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 arteries are connected to the veins via very small vessels called capillaries. Electrical and chemical signals to the heart cause it to pump the blood into the arteries. Further, a number factors can cause the blood vessels to expand (dilate) or contract. In doing so they can increase or decrease the amount of effort the heart needs to put forth to deliver the blood.