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.
This is because blood tends to move from high pressure to low pressure areas. At the same time, downward movement of the diaphragm causes pressure in the abdomen to rise, pushing venous blood back to the heart. The blood cannot flow backward because veins contain valves that only allow it to flow in one direction.