Concept
Version 9
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Structures of the Heart
Human Heart
(a) The heart is primarily made of a thick muscle layer, called the myocardium, surrounded by membranes. One-way valves separate the four chambers. (b) Blood vessels of the coronary system, including the coronary arteries and veins, keep the heart muscles oxygenated.
Source
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"OpenStax College, Mammalian Heart and Blood Vessels. October 17, 2013."
http://cnx.org/content/m44805/latest/Figure_40_03_02ab.png
OpenStax CNX
CC BY 3.0.