THE SYSTEMIC CIRCUIT
In the systemic circuit, oxygenated blood is pumped throughout the body to supply each of the trillions of individual cells with O₂ and to remove the CO₂ generated by aerobic respiration.

The left ventricle contracts, forcing deoxygenated blood through the aortic valve into the aorta. Immediately after exiting the heart, the aorta arches downward to carry blood to the lower part of the body. Branching off the arch are a group of arteries that supply the upper torso and brain. In both the brain and the rest of the body, the arteries branch repeatedly — ultimately becoming capillaries that contact every single cell. This contact provides every cell in the body with a supply of O₂ and a means of disposing of CO₂.

The now deoxygenated blood begins its return journey to the heart in veins. The returning veins eventually merge to form the superior and inferior venae cavae, which empty directly into the right atrium.

By the time blood exits the capillaries, virtually all of the left ventricle's pumping force has dissipated. Most of the force used to pump the blood back to the heart is actually generated by skeletal muscles. During activity, skeletal muscle contractions squeeze the veins, forcing the blood back to the heart. One-way valves in the veins ensure that blood only flows toward the heart.