The heart is the propulsive organ which is present in all animals that have a circulatory system which maintains the circulation of the blood by means of rhythmical contractions. The organ is a fixture amongst all vertebrates and it is composed of cardiac muscle, an involuntary striated muscle tissue that does not exist anywhere else in the body, and connective tissue.
The human heart weighs between 250 and 350 grams, and is placed in the front of the vertebral column, i.e. the backbone, behind the sternum, i.e. the breastbone, and it is enclosed in a double-walled sac called the pericardium. The pericardium serves to hold the heart attached to the surrounding structures as well as serving as a protective cover and helping to prevent the organ from being overfilled with blood.
The heart’s outer wall is made up of three layers, the epicardium, aka the visceral pericardium because it is the inner wall of the pericardium; the myocardium which is made up entirely of cardiac muscle; and the endocardium which is in contact with the blood that comes into and out of the heart and is merged with the endothelium, i.e. the inner lining of the blood vessels and the heart valves.
The heart is made up of four chambers, two superior atria (singular: atrium) and two inferior ventricles. The two atria receive incoming blood while the two ventricles handling the outgoing blood. The inflow and outflow of the blood is coordinated by four valves entering and exiting the heart: the tricuspid, the mitral, the aortic and the pulmonary valves. The ventricles are thicker and more powerful than the atria and the left ventricle is thicker and more powerful than the right one because of the additional force that it requires to pump.
The job of the right heart (right atrium and right ventricle) is to collect all the deoxygenated blood from the body in the right atrium by way of the superior and inferior vena cavae and transport that blood by way of the tricuspid valve through the right ventricle into the lungs where carbon dioxide is exchanged for oxygen by way of diffusion. The oxygenated blood now starts its return journey, going through the pulmonary vein to the left atrium from whence it goes on to the left ventricle by way of the mitral valve. From the left ventricle, the oxygenated blood is sent to the aorta where the blood is separated for use in the upper and lower parts of the body. Entering the arteries, the oxygenated blood travels through smaller arterioles to the tiny capillaries which provide sustenance to each individual cell. Then, the relatively deoxygenated blood enters the venules, tiny traffic lines that coalesce into the veins and, by way of the inferior and superior vena cavae reenter the right atrium and the whole process of circulation restarts.
The heart is undoubtedly the hardest working organ in the body. In humans, the heart is functional at about five weeks after the last normal menstrual period, i.e. about 21 days, give or take a few days, after conception, and, starting from that point, for a man or woman who lives to be 65 years, the heart would have beat nonstop some two and a half billion (2,500,000,000) times. No other organ works that much; certainly none that does not have a supporting partner.