Atria of the Heart

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What Are Heart Atria?

Interior Heart Anatomy
This cross-section of the human heart shows its interior anatomy including atria, ventricles, and heart valves. Encyclopaedia Britannica/UIG/Getty Images

The heart is an important organ of the circulatory system. It is divided into four chambers that are connected by heart valves. The upper two heart chambers are called atria. Atria are separated by an interatrial septum into the left atrium and the right atrium. The lower two chambers of the heart are called ventricles. Atria receive blood returning to the heart from the body and ventricles pump blood from the heart to the body.

Function of Heart Atria

The atria of the heart receive blood returning to the heart from other areas of the body.

  • Right Atrium: Receives blood returning to the heart from the superior and inferior venae cavae. The superior vena cava returns de-oxygenated blood from the head, neck, arm and chest regions of the body to the right atrium. The inferior vena cava returns de-oxygenated blood from the lower body regions (legs, back, abdomen and pelvis) to the right atrium.
     
  • Left Atrium: Receives blood returning to the heart from the pulmonary veins. The pulmonary veins extend from the left atrium to the lungs and bring oxygen-rich blood back to the heart.

Atrial Heart Wall

The wall of the heart is divided into three layers and is composed of connective tissue, endothelium, and cardiac muscle. The layers of the heart wall are the outer epicardium, the middle myocardium, and the inner endocardium. The walls of the atria are thinner than the ventricle walls because they have less myocardium. Myocardium is composed of cardiac muscle fibers, which enable heart contractions. The thicker ventricle walls are needed to generate more power to force blood out of the heart chambers.

Atria and Cardiac Conduction

Cardiac conduction is the rate at which the heart conducts electrical impulses. Heart rate and heartbeat rhythm are controlled by electrical impulses generated by heart nodes. Heart nodal tissue is a specialized type of tissue that behaves as both muscle tissue and nervous tissue. Heart nodes are located in the right atrium of the heart. The sinoatrial (SA) node, commonly called the heart's pacemaker, is found in the upper wall of the right atrium. Electrical impulses originating from the SA node travel throughout the heart wall until they reach another node called the atrioventricular (AV) node. The AV node lies on the right side of the interatrial septum, near the lower portion of the right atrium. The AV node receives impulses from the SA node and delays the signal for a fraction of a second. This gives atria time to contract and send blood to the ventricles prior to the stimulation of ventricular contraction.

Atrial Problems

Atrial fibrillation and atrial flutter are examples of two disorders that arise from electrical discharge problems in the heart. These disorders result in irregular heartbeat or heart quivering. In atrial fibrillation, the normal electrical pathway is disrupted. In addition to receiving impulses from the SA node, atria receive electrical signals from nearby sources, such as the pulmonary veins. This disorganized electrical activity causes atria not to contract fully and to beat irregularly. In atrial flutter, electrical impulses are conducted too quickly causing atria to beat very rapidly. Both of these conditions are serious as they can lead to decreased cardiac output, heart failure, blood clots, and stroke.