Heart
The heart is a muscular pump which serves to circulate the blood throughout the body. The structure of the heart in vertebrates (organisms with backbones) is remarkable similar. All vertebrates have at least one atrium (a chamber which receives blood returning from circulation throughout the body) and at least one ventricle (a chamber which pumps blood into the arteries which circulate it throughout the body). As organisms become more complex, so do their hearts. Therefore, fish have only one atrium and one ventricle; amphibians have two atria and one ventricle; reptiles have two atria and two ventricles, although with an incomplete wall separating the two ventricles); birds and mammals have two atria and two ventricles, with complete separation between all four chambers.
In the human heart, the major job of the right side of the heart is to receive oxygen-poor (deoxygenated) blood, and then pump it back to the lungs to load up on oxygen. The major job of the left side of the heart is to receive oxygen-rich (oxygenated) blood, and then pump it out to all the other organs and tissues of the body.
The receiving chambers of the heart are called the atria (singular=atrium). The pumping chambers of the heart are called ventricles. Specialized valves are closed to prevent backflow from the ventricles to the atria during pumping. The valve operating between the right atrium and the right ventricle is called the tricuspid valve; the valve operating between the left atrium and the left ventricle is called the mitral valve. Valves also exist between each ventricle and the exiting arteries.
The basic flow of blood through the heart is as follows: deoxygenated blood is picked up throughout the body by veins which flow into the major veins, the venae cavae the vena cavae empty into the right atrium the right atrium empties into the right ventricle the right ventricle pumps blood through the pulmonary artery (the only artery in the body to carry deoxygenated blood) to the lungs, where the blood loads up on oxygen and gets rid of the waste product, carbon dioxide the pulmonary vein (the only vein in the body to carry oxygenated blood) carries blood to the left atrium the left atrium empties into the left ventricle the left ventricle pumps oxygenated blood into the body's largest artery, the aorta; numerous arteries branching off of the aorta deliver the oxygenated blood to all of the organs and tissues of the body
The heart is composed of specialized muscle tissue. It beats due to complex electrical networks which stimulate the carefully organized contraction (systole) and relaxation (diastole) of the muscle fibers. The functioning of the heart can be studied using a device which detects the electrical activity of the heart and graphs it on paper. This device is called an electrocardiogram (ECG or EKG). The beating of the heart, its strength, and its rate are all affected by various nerves and hormones. This allows the heart to respond to all kinds of stress, including exertion, emotional stress, and changes in the bodily environment such as infections.
A number of conditions and diseases can affect the heart. Because the heart is essential to life, these disorders are all potentially life-threatening. Such disorders include: infections or inflammations of the sac around the heart (pericarditis), the lining of the heart (endocarditis), or the cardiac muscle itself (myocarditis); electrical derangements within the heart, called arrhythmias or blockade; stiffening of and fatty blockages within the arteries which supply oxygen to the heart muscle (coronary artery disease, or atherosclerosis, which can lead to heart attack); long-term high blood pressure and stiffening in the arteries of the body, which cause the heart to have to pump more strongly against this pressure, possibly causing the heart to enlarge.
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