Blood serves many functions in the human body. The primary function of blood is to transport nutrients and wastes to and from the cells, but blood also plays a major role in helping the body fight infections. Additionally, blood plays a role in maintaining homeostasis, regulating body temperature, and forming clots to reduce blood loss. Blood is made up of plasma⎯a liquid composed of water, salts, and proteins⎯and solids that include red blood cells, white blood cells, and platelets. Blood can only be made by the body and cannot be artificially synthesized. Currently, the only source of blood for patients who need it is from blood donors. Donors who give blood must be carefully tested for blood type, diseases, and disorders in order to ensure that a patient receives blood that is compatible and safe.

At A Glance

  • The major functions of blood include transportation of nutrients and wastes, protection from foreign cells, and regulation of body temperature.
  • Hematopoiesis describes the pathway from a hemocytoblast to each formed element.
  • The blood contains erythrocytes (red blood cells), leukocytes (white blood cells), platelets (thrombocytes), and plasma.
  • Plasma is the yellow liquid part of the blood that contains water, nutrients, functional proteins, electrolytes, gases, waste, and hormones.
  • Red blood cells (erythrocytes) contain the protein hemoglobin and are specialized for transporting oxygen.
  • White blood cells (leukocytes), provide a defensive function in the body and are categorized as granulocytes and agranulocytes.
  • Platelets, also called thrombocytes, clump together in the process of blood clotting (hemostasis).
  • Blood type is determined by the presence or absence of antigens, molecules that can cause an immune response, such as ABO antigens and Rh factor antigens on erythrocytes.