The human heart, a muscular organ located slightly to the left of the center in the chest, serves as the body’s primary pump. It continuously circulates blood, delivering oxygen and nutrients to every cell while removing waste products. This tireless action sustains all bodily functions, making its consistent operation fundamental to life.
The Heart’s Chambers and Walls
The human heart is divided into four distinct chambers, organized into an upper and lower pair. The two upper chambers are called atria (right and left), which receive blood. Below them are the two larger, more muscular lower chambers known as ventricles (right and left), responsible for pumping blood out of the heart.
These chambers are separated by muscular walls known as septa. The interatrial septum divides the two atria, while the much thicker interventricular septum separates the ventricles. The left ventricle possesses the thickest muscular wall, called the myocardium, reflecting its demanding role in pumping oxygenated blood to the entire body. The right ventricle has a less robust myocardial wall, as it only pumps deoxygenated blood to the nearby lungs.
Valves and Major Blood Vessels
Four specialized valves within the heart ensure that blood flows in only one direction, preventing backward movement. The tricuspid valve is situated between the right atrium and right ventricle, regulating blood flow. The pulmonary valve lies at the exit of the right ventricle, opening into the pulmonary artery.
On the left side, the mitral valve (bicuspid valve) controls blood movement from the left atrium to the left ventricle. The aortic valve, at the exit of the left ventricle, directs blood into the body’s main artery. These valves open and close in precise coordination, driven by pressure changes as the heart contracts and relaxes, facilitating efficient blood circulation.
Several major blood vessels connect directly to the heart, forming the initial pathways for circulation. Deoxygenated blood from the body enters the right atrium through the superior and inferior vena cava. The pulmonary artery emerges from the right ventricle, carrying deoxygenated blood to the lungs. Oxygenated blood returns from the lungs to the left atrium via the pulmonary veins. The aorta, the largest artery, originates from the left ventricle and distributes oxygenated blood to all systemic circulation.
The Pathway of Blood Flow
Blood circulation begins with deoxygenated blood returning from the body, entering the heart’s right atrium through the superior and inferior vena cava. As the right atrium fills and contracts, this blood is pushed through the tricuspid valve into the right ventricle. The right ventricle then contracts, propelling the deoxygenated blood through the pulmonary valve and into the pulmonary artery.
The pulmonary artery carries this blood to the lungs, where it releases carbon dioxide and picks up oxygen. This exchange occurs in tiny air sacs called alveoli, surrounded by capillaries. Once oxygenated, the blood returns to the heart, flowing through the pulmonary veins and entering the left atrium.
From the left atrium, the oxygen-rich blood passes through the mitral valve into the left ventricle. The left ventricle, the heart’s strongest chamber, contracts forcefully to pump this oxygenated blood through the aortic valve and into the aorta. The aorta then distributes the oxygenated blood to all parts of the body.
The Heart’s Electrical System
The heart’s rhythmic beating is governed by an intrinsic electrical system that generates and transmits impulses to coordinate muscle contractions. This system originates with the sinoatrial (SA) node, often called the heart’s natural pacemaker, located in the upper right atrium. The SA node spontaneously produces electrical signals, typically at a rate of 60 to 100 impulses per minute in a resting adult.
These electrical impulses spread across both atria, causing them to contract and push blood into the ventricles. The impulses then converge at the atrioventricular (AV) node, situated near the heart’s center, which briefly delays the signal to allow the ventricles to fully fill with blood. From the AV node, the electrical signal travels down specialized conduction pathways.
These pathways include the Bundle of His and the Purkinje fibers, which rapidly distribute the impulses throughout the ventricular walls. This rapid propagation ensures both ventricles contract almost simultaneously, efficiently pumping blood out of the heart. This synchronized electrical activity maintains continuous blood flow throughout the body.