The heart is a group muscles that pumps blood throughout the blood vessels by repeated, rhythmic contractions. It is found in all animals with a circulatory system. The vertebrate heart is principally composed of cardiac muscle and connective tissue. Cardiac muscle is an involuntary striated muscle tissue found only in this organ and responsible for the ability of the heart to pump blood. The structure of the heart can vary among the different branches of the animal kingdom. Cephalopods have two gill hearts and one systemic heart.
In vertebrates, the heart lies in the anterior part of the body cavity, dorsal to the gut. It is always surrounded by a pericardium, which is usually a distinct structure, but may be continuous with the peritoneum in jawless and cartilaginous fish. Hagfishes, uniquely among vertebrates, also possess a second heart-like structure in the tail. It is located anterior to the vertebral column and posterior to the sternum. It is enclosed in a double-walled sac called the pericardium.
It has two leaflets, the superficial is called the parietal pericardium and the inner one is the visceral pericardium. Between them there is some pericardium fluid which function is to permit them the easy slide with the heart movements. Outside the parietal pericardium there is a fibrous layer which is called the fibrous pericardium. This sac protects the heart and anchors it to the surrounding structures. The outer wall of the human heart is composed of three layers. The outer layer is called the visceral pericardium since it is also the inner wall of the pericardium.
The middle layer is called the myocardium and is composed of cardiac 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 of blood vessels and covers heart valves. The human heart has four chambers, two superior atria and two inferior ventricles. The atria are the receiving chambers and the ventricles are the discharging chambers. The pathway of blood through the human heart consists of a pulmonary circuit and a systemic circuit.
Deoxygenated blood flows through the heart in one direction, entering through the superior vena cava into the right atrium and is pumped through the tricuspid valve into the right ventricle before being pumped out through the pulmonary valve to the pulmonary arteries into the lungs. It returns from the lungs through the pulmonary veins to the left atrium where it is pumped through the mitral valve into the left ventricle before leaving through the aortic valve to the aorta.