The heart is a muscular organ in humans and other animals, which pumps blood through the blood vessels of the circulatory system. The heart is located between the lungs, in the middle of the chest. In humans, it is about the size of a fist and weighs about 300 grams (11 ounces).
The wall of the heart is made up of three layers:
-The outer layer (epicardium)
-A thick middle layer (myocardium)
-And an inner layer (endocardium).
The human heart has four chambers: two atria (upper chambers) and two ventricles (lower chambers). Blood enters the heart through two large veins, the superior vena cava and inferior vena cava. These veins empty into the right atrium. The muscles in the right atrium contract and push blood through a valve into the right ventricle. The muscles in the right ventricle contract and pump blood through another valve, calledthe pulmonic valve, into pulmonary arteries leading to lungs. In the lungs, carbon dioxide is removed from circulation and oxygen enriched blood returns to left atrium via pulmonary veins. From left atrium, blood passes though mitral valve into left ventricle which contracts to pump oxygenatedblood out of heart via aorta to body tissues for distribution of oxygen and nutrients. Deoxygenatedblood from body tissuesreturns toright atrium via venae cavaeand cycle repeats.