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Human Heart: Anatomy, Function & Facts

heart-in-body-02
Credit: Dreamstime

The human heart is an organ that pumps blood throughout the body via the circulatory system.

Human heart anatomy

In humans, the heart is roughly the size of a large fist and weighs between 9 and 12 ounces (250 and 350 grams). It has four chambers: two upper chambers (the atria) and two lower ones (the ventricles). The right atrium and right ventricle together make up the "right heart," and the left atrium and left ventricle make up the "left heart." A wall of muscle called the septum separates the two sides of the heart.

Human heart
The human heart is about the size of a fist.
Credit: tlorna | Shutterstock

A double-walled sac called the pericardium encases the heart, which serves to protect the heart and anchor it inside the chest. Between the outer layer, the parietal pericardium, and the inner layer, the serous pericardium, runs pericardial fluid, which lubricates the heart during contractions and movements of the lungs and diaphragm.

The heart's outer wall consists of three layers. The outermost wall layer, or epicardium, is the inner wall of the pericardium.  The middle layer, or myocardium, contains the muscle that contracts. The inner layer, or endocardium, is the lining that contacts the blood.

The tricuspid valve and the mitral valve make up the atrioventricular (AV) valves, which connect the atria and the ventricles. The pulmonary semi-lunar valve separates the left ventricle from the pulmonary artery, and the aortic valve separates the right ventricle from the aorta. The heartstrings, or chordae tendinae, anchor the valves to heart muscles.

The sinoatrial node produces the electrical pulses that drive heart contractions.

Human heart function

The heart circulates blood through two pathways: the pulmonary circuit and the systemic circuit.

In the pulmonary circuit, deoxygenated blood leaves the right ventricle of the heart via the pulmonary artery and travels to the lungs, then returns as oxygenated blood to the left atrium of the heart via the pulmonary vein.

Biodigital human cardiovascular system
The cardiovascular system circulates blood from the heart to the lungs and around the body via blood vessels.
Credit: The BioDigital HumanTM developed by NYU School of Medicine and BioDigital Systems LLC

In the systemic circuit, oxygenated blood leaves the body via the left ventricle to the aorta, and from there enters the arteries and capillaries where it supplies the body's tissues with oxygen. Deoxygenated blood returns via veins to the venae cavae, re-entering the heart's right atrium.

Electrical "pacemaker" cells cause the heart to contract, which happens in five stages. In the first stage (early diastole), the heart is relaxed. Then the atrium contracts (atrial systole) to push blood into the ventricle. Next, the ventricles start contracting without changing volume. Then the ventricles continue contracting while empty. Finally, the ventricles stop contracting and relax. Then the cycle repeats.

Valves prevent backflow, keeping the blood flowing in one direction through the heart.

Facts about the human heart

  • A human heart is roughly the size of a large fist
  • The heart weighs between 9 and 12 ounces (250 and 350 grams)
  • The heart beats about 100,000 times per day (about three billion beats in a lifetime)
  • An adult heart beats about 60 to 80 times per minute
  • Newborns hearts beat faster than adult hearts, about 70 -190 beats per minute
  • The heart pumps about 6 quarts (5.7 liters) of blood  throughout the body
  • The heart is located in the center of the chest, usually pointing slightly left

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Tanya Lewis, LiveScience Staff Writer

Tanya Lewis

Tanya has been writing for Live Science since 2013. She covers a wide array of topics, including neuroscience, biotech, robotics, astronomy and strange or cute animals. She received a graduate certificate in science communication from the University of California, Santa Cruz, and a bachelor of science with honors in biomedical engineering from Brown University, with postgraduate research experience in a neuroscience. She has previously written for Science News, Wired, The Santa Cruz Sentinel, the radio show Big Picture Science and other places. Tanya has lived on a tropical island, witnessed volcanic eruptions and flown in zero gravity (without losing her lunch!). To find out what her latest project is, you can visit her website or follow Tanya on twitter or .
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