The human heart is a muscular organ that serves as the body's pump, responsible for circulating blood throughout the body by rhythmic contraction and dilation of its four chambers. Blood carries oxygen, nutrients, and hormones to the cells, and removes waste and carbon dioxide. The heart works in a cycle of contraction and relaxation, called a cardiac cycle, which is regulated by electrical impulses generated by the sinoatrial (SA) node and coordinated by the atrioventricular (AV) node. Blood enters the heart through two large veins, the superior and inferior vena cava, and is pumped out of the heart to the rest of the body through the aorta and other blood vessels. The heart's ability to contract and relax in a coordinated and efficient manner is essential for maintaining the overall health and function of the body.