An open circulatory system is seen in Phylum Arthropoda and Class Insecta. Blood flows in a tube which functions as a heart and then leaves, entering the sinuses. Eventually, the blood re-enters the tube and is pumped toward the anterior direction once again. This system is inefficient and slow, but it is enough of a circulatory system to support an insect's life functions. A human has a closed circulatory system, as do all mammals and birds. This system has a heart which separates oxygenated and deoxygenated blood completely. One side of the heart pumps deoxygenated blood to the lungs to become oxygenated. The other side pumps oxygenated blood to the body's cells so they may receive the oxygen needed for respiration. In birds and mammals, which are both warm-blooded and have fast metabolic rates, a closed circulatory system insures rapid blood circulation, supplying food and nutrients to cells as well as waste removal from the cells. Arteries, veins and capillaries provide the continuous closed system of tubes, which the blood rapidly flows through.