Left Atrium

Click to View Larger Image

The left atrium is one of the four hollow chambers of the heart. It plays the vital role of receiving blood from the lungs via the pulmonary veins and pumping it to the left ventricle.

Anatomy

The left atrium is a small, hollow structure on the superior left side of the heart. It is separated from the right atrium by the interatrial septum and from the left ventricle by the bicuspid (mitral) valve. A thin, irregular pouch of cardiac tissue known as the left auricle extends from the left atrium along its anterior surface.

The heart wall of the left atrium is much thinner and weaker than that of the left ventricle. It is made of three distinct layers: the epicardium, myocardium, and endocardium.

• The epicardium is a thin layer of simple squamous epithelium that protects the heart wall by giving it smooth surface and by secreting pericardial fluid to lubricate the pericardial space.
• Deep to the epicardium is the myocardium that makes up the bulk of the heart wall and contains the cardiac muscle cells. Contraction of cardiac muscle cells in the myocardium produces the pumping force of the left atrium.
• Finally, the endocardium forms the innermost layer of the heart wall that is in contact with blood inside the left atrium. It is made of endothelium, a specialized simple squamous epithelium that prevents blood cells from sticking to the heart wall and forming dangerous blood clots.

Blood flow to the tissues of the left atrium is provided by the left coronary artery, which branches from the aorta at the surface of the heart. Most of the posterior tissues of the left atrium receive blood from the left circumflex coronary artery, one of the major branches of the left coronary artery. Deoxygenated blood is drained from the left atrium through the oblique vein of the left atrium, which joins with the larger coronary sinus to return blood to the interior of the heart.

Physiology

Blood is pumped by the right side of the heart to the lungs, where it passes through a capillary bed and picks up vital supplies of oxygen from the air in the lungs. This newly oxygenated blood collects in the pulmonary veins and travels to the left atrium of the heart. The left atrium acts as a collecting chamber for blood and allows it to pass into the larger and more muscular left ventricle. Stimulatory signals from the sinoatrial (SA) node in the right atrium act as a pacemaker and trigger the coordinated contraction of the myocardium of the atria, known as atrial systole. Atrial systole results in the active pumping of blood from the left atrium into the left ventricle to fill the ventricle completely. The left ventricle contracts a few milliseconds after the left atrium, forcing the bicuspid valve to close and pumping blood into the aorta and onwards towards the body's organ systems. While the ventricles are contracting, the left atrium relaxes to allow more blood from the lungs to refill its chamber and prepare for the next cardiac cycle.