heart- the hollow muscular organ that pumps the blood throughout the body. 

  The heart is a strong muscular pump with four chambers: right atrium, left atrium, right ventricle, left ventricle. The atria contract to push blood into the ventricles as the latter relax. Then the right and left ventricles contract to pump blood into the pulmonary arteries and the aorta. Meanwhile, the atria relax and fill with blood to start the cycle again. 
  
  The heart is a fist sized bag of tough walled cardiac muscle protectively encased by the thin, tough tissue of the pericardium, and shaped like a pear. The pulmonary artery is the only artery in the blood that carries oxygen depleted blood. It carries this blood into the lungs, where the blood releases carbon dioxide and picks up oxygen. The fresh blood with oxygen then returns to the left atrium through the four pulmonary veins.