The human heart is a remarkable muscular organ located in the chest cavity between the lungs. It consists of four chambers: two upper chambers called atria and two lower chambers called ventricles. The heart functions as a dual pump system, with the right side responsible for pumping blood to the lungs for oxygenation, while the left side pumps oxygen-rich blood throughout the body.
The heart operates two distinct circulation circuits. The pulmonary circulation involves the right side of the heart pumping deoxygenated blood through the pulmonary arteries to the lungs, where it picks up oxygen and returns via pulmonary veins to the left atrium. The systemic circulation involves the left side pumping oxygen-rich blood through the aorta to supply the entire body, with deoxygenated blood returning through the vena cava to the right atrium.
The heart contains four crucial valves that maintain unidirectional blood flow. The tricuspid valve controls flow from the right atrium to right ventricle, while the pulmonary valve regulates flow from the right ventricle to the lungs. On the left side, the mitral valve controls flow from left atrium to left ventricle, and the aortic valve regulates flow from the left ventricle to the body. These valves open and close in precise coordination to prevent blood from flowing backward.
The cardiac cycle represents the complete sequence of events during one heartbeat. It consists of systole, the contraction phase, and diastole, the relaxation phase. During atrial systole, the atria contract to fill the ventricles. This is followed by ventricular systole, where the ventricles contract forcefully to pump blood out of the heart. During diastole, all chambers relax and refill with blood. These coordinated contractions create pressure changes that drive blood circulation throughout the body.
The heart has its own electrical conduction system that controls the rhythmic beating. It begins with the sinoatrial node, the natural pacemaker, which generates electrical impulses. These signals travel to the atrioventricular node, which delays transmission to allow the atria to empty completely. The impulse then passes through the bundle of His and spreads via Purkinje fibers throughout the ventricular muscle, causing coordinated contraction. This electrical activity can be measured as an electrocardiogram or ECG.