82 THE INDIVIDUAL ORGANISM 



parts of the body and back to the heart, and accessory drainage tubes 

 (the lymphatics) that collect the leakage from the blood conduits and 

 return this fluid (lymph) to the main circulation. 



Blood. Although fluid, blood is a tissue, in which the cells are sepa- 

 rated by large amounts of a fluid intercellular substance, instead of by a 

 solid substance as in bone. Blood makes up about 8 per cent of the entire 

 body weight, and there are about 6 to 8 quarts of blood in the body of a 

 150-pound man. Somewhat more than half of this volume is made up of 

 the noncellular fluid portion, or plasma. About 90 per cent of plasma is 

 water; the remaining 10 per cent is a complex and somewhat variable 

 mixture of dissolved and suspended substances. These include proteins 

 and amino acids, carbohydrates (chiefly glucose), various salts, nitro- 

 genous wastes, vitamins, internal secretions (hormones), and immunity- 

 producing substances (antibodies). 



The cellular portion of blood comprises three kinds of cells — red 

 corpuscles (erythrocytes), white blood cells (leucocytes), and blood platelets. 

 Examples of these are shown in Fig. 2.3. 



Red Corpuscles. These are incomplete cells that lack nuclei, for reasons 

 that will be clear when we see how they are formed. They constitute the 

 bulk of the blood cells and give blood its characteristic red color. Each is 

 a biconcave disk, about *Kooo mm - m diameter. The number of corpuscles 

 varies from person to person and from time to time in the same individual 

 but averages between 4.5 and 5 million per cubic millimeter in a healthy 

 adult. The red color is due to the contained hemoglobin, a respiratory 

 pigment the functions of which have already been considered in our 

 discussion of the respiratory system. 



White Blood Cells. These are of several types, all of which fall into 

 one or the other of two general classes — granular and nongranular. These 

 cells are all nucleated, and none contains hemoglobin. Leucocytes are 

 much less numerous than the red corpuscles, averaging perhaps 6,000 

 to 8,000 per cubic millimeter of blood in a normally healthy adult. Under 

 certain conditions of bacterial infection (appendicitis, for instance) they 

 multiply until as many as 30,000 may be present per cubic millimeter. 

 The functions of the white cells are varied. A great many of them serve 

 as phagocytes, or "eating cells" (Greek, phago, "to eat," and kytos, 

 "cell"), which destroy invading bacteria, dead cells, and organic particles 

 by ingesting them. Some phagocytes are important in defense against 

 disease ; others aid in the healing of wounds and other injuries by removing 

 the dead cells and damaged portions of tissues. 



Platelets. These are colorless like the white cells but are very small, 

 only about half the diameter of a red corpuscle. In shape they are typically 

 biconvex disks. The platelets have an important role in the process of 

 blood clotting, as is described below. 



