Double circulation in humans refers to two different circuits of blood circulation, according to the BBC. Humans and all other mammals have double circulatory systems. One circuit takes blood to the lungs to pick up oxygen, and the other circuit brings oxygenated blood to all other tissues.
According to the BBC, blood is pumped in and out of the heart twice as it takes one full journey around the body. The more muscular left side pumps oxygenated blood through the aorta and, ultimately, to every capillary, and the right side pumps deoxygenated blood through the pulmonary artery to the lungs.