Blood must be brought close to the cells since nutrient and metabolic waste exchange takes place by passive diffusion.Diffusion is only efficient over short distances.A circulatory system uses bulk flow to reduce the effective distance between the transport medium and the various parts of an organism.Insects have an open circulatory system with blood circulating in large open spaces. Larger animals have closed circulatory systems with blood contained within tubes. This allows blood to be transported around the body under high pressure.Fish have a single circulatory system so blood pressure is reduced and blood does not flow very quickly.In a double circulatory system, blood flows through the heart twice for each circulation of the body.Mammals have a closed, double circulatory system. In mammals there are two circulations. Pulmonary and systemic. Oxygenated and deoxygenated blood do not mix so the tissues receive as much oxygen as possible.The heart can increase the pressure of the blood after it has passed through the lungs.If the pulmonary circulation is too high it may damage the delicate capillaries of the lungs.