The human heart is a muscular organ that functions as a pump, continuously circulating blood throughout the body. Its role is to deliver oxygen and nutrients to cells and tissues while removing carbon dioxide and other waste products. This rhythmic action sustains life, ensuring the body receives necessary supplies. The heart’s efficient pumping maintains blood pressure.
The Heart’s Anatomy
The heart is roughly the size of a clenched fist and resides in the chest, slightly to the left of the breastbone. It is divided into four chambers: two upper chambers called the atria and two lower chambers called the ventricles. The right atrium receives deoxygenated blood from the body, and the right ventricle pumps this blood to the lungs for oxygenation. Oxygenated blood returns to the left atrium from the lungs, and the left ventricle, which is the strongest chamber, then pumps this blood to the rest of the body.
Blood flow through these chambers is regulated by four valves: the tricuspid, mitral, pulmonary, and aortic valves. The tricuspid valve separates the right atrium and right ventricle, while the mitral valve lies between the left atrium and left ventricle. The pulmonary valve controls blood flow from the right ventricle into the pulmonary artery, and the aortic valve regulates flow from the left ventricle into the aorta. These valves open and close to ensure blood moves in one direction and prevent backflow.
How the Heart Beats
The heart’s rhythmic beating originates from specialized cells within its own electrical system. The process begins with the sinoatrial (SA) node, often called the heart’s natural pacemaker, located in the upper part of the right atrium. This small cluster of cells spontaneously generates electrical impulses. These electrical signals spread rapidly across both atria, causing them to contract and push blood into the ventricles.
The impulse then travels to the atrioventricular (AV) node, situated near the center of the heart, between the atria and ventricles. The AV node briefly delays the electrical signal, which ensures the atria have time to empty into the ventricles before they contract. After this brief delay, the impulse continues through a pathway called the bundle of His and into the Purkinje fibers, which rapidly distribute the signal throughout the ventricles. This coordinated electrical activity causes the ventricular muscle fibers to contract, ejecting blood into the arteries and out to the body and lungs.
This process involves two main phases: systole and diastole. Systole is the contraction phase, during which the heart muscle squeezes to pump blood out of the chambers. Diastole is the relaxation phase, where the heart muscles relax and the chambers refill with blood. These alternating contractions and relaxations form a complete cardiac cycle, ensuring efficient blood circulation.
What Makes the Heart Sounds
The “lub-dub” sounds of a heartbeat are not caused by the heart muscle contracting, but by the closing of the heart’s four valves. When listening with a stethoscope, the “lub” sound, known as S1, is heard first. This sound occurs when the tricuspid and mitral valves snap shut at the beginning of ventricular contraction.
The second sound, “dub,” or S2, follows the first. This sound is produced by the closing of the pulmonary and aortic valves as the ventricles finish contracting and begin to relax. These valve closures prevent the backward flow of blood, ensuring it moves in the correct direction. The vibrations created by this sudden closure generate the audible heart sounds.
Controlling the Heart’s Rhythm
While the heart possesses its own electrical system to initiate beats, its rate and rhythm are also regulated by external influences. The autonomic nervous system plays a role in this control. It comprises two branches: the sympathetic nervous system, which acts like an accelerator, and the parasympathetic nervous system, which acts like a brake.
During physical activity or stress, the sympathetic nervous system releases neurotransmitters like norepinephrine and epinephrine, often called adrenaline, which increase the heart rate and the force of contractions. During rest, the parasympathetic nervous system releases acetylcholine, slowing the heart rate. Hormones also influence heart rate; for example, thyroid hormones can affect the heart’s speed and force. This interplay allows the heart to adapt its pumping action to meet the body’s changing demands, whether during intense exercise or quiet sleep.