Haematology. The fluid which circulates through the heart, arteries, veins and capillaries. Blood functions to provide nutrition and respiration for tissues. It also transports waste from the tissues to the excretory organs. Blood provides thermal regulation to the body and transport of enzymes and hormones. It is composed of red blood cells (erythrocytes), white blood cells (leukocytes) and blood platelets (thrombocytes), and plasma.