Nina studies an artificial heart model. The model has tubes that supply an electric current to the model. Nina switches on the current that provides an impulse to the heart. She observes that the heart contracts and relaxes. Compare this model with that of a human heart, and place the steps describing how blood flows through the human heart in order.

The human heart has it own nervous system, nodal system. It's made of nodes (that produces nerve impulses) and nerve fibers (running to all parts of the heart). Our nervous system can only increase or decreasing the nodes firing rates.

Starting with deoxygenated blood (oxygen poor blood), it travels from the superior and inferior vena cava into the right atrium, then into the right ventricle via the tricuspid value. There, it enters the pulmonary arteries via the pulmonic value. From there, it heads to the lungs and become oxygenated (rich in oxygen). It re-enters the left atrium of the heart via the pulmonary veins. There, it now travels to the left ventricle via the mitral valve and then into the aorta via the aortic value. From there it travels to the rest of the body supplying cells with oxygen and return back via the vena cava.