According to For Dummies, deoxygenated blood is blood that has no oxygen. Blood becomes deoxygenated after receiving carbon dioxide in exchange for carbon dioxide, which occurs at the cell membrane during respiration and circulation.
For Dummies says that deoxygenated blood enters the right atrium of the heart during circulation, at which point it is forced through the pulmonary valve and into the pulmonary artery. Here, the deoxygenated blood travels through the pulmonary artery to the lungs where it receives oxygen and becomes oxygenated blood again. From here, the process starts all over again, and the oxygenated blood leaves the pulmonary artery to bring oxygen to the cell membranes once more.