The human heart is composed of four chambers: two on top, called the atria, and two on bottom called the ventricles, according to LiveScience. The atria receive deoxygenated blood from the body, and the ventricles pump oxygen-rich blood back out into the body, states Johns Hopkins Medicine.
The pericardium encapsulates the heart, both protecting the heart and securing it inside the thoracic cavity, explains LiveScience. The pericardium contains two layers: the parietal pericardium, or the inner layer, and the serous pericardium, or outer layer. In between these two layers runs pericardial fluid, which provides lubrication for heart contractions and lung and diaphragm movements.
The atria and ventricles are separated by two atrioventricular valves: the tricuspid valve, which is on the right side of the heart, and the mitral valve, which is on the left side of the heart, notes InnerBody. Both valves allow blood to flow in only one direction from the atria to the ventricles to prevent regurgitation of blood. Each side of the heart also has a semilunar valve, which stops blood from flowing backing into the ventricles. The pulmonary valve is the semilunar valve on the right side of the heart, and the aortic valve is on the left.
The sinoatrial node, located in the wall of the right atrium below the superior vena cava, is composed of a bundle of cells that tell the atria to contract and set the heartbeat, explains InnerBody.