ABSTRACT
Blood is the medium for the transport of the needed essentials of life. The blood circulates within a closed system called the vascular system. Contained by the blood are the red blood cells (the erythrocytes), the white cells (including antibodies and macrophages), platelets, brin, clotting factors, various metabolites, electrolytes, various hormones and cytokines, various lipids, and eicosanoids. Some of these are listed in Table 4.1. Chapter 1 contains several tables giving the normal levels of some of these components. As can be seen, the blood is a rich supplier of everything the body needs to stay alive and to respond to the external environment. The blood serves a fundamental role in the maintenance of homeostasis and, because it is a readily accessible tissue, it has been much studied. The process of blood clotting has been revealed through the study of clotting components; the identication of small proteins that serve as messengers (the cytokines) has been possible because blood samples can be drawn and studied in response to a variety of treatments. The same is also true for studies of the function of the eicosanoids, hormones, and metabolic ux. We have learned so much about the function of the body through studies of the various components of blood.
