Description:

 

The heart is a 4-chambered organ made of three layers of tissue: from outside to inside, they are the epicardium, myocardium, and endocardium. Surrounding the entire heart is the pericardium. The four chambers are the right atrium and ventricle and the left atrium and ventricle. The job of the upper atria is to either receive deoxygenated blood from the vena cava (for the right one) or to receive oxygenated blood coming from the left ventricle (for the left one). The job of the lower ventricles is to either pump deoxygenated blood from the right atrium to the pulmonary arteries(for the right one) or to pump oxygenated blood out through the aorta (for the left one). Between these four chambers are valves that prevent blood from moving in the opposite direction. Between the atria and ventricles, there are four valves to prevent the backward flow of blood: The tricuspid valve between the right atrium and ventricle, the bicuspid valve between the left atrium and ventricle, the pulmonary semilunar valve between the right ventricle and pulmonary vein, and the aortic semilunar valve between the left ventricle and aorta.

The cardiac cycle is the contraction and corresponding relaxation of the myocardium, leading to a heartbeat. During diastole, ventricles relax as the atria pump blood into them. During systole, ventricles contract as they pump blood out, letting the atria become filled. The pressures created by the blood cause diastolic and systolic pressure, respectively. To coordinate the flow of blood, the heart uses electrical signals originating from the sino-atrial (SA) node on the right atrium, travels to the atrioventricular (AV) node, travels through Purkinje fibers, and cause ventricle contraction. The nodes are known as gatekeepers because they delay contraction.