deoxygenated blood

Deoxygenated blood is blood that has delivered its oxygen to the body's tissues and organs. It is darker red because it lacks most of the oxygen that was once attached to hemoglobin in the blood. This blood returns to the lungs via the veins, where it releases carbon dioxide—a waste product—and picks up fresh oxygen. The oxygenated blood is then pumped back to the body to nourish tissues. Essentially, deoxygenated blood is the used, oxygen-depleted blood returning to the lungs to be refreshed for another cycle.