BLOOD. 123 



extent of the movement ; as a result, the fluid in the thoracic duct 

 outside of the thorax, being under a higher pressure, flows more 

 rapidly into the venous system. At the time of an expiration, the 

 pressure rises and the flow is temporarily impeded, only to begin 

 again at the next inspiration. 



BLOOD. 



The blood is a nutritive fluid containing all the elements necessary 

 for the repair of the tissues ; it also contains principles of waste 

 absorbed from the tissues, which are conveyed to the various excre- 

 tory organs and by them eliminated from the body. 



The total amount of blood in the body is estimated to be about 

 one thirteenth of the body-weight ; from ten to twelve pounds in an 

 individual of average physical development. The quantity varies 

 during the twenty-four hours, the maximum being reached in the 

 afternoon, the minimum in the early morning hours. 



Blood is an opaque, red fluid, having an alkaline reaction, a saline 

 taste, and a specific gravity of 1055. 



The opacity is due to the refraction of the rays of light by the 

 elements of which the blood is composed. The color varies in hue, 

 from a bright scarlet in the arteries to a deep purple in the veins, 

 due to the presence of a coloring-matter hemoglobin in different 

 degrees of oxidation. 



The alkalinity is constant, and depends upon the presence of the 

 alkaline sodium phosphate, Na 2 HPO 4 . 



The saline taste is due to the amount of sodium chlorid present. 



The specific gravity, within the limits of health, ranges from 1045 

 to 1075. 



The odor of the blood is characteristic, and varies with the animal 

 from which it is drawn ; it is due to the presence of caproic acid. 



The temperature of the blood ranges from 98 F. at the surface 

 to 107 F. in the hepatic vein; it loses heat by radiation and evapora- 

 tion as it approaches the extremities and as it passes through the 

 lungs. 



Blood Consists of Two Portions: 



i. The liquor sanguinis or plasma, a transparent, colorless fluid, in 

 which are floating 



