The heart is a muscular organ located near the center of the chest that powers the circulatory system. Its function is to continuously pump blood throughout the body, ensuring that oxygen and nutrients are delivered to every cell. This action is managed by a coordinated structure of chambers, valves, and major blood vessels working together in a two-part circuit. The right side handles blood needing oxygenation, and the left side manages the distribution of oxygen-rich blood to the body.
The Four Chambers and Internal Divisions
The heart contains four hollow spaces, or chambers, that collect and pump blood in a specific sequence. The two upper chambers are the atria, which function as receiving reservoirs for blood entering the heart. The two lower, more muscular chambers are the ventricles, which are responsible for forcefully pumping blood out of the heart.
The right atrium receives deoxygenated blood returning from the body, while the left atrium receives oxygenated blood from the lungs. The right ventricle sends the deoxygenated blood toward the lungs. Conversely, the left ventricle pumps the newly oxygenated blood out to the rest of the body. This separation of function makes the heart work as two synchronized pumps.
A thick wall of muscle tissue, known as the septum, divides the heart vertically into right and left halves. This division is physically important because it prevents the oxygenated blood on the left side from mixing with the deoxygenated blood on the right side, ensuring maximum efficiency of oxygen transport. The septum is composed of an interatrial section separating the upper chambers and a much thicker interventricular section separating the lower chambers.
The muscular wall of the heart’s chambers is called the myocardium. The thickness of this tissue varies based on the work required of each chamber. The walls of the atria are relatively thin because they only pump blood a short distance into the adjacent ventricles. The left ventricle wall is the thickest of all, being approximately three times thicker than the right. This is necessary because the left ventricle must generate enough pressure to circulate blood through the entire systemic circuit of the body.
Gates and Stops: The Heart’s Valves
Four specialized valves regulate the flow of blood through the chambers and into the major vessels, functioning like one-way doors that prevent backflow. These valves are grouped into two pairs: the atrioventricular (AV) valves and the semilunar valves. Their precise opening and closing, which is triggered by pressure changes within the chambers, creates the familiar “lub-dub” sound of a heartbeat.
The AV valves are situated between the atria and the ventricles. On the right side, the tricuspid valve separates the right atrium from the right ventricle. On the left side, the mitral valve controls the flow of oxygenated blood from the left atrium into the left ventricle. These valves snap shut when the ventricles contract, creating the first heart sound (“lub”) and ensuring blood is pushed forward out of the heart.
The second set of regulators are the semilunar valves, which are located at the exit of the ventricles and into the major arteries. The pulmonary valve is positioned between the right ventricle and the pulmonary artery. The aortic valve is located between the left ventricle and the aorta.
Both the pulmonary and aortic valves open to allow blood to exit the heart during ventricular contraction. They close immediately when the ventricles relax, preventing blood from flowing backward from the arteries into the ventricles. The simultaneous closure of these two valves generates the second heart sound (“dub”).
The Highway System: Major Blood Vessels
A network of large blood vessels serves as the primary input and output lines directly connected to the heart. These vessels, collectively referred to as the great vessels, manage the movement of blood into and out of the heart, to and from the body and lungs.
Deoxygenated blood returns from the body via the two largest veins, the superior and inferior vena cavae, which merge directly into the right atrium. The superior vena cava collects blood from the upper body, while the inferior vena cava collects blood from all regions below the diaphragm. This blood is then pumped out of the right ventricle and into the pulmonary artery.
The pulmonary artery is unique because, despite being an artery, it carries deoxygenated blood away from the heart and directly to the lungs for gas exchange. Once the blood is oxygenated, it returns to the heart’s left atrium through the pulmonary veins. These veins are also exceptions to the general rule, as they carry oxygen-rich blood toward the heart.
The final major output line is the aorta, the body’s largest artery. The aorta emerges directly from the left ventricle and arches over the heart, distributing oxygenated blood to the entire systemic circulation. Its structure ensures that blood is delivered at high pressure to the smaller arteries that branch off to supply all organs and tissues.