Circulation – blood is pumped by the heart, obtaining oxygen in the lungs and delivering oxygen via the blood transport system through the arteries (bright red blood to the brain and other body parts). The arteries (bluish blood) is the return path to the heart and lungs. The average human has about 9 pints of blood (4 to 5 litres) in the body, and when we donate blood about 450 ml is taken (about 7.5%), which is usually replenished in about 24 hours. The blood is also the transport system for nutrients, and for removing waste from the body (via the liver and kidneys).
Hormones from the endocrine system are circulated around the body via the bloodstream, and are responsible for regulating slow responses in the body such as growth and pregnancy. This is in contrast to the nervous system which is used to effect very fast responses to external stimuli.