The diagram below shows a model of the circulatory system. Red represents blood that has high oxygen content and blue represents blood that has low oxygen content. When one part of the heart contracts, blood moves from the heart to the lungs. Which statement best explains this motion of the blood?
A. The heart only contracts when it contains blood with high oxygen content.
B. The lungs cause the heart to contract when they need more blood.
C. The lungs move to inhale and exhale gases when the heart contracts.
D. The heart supplies the force that moves blood through the body.