Why are hematologic studies of blood important in the diagnosis of disease?

Human blood is composed of two essential elements: the liquid consisting of water, organic matter and minerals in the solid soluble and consists of cellular elements (red cells, white cells and platelets). In case the blood harvested from the patient's vein is placed in a test tube coagulates after a few minutes. Then it is separated into a liquid layer, the upper, which is called serum and a lower layer, which is solid clot. If the tube that was harvested blood of a substance anticoagulant (which prevents clotting) when blood no longer coagulates, but separates everything into two layers: the upper layer liquid consisting of plasma and lower layer, solid, constituted coagulated celuare the elements.
From these two components serum and blood plasma will perform biochemical and serological main medical tests. Of blood cellular components is achieved hematology. Hematology investigating the number, shape, and composition characteristics of cellular elements of the blood.