Atrium (heart)

Diagram of the human heart

The atria (singular: atrium) are two of the four chambers of the human heart, the other two being the ventricles. They are thin-walled, upper chambers that play a crucial role in the circulation of blood within the heart.

Structure

The human heart has two atria:

• Right Atrium: The right atrium receives deoxygenated blood from two large veins, the superior vena cava and the inferior vena cava. It then contracts to push this blood into the right ventricle.

• Left Atrium: The left atrium receives oxygenated blood from the pulmonary veins. It contracts to pump this oxygen-rich blood into the left ventricle.

Function

The primary functions of the atria include:

• Receiving Blood: The right atrium receives deoxygenated blood returning from the body, while the left atrium receives oxygenated blood from the lungs.

• Transfer to the Ventricles: The atria contract to push blood into the respective ventricles. The right atrium sends blood to the right ventricle, which pumps it to the lungs for oxygenation. The left atrium sends blood to the left ventricle, which then pumps it into the systemic circulation to supply oxygen to the body's tissues.

• Atrial Kick: The atria contribute to the final stage of ventricular filling by providing an "atrial kick," a contraction that ensures the ventricles are adequately filled before they contract.

Electrical Signaling

The heart's electrical conduction system initiates and coordinates the contractions of the atria and ventricles. The sinoatrial (SA) node, often called the heart's natural pacemaker, generates electrical signals that stimulate atrial contraction.

Clinical Significance

Atrial function is crucial for effective heart function. Issues with the atria, such as arrhythmias or atrial fibrillation, can disrupt the normal heart rhythm and impair cardiac output. Conditions affecting the atria may require medical intervention, including medication or procedures like cardioversion.

Summary

The atrium or auricle is the upper chamber through which blood enters the ventricles of the heart. There are two atria in the human heart - the left atrium receives blood from the pulmonary (lung) circulation, and the right atrium receives blood from the venae cavae (venous circulation). The atria receive blood while relaxed (diastole), then contract (systole) to move blood to the ventricles. All animals with a closed circulatory system have at least one atrium. Humans have two atria.