A normally beating heart contracts and relaxes to a regular beat. (vetmed.auburn.edu)
It produces the electrical signal that makes your heart contract and relax like an angry milkmaid's fist. (discovermagazine.com)
The researchers investigated factors such as smallest and largest sizes of the left atria, the percentage of blood pushed out of the left atrium when the heart contracted and the amount of strain on the left atrium during heart contraction. (sciencedaily.com)