Double closed circulation, or closed circulation, is that in which the blood circulates through two associated and parallel vascular systems: one that carries blood to and takes blood from the peripheral tissues (the systemic circulation) and the other that carries blood to and takes blood from the tissues that perform gas exchange with the environment, e.g, the lungs (pulmonary circulation). Double circulation occurs in  amphibians, reptiles, birds and mammals. Simple closed circulation, or simple circulation, is the one in which the tissues that perform gas exchange are associated in series with the systemic circulation, as in fishes.