Left atrium

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Anterior (frontal) view of the opened heart. White arrows indicate normal blood flow.

Gray's Fig. 496 - Interior of left side of heart.

The left atrium is one of the four chambers in the human heart. It receives oxygenated blood from the four pulmonary veins, and pumps it into the left ventricle.

Blood is pumped through the left atrioventricular orifice, which contains the mitral valve. The left atrium is larger than the right. A normal left atrium may be up to 5.5cm in maximum diameter; any larger than this is a sign of cardiac failure. This may occur in cases of mitral regurgitation.

The left atrium of a human faces more or less posteriorly. It is named 'left' based on the chamber's embryological and (putative) evolutionary origin. The term 'base' of the heart sometimes refers to the left atrium, though 'base' is an ambiguous term.

There is a foramen ovale (oval hole) between the right and left atrium in the fetus. After birth, this should close over and become the fossa ovale. If it does not, this is an atrial septal defect (hole in the heart). In the fetus, the right atrium pumps blood into the left atrium, bypassing the pulmonary circulation (which is useless in a fetus). In an adult, a septal defect would result in flow in the reverse direction - from the left atrium to the right - which will reduce cardiac output, potentially cause cardiac failure and in severe or untreated cases, death.

Attached to the left atrium is the left auricular appendix (auricle). This auricle is a common site for formation of thrombi, which may embolise causing stroke or ischemic gut. Atrial fibrillation makes this more likely.

The left atrium is supplied mainly by the left circumflex coronary artery, though the branches are too small to be identified in a cadaveric human heart and are not named. The oblique vein of the left atrium is partly responsible for venous drainage; it derives from the embryonic left superior vena cava.

[edit] Animals

Many other animals, including mammals, also have four-chambered hearts, and have a left atrium. The function in these animals is similar. Some animals (amphibians, reptiles) have a three-chambered heart, in which the blood from each atrium is mixed in the single ventricle before being pumped to the aorta. In these animals, the left atrium still serves the purpose of collecting blood from pulmonary veins.