The wall of the heart is made up of three distinct layers :

* An outer epicardium, consisting of a layer of flattened epithelial cells and supporting connective tissue

* A very thick , muscular myocardium consisting of cardiac muscle

* An inner endocardium , consisting of flattened epithelial cells supported by a delicate layer of connective tissue.

Mammals have a double circulatory system, which consists of the systemic circulatory , in which blood is pumped from the left side of the heart to the carious tissues and organs of the body , and the pulmonary circulation , in which blood is pumped from the right side of the heart and major blood vessels pulmonary capillaries of the lungs.