Through the circulatory system, blood adapts to the body's needs. When you are exercising, your heart pumps harder and faster to provide more blood and hence oxygen to your muscles. During an infection, the blood delivers more immune cells to the site of infection, where they accumulate to ward off harmful invaders.

All of these functions make blood a precious fluid. Each year in the USA, 30 million units of blood components are transfused to patients who need them. Blood is deemed so precious that is also called "red gold" because the cells and proteins it contains can be sold for more than the cost of the same weight in gold.

Leukocytes, also called White Blood Cells (WBCs), are associated with the immune system. There are three main classifications of leukocytes: lymphocytes, granulocytes, and agranulocytes. Granulocytes, as their name suggests, are white blood cells that have granules present in their cytoplasm. Agranulocytes do not have these granules. Using a  special histological staining method (Wright’s Stain), leukocytes can be seen under a microscope. This stain includes azure B to stain the granules in the cytoplasm of white blood cells so that they can be differentiated.

Leukocytes are classified by both the shape of their nuclei and the presence or absence of azurophilic granules in their cytoplasm. Neutrophils, basophils, and eosinophils are all polymorphonuclear granulocytes. Polymorphonuclear cells have nuclei with multiple lobes. Monocytes and lymphocytes are mononuclear agranulocytes. Mononuclear cells have a single nucleus. The buffy coat, seen when blood is spun down in a centrifuge to separate its cellular components, is generally made up of 60-70% neutrophils, 20-25% lymphocytes, 3-8% monocytes, 2-4% eosinophils, and less than 1% basophils. 

Platelets are made in your bone marrow along with your white and red blood cells. Your bone marrow is the spongy center inside your bones. Another name for platelets is thrombocytes. Healthcare providers usually call a clot a thrombus. Once platelets are made and circulated into your bloodstream, they live for 8 to 10 days.

Under a microscope, a platelet looks like a tiny plate. Your healthcare provider may do a blood test called a complete blood count to find out if your bone marrow is making the right number of platelets: