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.
The left ventricle is subsequently squeezed causing blood to flow up the aorta and out to the body. When the heart is contracting like this it is in Systole. Similarly, valves in the aorta close due to the pressure of the blood (caused by gravity).