In a closed circulatory system, blood is the transport medium and is confined in blood vessels and does not come directly in contact with tissues and cells.The heart pumps blood under high pressure throughout the body and blood returns directly to the heart.Substances leave the blood through gaps in the walls of blood vessels and enter the cells by diffusion.The mass transport of substances under pressure in closed vessels increases the rate at which substances can be delivered to tissues and cells.Diffusion takes place at the cells where diffusion distances are very short so the smallest blood vessels must run very close to all the cells.