Animal Circulatory Systems An efficient circulatory system has: a fluid, e.g., blood, to carry the materials to be transported; a system of vessels to distribute the blood; a pump to push the blood through the system; exchange organs to carry out exchanges between the blood and external environment, e.g., lungs and intestine to add materials to the blood; lungs and kidneys to remove materials from the blood. The most crucial demand on the circulatory system is the transport of oxygen and carbon dioxide to and from a gas exchange organ: lungs or gills and the tissues. All exchanges between blood and cells occur in the capillaries. The force of the pump that pushes blood through the arteries is dissipated as the blood flows through capillaries.