Of What Are the Body Fluids Composed? 



Blood In all animals above the corals and sea-anemones, and certain 

 kinds of worms, there is present a circulating mass of liquid which is com- 

 monly called blood, although not all kinds of blood are alike (see pages 205- 

 207). The blood of backboned animals has a rather complex structure, and 

 is associated with an elaborate system of vessels and a pumping organ, called 

 the heart. 



The fluid portion of the blood is a colorless liquid, called the plasma, and 

 consists chiefly of water. In this are dissolved various salts, organic food sub- 

 stances, some oxygen, some carbon dioxide, certain enzymes, and other or- 

 ganic substances derived from various organs and tissues of the body. 



Floating in the plasma are large numbers of corpuscles — that is, "small 

 bodies". The most easily seen are the so-called red corpuscles. About 

 3200 of these corpuscles placed side by side would stretch an inch. In addi- 

 tion to the red corpuscles there are also colorless bodies of irregular shape, 

 the white corpuscles, of several distinct sizes and other characteristics. Some- 

 what resembling the red corpuscles in appearance are the very small color- 

 less "platelets" (see illustrations below and opposite). 



The Lymph The blood, consisting of plasma and corpuscles, fills a 

 set of tubes which have no openings through their walls. The system is 

 therefore called a closed blood system, to distinguish it from the blood 

 systems of clams, crustaceans, and certain other animals, in which some of 

 the blood tubes open into various spaces among the tissues. Outside the 

 blood vessels, filling the spaces among tissue masses and cells, is a colorless 

 liquid called lymph. It is from the lymph that the cells obtain their food 

 supplies, water, salts and oxygen. And it is to the lymph that they discharge 



,s *W ^^0^ 



(O Geneial Biological Siipph House. \m 



HUMAN BLOOD 

 186 



Under a microscope, human 

 blood appears to consist of 

 a colorless liquid with many 

 small bodies floating in it. 

 The more numerous particles 

 are the disk-shaped yellowish, 

 or "red", corpuscles, having 

 rounded edges. Some of the 

 white, or colorless, corpuscles, 

 which resemble the ameba, 

 are barely larger than the red 

 ones, others many times as 

 large. And there are disk- 

 shaped platelets,much smaller 

 than the red corpuscles 



