
The
circulatory system consists of the
heart and blood vessels (
arteries,
veins,
capillaries). The heart propels the circulation of the
blood, which serves as a "transportation system" to transfer
oxygen, fuel, nutrients, waste products, immune cells, and signalling molecules (i.e.,
hormones) from one part of the body to another. The
blood consists of fluid that carries
cells in the circulation, including some that move from tissue to blood vessels and back, as well as the
spleen and
bone marrow.