CHAPTER XII 

 THE BLOOD 



The blood may be defined as the nutritive fluid of the body since it con- 

 tains all those materials that are necessary to the maintenance of the nutri- 

 tion. The presence and proper circulation of the blood in the living or- 

 ganism are essential for the maintenance of tissue irritability and for the 

 manifestation of the activities of all physiologic mechanisms. The escape 

 of the blood from the vessels, especially in the higher animals, is followed by 

 cessation of the physiologic activities of all the tissues within a short period. 

 The irritability, however, persists for a variable length of time though it too 

 gradually declines and finally disappears. The immediate dependence of 

 the functional activities of the tissues and organs on the presence of the 

 blood can be demonstrated by the following experiment: If the nozzle of 

 a syringe, adapted to the size of the animal, be introduced through the 

 jugular vein into the right side of the heart and the blood be suddenly with- 

 drawn, there is an immediate cessation in the activity of all the organs; the 

 return of the blood to the vessels within a limited period of time is promptly 

 followed by a renewal of their activity. 



Though contained within a practically closed system of vessels, the blood 

 is brought into intimate relation with all the tissue elements through the 

 intermediation of the capillaries. As the blood flows through these delicate 

 vessels, portions of its soluble nutritive constituents, including oxygen, are 

 given up to the tissues, by which they are utilized for growth, repair, and 

 the liberation of heat. At the same time the tissues yield up to the blood 

 a series of decomposition or katabolic products, resulting from their activity, 

 which vary in quantity and quality according as the blood traverses the 

 muscles, nerves, glands, or other tissues. 



The blood may be regarded, therefore, as a reservoir of nutritive materials 

 prepared by the digestive apparatus and absorbed from the intestinal canal; 

 of oxygen, absorbed from the respiratory surface of the lungs; of katabolic 

 products, produced by and absorbed from the tissues. Though the blood 

 varies in composition in different parts of the body in consequence of the 

 introduction of both nutritive material and katabolic products, it neverthe- 

 less presents certain average physical, morphologic, and chemic properties 

 which distinguish it as an individual tissue. 



The Physical Constitution of Blood. A microscopic examination of 

 the blood as it flows through the capillary vessels of the web of the frog' or 

 the mesentery of the rabbit shows that it is not a homogeneous fluid, but 

 that it consists of two distinct portions, viz.': (i) a clear, transparent, slightly 

 yellow fluid, the plasma or liquor sanguinis: (2) small particles termed 

 corpuscles floating in it, of which there are two varieties, the red or the 

 erythrocytes and the white or the leukocytes. By appropriate methods it 

 can be shown that a third corpuscle, colorless in appearance and smaller 

 in size than the ordinary white corpuscle, is present in the blood-stream 



