The heart is a cone-shaped organ located in the chest cavity between the lungs. It is surrounded by fluid-filled sac called the pericardium. The heart has four chambers - two upper atria that receive blood and two lower ventricles that pump blood out. Blood flows through valves from the atria to ventricles and then out arteries or pulmonary artery. The heart's wall has three layers - outer epicardium, middle muscular myocardium, and inner endocardium. The heart continuously contracts and relaxes in a cardiac cycle of diastole and systole to pump blood through the body.

1. dr shabeel pn

21. Cardiac Conduction system

26. Cardiac Cycle

27. The cardiac cycle is the sequence of events that occur when the heart beats. There are two phases of this cycle: Diastole - Ventricles are relaxed. Systole - Ventricles contract.

28. Image courtesy of MedValet Cardiac Cycle

30. Cardiac Cycle Diastole Phase SA node contracts. Atria contract. Ventricles fill with more blood. Contraction reaches AV node . Late Diastole Atria and Ventricles are relaxed. Semilunar valves are closed. Atrioventricular valves are open. Ventricles continue to fill with blood. Mid Diastole Ventricles relax. Semilunar valves close. Atrioventricular valves open. Ventricles fill with blood. Early Diastole

32. Contraction passes from AV node to Purkinje fibers and ventricular cells. Ventricles contract. Atrioventricular valves close. Semilunar valves open. Blood is pumped from the ventricles to the arteries. Systole Cardiac Cycle Systole Phase