In addition to the cellular components of the blood there are small particles of cellular origin, called platelets or thrombocytes. They are formed by fragmentation of large cells, megakaryocytes, in the bone marrow, which have an unusual maturation from the stem cells.
Formation is under control of thrombopoietin.
It is a vehicle for all the raw materials of the tissues - water, oxygen, glucose, etc and for removing waste products. The principal plasma proteins are albumin, globulin and fibrinogen and are all formed by the liver except g globulin which is formed by lymphoid tissues. The molecules of protein are too big to pass through the walls of the capillaries, but the other constituents are free to do so. Their tendency to escape under blood pressure is counteracted by the osmotic pressure of the proteins. This is the first function of the plasma proteins and the main function of albumin .