When a heart beats, electrical impulses stimulate the muscles. The electrical pulses cause the heart to contract and retract. When the heart muscles contract and pull inward, the heart —
A applies a pushing force on the blood in the heart, forcing the blood outward through the body.
B applies a pulling force on the blood in the heart, forcing the blood outward through the body.
C is affected by a strong gravitational force that pushes the blood upward and out of the heart.
D is affected by a strong frictional force that pulls the blood upward and out of the heart.