Hematopoietic System

The hematopoietic system is re­sponsible for the production of blood cells. The hemato­poietic system is composed of pluripotent stem cells that differentiate to form the red blood cells (RBCs), white blood cells (WBCs), and platelets through a process called hemato-poiesis. New blood cells are constantly being generated to re­place older blood cells that are broken down. A balance between new blood cell production and blood cell break­down is necessary for normal circulating volume. As red blood cells are broken down, one of the major byproducts of the breakdown of hemoglobin is bilirubin (see Chap­ter 16). The lipid-soluble form of bilirubin, which is called unconjugated bilirubin or indirect reacting bilirubin, combines with plasma proteins for transport in the blood and inter­stitial fluids. Unconjugated bilirubin is removed from the blood by the liver and conjugated to form a water-soluble form of bilirubin. Conjugated (direct-acting) bilirubin is ex­creted from the liver cells into the biliary system and then into the intestinal tract. Normally, about two thirds of the unconjugated bilirubin produced by a term newborn can be effectively cleared by the liver. However, the relative imma­turity of the newborn liver and the shortened life span of the fetal red blood cells may predispose the term newborn to hyperbilirubinemia and yellowish pigmentation of the skin, known as jaundice (Fig. 2-6). With the establishment of suf­ficient enteral nutrition, regular bowel elimination, and nor­mal fluid volume, the liver is usually able to clear the excess bilirubin. The presence of jaundice with elevation of either indirect or direct bilirubin that is present at birth, appears within the first 24 hours of life, or is persistent indicates a pathophysiologic cause and should be investigated.