220 HUMAN BIOLOGY 



ways in which the internal environment of the cell is made 

 favorable by keeping them on the move and constantly 

 fresh and uniform. 



The Nature of Blood and Tissue Fluid. The blood, con- 

 stituting about 8 per cent of the body weight, is a remarkable 

 substance consisting of immense numbers of red corpuscles 

 (a drop of blood contains millions of them) and also minute 

 motile white corpuscles, floating in a thickish watery 

 solution of salts, sugar and albuminous material, the plasma. 

 The plasma constitutes somewhat more than half of the 

 total blood mass. The red corpuscles are able to take on 

 very quickly in the lungs a load of oxygen, which is more 

 or less unloaded in other parts of the body where the cells 

 are in need of it. On the way back from these cells to the 

 lungs the red corpuscles carry one of the waste products of 

 activity, the carbon dioxide which results from oxidation 

 or burning, a process that yields heat and mechanical work 

 in the activities of the organism. The motile white corpuscles 

 serye as scavengers and protectors against inert foreign 

 particles and invading germs which, if they should accumu- 

 late, would pollute the stream. The plasma is a conveyor 

 of all manner of food materials provided by the fmal digestive 

 processes in the intestines. These materials are carried, like 

 oxygen, to the remote cells for use in case of need or to 

 special places where they are stored for future use. The 

 plasma also carries from the cells the waste materials, 

 apart from carbon dioxide, which result from the wear and 

 tear of activity, and delivers them to the kidneys through 

 which they are discharged from the body. 



The plasma also has the extraordinary capacity to change 

 from a fluid to a jelly when it comes into contact with an 

 injured region. If the blood vessels are damaged, for example, 

 and there is danger of loss of blood through the opening, 

 the jellifying or clotting of the plasma forms a plug which 

 more or less promptly closes the opening and prevents what 

 might be a serious bleeding. 



The tissue fluid diff'ers from blood chiefly in containing no 

 red corpuscles and less albuminous material than the 

 plasma. It does contain, however, white corpuscles, and 



