MILK, BLOOD AND URINE 223 



which prevent their activity. It is also thought that they take 

 part in the production of immune substances in the blood. 

 Other theories are that they aid in the absorption of fats and 

 peptones, that they take part in the process of blood coagula- 

 tion, and that they maintain the protein content of the blood 

 plasma. 



Blood Plasma. — The blood plasma, as we have stated, is 

 the liquid portion which holds the corpuscles, plates and dust 

 in suspension. In this blood plasma there are held in true 

 solution both organic and inorganic substances. The amount 

 of solid matter contained in the plasma is about 8.0 per cent. 

 The organic constituents amount to about 7.0 per cent and the 

 inorganic to about i.o per cent. 



The two most important organic constituents are glucose 

 sugar and proteins. The former is in very small amounts, only 

 about 0.1 per cent, but is very important, as it is the form in 

 which carbohydrate food, and perhaps all food, is finally oxidized 

 in the cells. The protein constituents are in larger amounts 

 and are four in number, viz. (i) fibrinogen, (2) a nucleo protein, 

 (3) 3- globulin, and (4) serum albumin. Other organic constit- 

 uents present in small amounts are enzymes, fats, lecithin, 

 cholesterol, sarco-lactic acid, urea, uric acid and some others. 



The inorganic constituents of blood plasma include chlorides, 

 carbonates, sulphates and phosphates of the metals sodium, cal- 

 cium, magnesium, potassium and iron. 



Blood Serum. — When fresh-drawn blood is exposed to the 

 air, the striking phenomenon termed clotting takes place. In 

 this clotting of the blood one of the soluble protein constituents 

 of the blood plasma, the fibrinogen, is converted into an in- 

 soluble protein termed fibrin. This insoluble fibrin in separat- 

 ing carries down with it the blood corpuscles and other sus- 

 pended matter in the form of a thick clot. As this clot sepa- 

 rates there is left a light yellow liquid known as blood serum. 

 The serum differs from plasma in containing no fibrinogen. 

 In it there are present, however, all of the other soluble con- 

 stituents of the plasma. 



