste, such as carbon dioxide, to the lungs.

Mammalian and avian hearts have four chambers, two atria and two ventricles. The atria occupy the top of the heart, while the ventricles sit below them, connected by valves. Deoxygenated blood enters the right atrium and then travels through the tricuspid valve to the right ventricle. From there, the heart sends blood through the pulmonary valve in two directions to the lungs. The lungs trade carbon dioxide for new oxygen before the blood goes back to the ticker. It enters the left atrium, moves through the mitral valve to the left ventricle, and goes for another lap around the body, starting in the aorta. In most humans, the left side of the heart is larger than the right because the pump needs to propel blood farther. The right side only needs to transport blood to the lungs, while the left side is responsible for every other hungry cell.

Our hearts sit in a sac called the pericardium. The organ rests in a central cavity called the mediastinum. They are approximately the size of one’s balled fist. Like the symbol, the heart does have a tip, which we call the apex.

The location of the human heart - graphic by BodyParts3D/Anatomography
Diagram of a human heart - graphic by Wapcaplet
A human heart during an autopsy - photo by Stanwhit607

The human heart is an incredible machine. The average heart beats approximately 100,000 times a day. It pumps about 2,000 gallons of blood in that time, pushing the fluid more than 12,000 miles! The circumference of the planet is close to 25,000 miles. The entirety of our circulatory system stretches 60,000 miles. In a year, the average heart pulses 35 million times; in the average lifetime, that comes out to 2.5 billion ticks!

Not all animal hearts are created equally. Fish hearts have two chambers. Reptiles have three. Giraffes, with their enormous necks, feature gargantuan blood pressure. The healthy human touts a reading of 120/80. Giraffes pack measurements as high as 280/180! A cow’s heart is the size of a human head. Not to be outdone, the blue whale’s heart weighs 400 pounds. Cheetahs, the fastest creatures on land, can see their pulses reach 250 beats per minute in mere seconds during a sprint. Octopuses think the rest of us are all short on love: they have three hearts.

In addition to the universality of the heart as a symbol, the sound of a working heart is one of the planet’s most unmistakable vibrations. The sounds come from the valves opening and closing. Ventricular contraction produces a “lub,” while ventricular diastole creates a “dub.” If one listens closely enough, a slight double tone emerges on each sound, which reflects the difference in the timing of the halves of the heart.

A model of a blue whale's heart - photo credit: © AMNH/D. Finnin

Despite a rudimentary understanding of anatomy, ancient cultures recognized the beating center of the chest as an important aspect of life.

Nearly every religion places great importance on the heart. The word for “heart” in the Hebrew Bible means both the organ and the seat of emotion and mind. Ancient Egyptian religion centered on the heart. The word ib forms many expressions, such as Awi-ib, which means “happy” and translates directly as “long of heart.” The heart was the key to the afterlife, and it was the only organ not to be removed during mummification. Aztecs, who sacrificed hearts to the gods, believed them to be fragments of the sun’s heat. Modern Nahua still call the sun tona-tiuh, which means “heart-soul.” Catholics venerate the Sacred Heart of Jesus and the Immaculate Heart of Mary. The heart is the divine center, or atman, in Hinduism. To the Buddha, the heart was the “diamond of purity and essence.” 

By the time we reached the ancient Greeks, the medical implications of the heart started to emerge to human brains. Plato connected the heart to blood circulation, Aristotle to blood creation, and Hippocrates linked the heart and lungs. Erasistratus even coined the heart a pump. In the second century, Galen got a few things right – vessels carry blood; veinous blood darker; arterial blood lighter – and others very wrong – artery action moved blood, not the heart; blood moved between chambers through pores. Not for another century did we start to understand the heart in a proper scientific sense.

Heart diagram by Leonardo da Vinci, 15th century

Though connections to importance and centrality are rampant throughout the millennia, the association between the ideograph of the heart and romantic love is harder to pin. Before the Middle Ages, the heart shape typically represented plants or leaves. In the 13th and 14th centuries, perhaps because of the proliferation of the ideas of courtly love, chivalry, and heraldry, a tie to romance began to emerge.

From there, it’s only a few metaphorical valve beats to the mid-1800s and conversation hearts.

Image created by Wally Spence

And a couple “dub-lubs” to St. Valentine and Cupid becoming omnipresent.

On Valentine’s Day – or any day, really – check loved ones for ticks and send them a card with the equation 
(x2 + y2 − 1)3 − x2y3 = 0.

Nothing says “I love you” quite like exponents.

Graphic by Eviatar Bach

Further Reading and Exploration

The Heart – National Heart, Lung, and Blood Institute

Amazing Heart Facts – PBS NOVA

16 Surprising Facts About Your Heart – Unity Point Health

Animal Circulatory Systems – Biology Pages

Fun Facts About Animal Hearts – Morris Animal Foundation

The Shape of My Heart – Slate

Heart in History – American Experience

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