The left atrium is one of the four chambers

Heart chamber

aHeart chamber is a general term used to refer to any chambers of the mammalian heart. The heart consists of four chambers: the right and left atrium and the right and left ventricle. The top chambers are connected to the bottom chambers by valves and are separated by the coronary sulcus...

 in the human heart

Human heart

The human heart is a muscular organ that provides a continuous blood circulation through the cardiac cycle and is one of the most vital organs in the human body...

. It receives oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

ated blood

Blood

Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 from the pulmonary veins, and pumps it into the left ventricle

Left ventricle

The left ventricle is one of four chambers in the human heart. It receives oxygenated blood from the left atrium via the mitral valve, and pumps it into the aorta via the aortic valve.-Shape:...

, via the mitral valve.

Foramen ovale

There is a foramen ovale (oval hole) between the right and left atrium in the fetus. After birth, this should close. If it does not, this is an atrial septal defect

Atrial septal defect

Atrial septal defect is a form of congenital heart defect that enables blood flow between the left and right atria via the interatrial septum. The interatrial septum is the tissue that divides the right and left atria...

 (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.

Blood supply

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

Oblique vein of the left atrium

The Oblique Vein of the Left Atrium is a small vessel which descends obliquely on the back of the left atrium and ends in the coronary sinus near its left extremity; it is continuous above with the ligament of the left vena cava , and the two structures form the remnant of the left Cuvierian duct....

 is partly responsible for venous drainage; it derives from the embryonic left superior vena cava

Superior vena cava

The superior vena cava is truly superior, a large diameter, yet short, vein that carries deoxygenated blood from the upper half of the body to the heart's right atrium...

.

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.