Section 1
The Heart
By Boundless
![Thumbnail](../../../../../../figures.boundless-cdn.com/5268/raw/e-human-heart-28cropped-29.jpg)
The heart is an organ responsible for pumping blood through the blood vessels using rhythmic contractions of cardiac muscle.
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The pericardium is a thick, membranous, fluid-filled sac which encloses, protects, and nourishes the heart.
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The heart wall is comprised of three layers: the outer epicardium, the middle myocardium, and the inner endocardium.
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The heart has four chambers. The two atria receive blood into the heart and the two ventricles pump blood into circulation.
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Great vessels are the major vessels which directly carry blood into or out of the heart.
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The myocardium (cardiac muscle) is the thickest section of the heart wall and contains cardiomyocytes, the contractile cells of the heart.
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The cardiac skeleton, also known the heart's fibrous skeleton, consists of dense connective tissue in the heart that separates the atria from the ventricles.