The heart is basically a pump about the size of a clenched fist. It lies toward the front of the middle of chest with the wide end pointing toward the right shoulder and the small end pointing down and to the left.
A thin sac called the pericardium encloses the heart. It is made of a tough, fibrous tissue and keeps the heart from rubbing against the sternum and the lungs. Besides its toughness, the pericardium protects the heart by secreting a lubricating liquid.
The inner wall of the heart has three layers: the epicardium, the myocardium and the endocardium. The epicardium is the part of the pericardium that produces the lubricating liquid. The myocardium is the layer that holds the actual muscle of the heart, and the endocardium is a thin layer made up of squamous epithelial tissue. It is called squamous because the cells resemble the scales of a fish or dragon.
Inside the heart are four chambers with two vertical pairs separated by a thick muscle called the septum. The upper chambers on each side of the septum are called atria. They collect the blood that comes into the heart. Below each atrium is a ventricle, which pumps blood into the arteries. In contrast to the atria, the ventricles have thick, strong walls. Valves also work to control how blood flows through the heart.