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The heart is a hollow muscular organ that pumps blood throughout the blood vessels to various parts of the body by repeated, rhythmic contractions.[1] It is found in all animals with a circulatory system, which includes the vertebrates.[2]

The adjective cardiac means "related to the heart" and comes from the Greek καρδιά, kardia, for "heart". Cardiology is the medical speciality that deals with cardiac diseases and abnormalities.

The vertebrate heart is principally composed of cardiac muscle and connective tissue. Cardiac muscle is an involuntary striated muscle tissue specific to the heart and is responsible for the heart's ability to pump blood.

The average human heart, beating at 72 beats per minute, will beat approximately 2.5 billion times during an average 66 year lifespan, and pumps approximately 4.7-5.7 litres of blood per minute. It weighs approximately 250 to 300 grams (9 to 11 oz) in females and 300 to 350 grams (11 to 12 oz) in males.[3]

Structure The structure of the heart can vary among the different animal species. (See Circulatory system.) 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. Hagfish, uniquely among vertebrates, also possess a second heart-like structure in the tail.[4]

In humans Main article: Human heart

Structure diagram of the human heart from an anterior view. Blue components indicate deoxygenated blood pathways and red components indicate oxygenated pathways. The adult human heart has a mass of between 250 and 350 grams and is about the size of a fist.[5] It is located anterior to the vertebral column and posterior to the sternum.[6]

It is enclosed in a double-walled sac called the pericardium. The pericardium's outer wall is called the parietal pericardium and the inner one the visceral pericardium. Between them there is some pericardial fluid which functions to permit the inner and outer walls to slide easily over one another with the heart movements. Outside the parietal pericardium is a fibrous layer called the fibrous pericardium which is attached to the mediastinal fascia.[7] 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 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 contractile cardiac muscle. The inner layer is called the endocardium and is in contact with the blood that the heart pumps.[8] Also, it merges with the inner lining (endothelium) of blood vessels and covers heart valves.[9]

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. During each cardiac cycle, the atria contract first, forcing blood that has entered them into their respective ventricles, then the ventricles contract, forcing blood out of the heart. The pathway of the blood consists of a pulmonary circuit and a systemic circuit[10] which function simultaneously. Deoxygenated blood from the body flows via the vena cava into the right atrium, which pumps it through the tricuspid valve into the right ventricle, whose subsequent contraction forces it out through the pulmonary valve into the pulmonary arteries leading to the lungs. Meanwhile, oxygenated blood returns from the lungs through the pulmonary veins into the left atrium, which pumps it through the mitral valve into the left ventricle, whose subsequent strong contraction forces it out through the aortic valve to the aorta leading to the systemic circulation.[11][12]Bold text