CHAPTER IV 



THE BLOOD 



THE blood is the fluid medium from which all the tissues of the body are 

 nourished. By means of the blood, materials absorbed from the alimentary 

 canal as well as oxygen taken from the air in the lungs are carried to the 

 tissues, while substances which result from the metabolism of the tissues are 

 carried to the excretory organs to be removed from the body. The blood 

 acts as a medium of exchange between the various tissues themselves. A 

 good example is found in the distribution by the blood of the products of 

 certain glandular activities, the internal secretions. The blood is an im- 

 portant factor in the regulation of the body temperature. 



The blood is a somewhat viscid fluid, and in man and in all other 

 vertebrate animals, with the exception of the two lowest, is red in color. 

 The exact color of blood is variable; that taken from the systemic arteries, 

 from the left side of the heart and from the pulmonary veins is of a bright 

 scarlet hue; that obtained from the systemic veins, from the right side of 

 the heart, and from the pulmonary artery is of a much darker color, which 

 varies from bluish-red to reddish-black. At first sight the red color appears 

 to belong to the whole mass of blood, but on further examination this is 

 found not to be the case. In reality blood consists of an almost colorless 

 fluid, called plasma or liquor sanguinis, in which are suspended numerous 

 minute masses of protoplasm, called blood corpuscles. The corpuscles are 

 of the two varieties, the white ameboid corpuscles, or leucocytes, and the 

 red corpuscles, erythrocytes. The latter compose by far the larger mass of 

 blood-cells. They contain the red pigment, hemoglobin, to which the color 

 of the blood is due. 



The plasma or fluid part of the blood is a remarkably complex chemical 

 mixture. It is kept in constant rapid circulation through the blood vessels 

 of the body and is, therefore, thoroughly mixed and homogeneous in 

 character. 



Quantity of the Blood. The quantity of blood in any animal under 

 normal conditions bears a fairly constant relation to the body weight. The 

 amount of blood in man averages T ^- to y of the body weight. In other 

 mammals the proportion of blood is also fairly constant, varying from 

 T2 to TO" f the body weight. In many of the lower vertebrates, the fishes 

 for example, the relative quantity of blood is very much less. 



An estimate of the quantity in man which corresponded very nearly 

 with this proportion has been more than once made by methods illustrated 



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