n.
Circulatory fluid (see circulation ) in multicellular animals.
In many species it also carries hormone s and disease-fighting substances. Blood picks up oxygen from the lungs and nutrients from the gastrointestinal tract and carries them to cells throughout the body for metabolism . It picks up carbon dioxide and other wastes from those cells and transports them to the lungs and excretory organs. Blood composition varies among species. Mammalian blood consists of plasma , red and white cells ( erythrocyte s and leukocyte s), and platelet s (thrombocytes). Blood disorders include polycythemia (abnormal increase in the number of circulating red blood cells), anemia , leukemia , and hemophilia . See also ABO blood-group system ; blood analysis ; blood bank ; blood pressure ; blood transfusion ; blood typing ; Rh blood-group system .