386 FOODS CIRCULATED AND USED IN THE BODY 



heat to run an engine. As a draft of air is required to make a fire 

 under the boiler, so, in the human body, plenty of oxygen must be 

 given to the tissues so that food may be oxidized there, releasing 

 energy for work and forming the wastes, carbon dioxide, water, 

 and urea (nitrogen product). This oxidation takes place in all 

 the cells of the body, be they portions of a muscle, a gland, or the 

 brain. Here again the blood plays its part, for it carries the 

 oxygen to the cells and takes away the waste products to be 

 excreted either through the skin or the kidneys. The smooth 

 running of this body machine of ours is only continued because 

 of the exchanges of food and wastes made possible by means of 

 this wonderful system of tubes and pumps which makes up the 

 circulatory system. 



PROBLEM I. WHAT IS THE COMPOSITION AND WHAT ARE 

 THE USES OF THE PARTS OF THE BLOOD? 



Composition of the blood. We learned in a former unit that 

 the chief function of the digestive organs is to change foods so that 

 they can pass into the blood. The chemical composition of the 

 blood is very complex and varies in different parts of the body. 

 The fluid part is the plasma, which consists of about 90 per cent 

 water and the various organic food substances, digested sugars, 

 fats, amino acids, mineral salts, and numerous other substances, 

 among which are enzymes and hormones. The blood also holds 

 three kinds of bodies : the red corpuscles, the white corpuscles, 

 and the blood platelets. 



Laboratory Exercise. To study the corpuscles of the blood. Place 

 a drop of frog's blood on a glass slide. Cover and examine under a 

 compound microscope. What are the color and shape of the corpus- 

 cles that are most numerous and most easily seen? What are the 

 other irregular-shaped corpuscles, more transparent and not so easily 

 seen? Are corpuscles cells? Can you prove your statement? 



Using a slide containing a drop of your own blood, note that red 

 corpuscles have no nucleus. Are they cells? Do you find colorless 

 corpuscles as well? How do they compare with the red in number? 

 Compare the structure of blood corpuscles in man with those of a frog. 



So small and so numerous are the red corpuscles that about five 

 million of them are found in a cubic millimeter of normal blood. 



