The blood plays a central role in the body's respiration, nutrition, waste elimination, thermoregulation, immune defence, acid-base balance, water balance, and internal communications. Blood carries oxygen from the lungs to other organs. It carries nutrients from the digestive system and storage depots to other organs. Blood carries carbon dioxide from all organs to the lungs for removal. It carries other wastes to the liver and kidneys for detoxification or removal.
Blood carries hormones from the endocrine glands to their target cells.
It also carries metabolic heat to the skin for removal and helps to stabilise body temperature. Blood plays several roles in inflammation. Leukocytes destroy some microogarnisms and cancerous cells. Platelet factors initiate clotting and minimise blood loss. Blood transfer water to and from the tissues and helps to stabilise the water content of cells.