HOW THE HUMAN BODY USES FOOD 



95 



How is digested food absorbed? Absorption of the 

 digested food takes place almost entirely through the 

 walls of the small intestine by osmosis of the soluble 

 foods. The intestines are well adapted for this pur- 

 pose. In an adult they are about twenty feet long, and 

 the inner surface is greatly increased by a succession 

 of circular ridges and depressions, and by millions of 

 finger-like projections which extend into the interior 

 of the intestines. These projections, known as villi 

 (sing., villus) are lined with networks of fine blood 

 vessels into which the digested food passes as it is ab- 

 sorbed by the villi. See Figure 134. 



How is food circulated in the body? After food has 

 been digested and absorbed it must be distributed to 

 all parts of the body where the cells use it. The cells 

 of the muscles, bones, nerves, glands, etc., must all 

 receive nourishment. 



The distribution of the food is carried on by our 

 circulatory system (see Figure 135). The heart pumps 



blood to all parts of the body 

 through blood vessels known 

 as arteries. The main arteries 

 branch first into the smaller 

 arteries and then into tiny 

 blood vessels called capil- 

 laries which extend to prac- 

 tically every part of the 

 body. You know this to be 

 true, for if you prick or cut 

 yourself almost anywhere 

 on the surface of your body, 

 blood will appear. The cells 

 of the body, which are in 

 close contact with the capil- 

 laries, receive the food from 

 the blood in them. The 

 blood in turn takes on waste 

 materials from the cells. The capillaries join to form 

 small veins. The smaller veins form larger veins which 

 lead back to the heart. 



This wonderful process of circulation was discov- 

 ered by William Harvey, an English physician, about 

 three hundred years ago. It is one of the greatest dis- 

 coveries ever made in science, and yet when Harvey 

 first announced his discovery most people did not 

 believe him ; in fact, he was ridiculed by many. Do you 

 think that it is scientific to resist the introduction of 

 new knowledge? 



Blood is composed of a liquid called plasma in which 

 are located red and white corpuscles. The red cor- 

 puscles give the blood its color as they are more 

 numerous than the white ones. Their chief constitu- 

 ent is a complex substance called haemoglobin which 

 carries oxygen to the cells of the body. The white cor- 

 puscles aid the body in resisting disease germs. They 



engulf bacteria that enter the blood and assemble in 

 places where infection occurs to help the body resist 

 the invasion of germs. 



Science Service 



FIG. 137. WILLIAM HARVEY 



His discovery of the 

 process of blood circula- 

 tion is one of the early 

 great achievements of 

 modern experimental sci- 

 ence. 



Courtesy P. Blakiston's Son 

 FIG. 138. WHITE AND RED CORPUSCLES UNDER THE MICROSCOPE 



When the human body is cut, blood flows from the 

 wound. If only the capillaries are severed, the blood 

 escapes slowly. If the cut is deep 

 and arteries or veins are severed, 

 blood flows rapidly. Blood from 

 veins flows steadily and is darker in 

 color than blood from arteries. 

 Blood from arteries pulsates, be- 

 cause of the pumping action of the 

 heart. Blood contains a substance 

 which thickens when exposed to 

 the air. This causes blood to clot 

 and prevents us from bleeding to 

 death. When an artery is cut, the 

 blood may not clot rapidly enough, 

 to stop bleeding. When this hap- 

 pens a cloth tied tightly near the 

 cut, on the side nearer the heart, 

 may slow the flow of blood suffi- 

 ciently to allow it to clot. The cloth 

 tied near the wound should be loos- 

 ened again after about ten minutes 

 or serious complications may de- 

 velop. 



How is energy released to the body? Energy is re- 

 leased to the body by oxidation. This takes place in 

 every cell of the body. Oxidation is made possible 

 through breathing. Breathing refers to the process 

 of taking air into the lungs and expelling the waste 

 products. In the lungs, oxygen is taken up by the 

 blood and carried to all the cells of the body, where 

 oxidation takes place, forming carbon dioxide, which 

 is returned to the lungs by the blood. The carbon 

 dioxide is thrown off as a waste product through 



FIG. 139 

 TOURNIQUET 



