Blood is a special type of connective tissue: the intercellular matrix, the plasma, is a fluid, and the cells are red and white blood corpuscles. Small fragments of cells, the platelets, and large proteins such as fibrinogen, albumin and globulin are non-cellular elements carried in the plasma. Both red and white blood cells are derived from the same primitive cell, the haemocytoblast. The red blood cells, or erythrocytes, are contained within the blood vessels and carry oxygen and carbon dioxide. The white blood cells, or leucocytes, are part of the body’s defence mechanism and use the circulation as a means of transport to particular sites where they leave the blood vessel and enter the tissues. The cellular elements account for 40% of whole blood and the plasma for 60%.