In humans

In humans

Main article: Human heart

Structure diagram of the human heart from an anterior view. Blue components indicate de-oxygenated blood pathways and red components indicate oxygenated pathways.

The human heart has a mass of between 250 and 350 grams and is about the size of a fist.^[7] It is located anterior to the vertebral column and posterior to the sternum.
It is enclosed in a double-walled sac called the pericardium. The superficial part of this sac is called the fibrous pericardium. This sac protects the heart, anchors its surrounding structures, and prevents overfilling of the heart with blood.
The outer wall of the human heart is composed of three layers. The outer layer is called the epicardium, or visceral pericardium since it is also the inner wall of the pericardium. The middle layer is called the myocardium and is composed of muscle which contracts. The inner layer is called the endocardium and is in contact with the blood that the heart pumps. Also, it merges with the inner lining (endothelium) of blood vessels and covers heart valves.^[8]