*Blood serves several functions, apart from transporting oxygen and nutrients, including roles in wound healing, inflammation, and immunity, by delivering components (cells and soluble factors such as antibodies and cytokines) to appropriate locations. The various cells (erythrocytes, leucocytes, and platelets) are suspended in plasma. The capacity of the blood to clot is important for haemostasis and a complex cascade of blood clotting factors (factors I–XII) are involved, together with platelets (see also Hageman factor; protein C; protein S; thrombomodulin; von Willebrand factor). The fluid left after clotting has occurred is serum. Clot formation can be initiated by the contact activation pathway or the tissue factor pathway. Blood clots (thrombi) can, however, form inappropriately and cause embolisms or in extreme cases disseminated intravascular coagulation. Tests for clotting time are standardized using the international normalized ratio. Various bleeding disorders (haemophilias), are caused by deficiencies in components of the clotting system (see Christmas disease). Defects in platelet function can also lead to bleeding disorders or excessive tendency to clotting (see Bernard–Soulier syndrome; Glanzmann's thrombasthenia; hereditary haemorrhagic telangiectasia; thrombophilia; thrombotic thrombocytopenic purpura). Red blood cell numbers (see haematocrit) can be reduced in anaemia, reducing the oxygen-transport capacity, or increased in polycythemia. Mutations in haemoglobin (see thalassaemia) can also affect oxygen-carrying capacity. Leucocyte and platelet numbers can be abnormally low (leucopenia) or raised (leucocytosis) in various forms of leukaemia, myelodysplasia, and thrombocythaemia. Various other disorders or abnormalities are recognized: see acidosis; alkalosis; azotaemia; bacillaemia; bacteraemia; galactosaemia; hyperaemia; hypercalcaemia; hypercapnia; hypercholanemia; hypercholesterolaemia; hyperCKmia; hyperferritinaemia–cataract syndrome; hypergammaglobulinaemia; hyperglycaemia; hyperkalaemia; hyperleucine–isoleucinaemia; hyperlipidaemia; hyperphosphataemia; hypertension; hypertriglyceridaemia; hyperuricaemia; hypervalinaemia; hypocalcaemia; hypogammaglobulinaemia; hypoglycaemia; hyponatraemia; hypotension; hypovolaemia; viraemia. See also blood group antigens and entries with the prefix haem-.
Subjects: Medicine and Health.