ERA. 1  baseball, baseball game - a ball game played with a bat and ball between two teams of nine players; teams take turns at bat trying to score runs; he played baseball in high school; there was a baseball game on every empty lot; there was a desire for National League ball in the area; play ball!.
The contraction of the heart muscle starts in the two atria, which push the blood into the ventricles. Then the walls of the ventricles squeeze together and force the blood out into the arteries: the aorta to the body and the pulmonary artery to the lungs.
Their purpose is to allow blood to move forwards through the heart and to prevent it flowing backwards into the previous chamber. How does the heart work? The heart muscle contracts in two stages to squeeze blood out of the heart. This is known as systole. In the first stage, the upper chambers (atria) contract at the same time, pushing blood down into the lower chambers (ventricles).