CHAPTER IV. 



THE BLOOD. 



Blood might well be considered under connective tissue as a 

 type in which the intercellular material, the plasma, is a fluid 

 carrying a variety of free cells. Both the fluid and the cellular 

 elements circulate through the body in endothelial-lined vessels and 

 act as media for metabolic exchanges. The blood also plays a vital 

 part in integrating and regulating the activities of the other tissues 

 through the endocrine secretions which it carries. 



THE PLASMA. 



Although the plasma ordinarily presents no structural features 

 in histological preparations, its physiological importance should 

 not be overlooked. Biochemical and biophysical studies show the 

 plasma to be composed largely of water with numerous substances 

 in solution. Various salts, sugar, proteins, and fat are held at 

 relatively constant concentration levels in the blood. During cir- 

 culation they are made available to the various cells of the body 

 by dift'usion through the capillary walls into the tissue juice at the 

 same time that metabolic wastes are entering the blood through the 

 capillaries from the tissue juice. The balance is kept constant by 

 a continuous replenishing of the various nutritive substances 

 coupled with an equivalent removal of wastes. Since some knowl- 

 edge of the role of the plasma in relation to processes concerned 

 with the internal environment of the organism is essential to an 

 understanding of the functioning of the tissues and organs, the 

 student is advised to consult the references given at the end of the 

 chapter. 



THE BLOOD CELLS. 



The cells of the blood are divided into two main groups: the 

 the erythrocytes, or red blood cells, and the leukocytes, or white 

 blood cells. In addition to these there may be small cytoplasmic 

 bodies, the platelets in the case of mammals, and spindle cells or 

 thrombocvtes in lower forms. 



