Blood, including red cells, white cells, platelets, and a collection of specialized proteins, serves an essential physiologic function as it carries molecules from one part of the body to another. This chapter discusses the biochemical underpinnings of how blood transports oxygen from the lungs and iron to the tissues that need them without causing damage to the tissues that do not. This transport system also insures adequate waste removal from the human body. In addition, the biochemical properties of clotting, the system that maintains the integrity of the blood vasculature, are discussed. In describing the normal way in which blood carries out these functions, the discussion will illustrate, from a biochemical perspective, the ways diseases may result when these processes go awry.
These three important roles, however, are just a small fraction of the critical functions that blood carries out to maintain the proper performance of the human body. The core biochemical principles of these three systems—the role of allostery in regulating ligand binding in the oxygen–hemoglobin system, the specialized functions of proteins for iron transport and storage, and the importance of biochemical cascades as illustrated by the clotting system—apply broadly to other functions of the blood and other organ systems in general. Thus, by understanding these fundamental biochemical mechanisms, a general foundation for understanding the normal physiology and pathophysiology of other systems can be established.
BASIC COMPONENTS OF BLOOD
Hematology, the study of blood, has been of central importance in medicine throughout history. For much of history, health and illness were considered a reflection of different “humors” in the body: black bile, yellow bile, phlegm, and blood. Good health was attained when all the humors were in harmonic balance and illness was considered as the result of an imbalance in the humors. Thus, the goal of healers was to restore the balance of the humors in those who were ill. To manipulate blood, healers employed methods such as applying leeches, cupping, and bloodletting in hope of achieving health through creating proper balance of humors.
A human adult contains approximately 5 liters (l) of blood, with the entire volume circulating through the body every 1–2 min. Centrifugation separates blood into a cellular layer, which collects at the bottom end of the collection tube, and a noncellular layer, located at the top of the tube (Figure 14-1). If whole blood clots before centrifugation, the remaining noncellular component is called serum. On the other hand, if anticlotting additives are added to the test tube, the noncellular layer is called plasma. The cellular and noncellular layer each account for approximately 50% of the total blood volume.
Separation of Blood into its Basic Components. Upon centrifugation, blood separates into a noncellular “plasma” layer at the top of the tube; a middle “buffy coat,” composed of platelets and white blood cells; and a bottom layer of ...