left atrium
The left atrium is the heart's receiving room for freshly oxygenated blood returning from the lungs. Imagine the lungs as a refueling station: once blood is recharged with oxygen, it flows back into the left atrium before being handed to the heart's main pumping chamber.
Anatomically, the left atrium is the upper left chamber of the heart. It receives oxygen-rich blood from the lungs through four pulmonary veins, then delivers that blood through the mitral valve into the left ventricle below it.
Although it is a low-pressure chamber, the left atrium matters a great deal in disease. It sits just behind the heart and can enlarge when the mitral valve leaks or narrows, or when the left ventricle stiffens. An enlarged left atrium is a common breeding ground for atrial fibrillation, the most common sustained heart rhythm disturbance.
A small pouch off the left atrium, the left atrial appendage, is where clots tend to form during atrial fibrillation, which is why stroke prevention is a major concern in that condition.