1570s, from Latin atrium "central court or main room of an ancient Roman house, room which contains the hearth," sometimes said (on authority of Varro, "De Lingua Latina") to be an Etruscan word, but perhaps from PIE *ater- "fire," on notion of "place where smoke from the hearth escapes" (through a hole in the roof). Anatomical sense of "either of the upper cavities of the heart" first recorded 1870. Meaning "skylit central court in a public building" first attested 1967.
1. He has found that smoke and toxic gases can be transported swiftly up through the atrium.
2. From the left atrium the blood passes through the bicuspid valve into the right ventricle.
3. Left ventricular failure results in distention of the left atrium, engorgement of the pulmonary veins.
4. After this courtyard inspection discovery, in patient heart left atrium steadily tumor.
5. The upper chamber on each side is called the atrium.