Humans have between 4 and 7 l of blood. Fifty five percent of this volume is plasma which is an aqueous solution of proteins, blood clotting factors, dissolved ions, wastes, dissolved gases, glucose and cholesterol. Blood also contains red blood cells (erythrocytes), platelets and five types of white blood cells (leucocytes). They are neutrophils, lymphocytes, monocytes, eosinophils and basophils. In addition blood has three buffer systems: the phosphate, the carbonic acid/bicarbonate system and the protein buffer which maintains blood pH between 7.35 and 7.45. Blood transports O2 from the lungs to the body’s cells by binding O2 to haemoglobin, CO2 from cells to the lungs, nitrogenous waste from cells to kidneys and sweat glands, absorbed nutrients from the small intestine to the liver and to cells and hormones from endocrine system to target cells. Blood also transfers heat around the body. Blood is able to coagulate to minimise blood loss when a vessel is damaged and contains antibodies and complement proteins to defend against bacteria. Blood cells are made in the active (red) bone marrow from stem cells called haemocytoblasts by the process known as haemopoiesis. The sequence of events that lead to blood clotting is known as haemostasis, while bleeding is known as a haemorrhage. Oxygen is transported attached to haemoglobin within the RBC – do you see a pattern yet in the words that refer to blood? Blood coagulation occurs when a clot of insoluble fibrin is formed from fibrinogen. The transformation of fibrinogen is produced by the enzyme thrombin. However thrombin is present in blood as the inactive prothrombin (factor II). Prothrombin is converted to thrombin by prothrombinase. However this enzyme is not present in blood either until activated by factor X. Factor X is unable to activate prothrombin until it unites with molecules that are produced either by the “intrinsic pathway” or the “extrinsic pathway” following damage to a blood vessel. This complicated arrangement ensures that all of the factors required to coagulate blood are present in plasma as inactive forms but are ready to go  – they don’t start the coagulation process until a blood vessel has been damaged.