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.