The left side of the heart receives unoxygenated blood back from the body, via the superior and inferior Vena Cava, and then forces the blood into the pulmonary circulation, via the pulmonary arteries. A good rule of thumb to remember the circulation involved is Left (as in side of the heart) Lung.
The right side receive blood from the lungs that is oxygenated and forces the blood out the Aorta in to the systemic circulation (the body)
