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.
Arteries do not have valves because the heart provides the pressure needed to send blood through the system. Veins have valves because venous pressure is often not great enoug â¦ h (as the blood must overcome gravity and other forces) to return the blood to the heart. + 103 others found this useful. Natii Carazo.