The myocardium (mi"o-KAR'de-um) is the heart's muscular wall. It contracts to pump blood out of the heart, then relaxes as the heart refills with returning blood. Its outer surface is called the epicardium (ep"ih-KAR'de-um). Its inner lining is the endocardium (en"do-KAR'de-um).
Muscle tissue, known as myocardium or cardiac muscle, wraps around a scaffolding of tough connective tissue to form the walls of the heart’s chambers. The atria, the receiving chambers of the heart, have relatively thin walls compared to the ventricles, the pumping chambers. The left ventricle has the thickest walls—nearly 1 cm (0.5 in) thick in an adult—because it must work the hardest to propel blood to the farthest reaches of the body.
World Health Organization
Circulatory System
Circulatory System, or cardiovascular system, in humans, the combined function of the heart, blood, and blood vessels to transport oxygen and nutrients to organs and tissues throughout the body and carry away waste products.
Disorders of the circulatory system include any injury or disease that damages the heart, the blood, or the blood vessels. The three most important circulatory diseases are hypertension, arteriosclerosis, and atherosclerosis.
Respiratory System, in anatomy and physiology, organs that deliver oxygen to the circulatory system for transport to all body cells. Oxygen is essential for cells, which use this vital substance to liberate the energy needed for cellular activities.
