Blood consists of blood cells, nutrients, protein, and electrolytes, each of which plays a very important role in blood functions. As shown in the below blood composition diagram, blood is a specialized body fluid and has four major components: plasma, red blood cells, white blood cells, and platelets. In a biological body system, blood has many different functions, including transporting oxygen and nutrients to the lungs and tissues. Blood is composed of 55 percent plasma and 45 percent formed elements. A blood composition diagram shows red blood cells are formed in the red bone marrow of bones and stem cells in the red bone marrow are called hemocytoblasts. They give rise to all of the formed elements in the blood.