BODY TISSUES 295 



ihouoht to contribute tlnoniboplastin, the subslanc:e essential lot the 

 conversion ol i^rotltrombin to thrombin in the initiation ol the 

 blood-dotting process. 



Plasma and serum. Ihe fiiiicl portion ol inicoagulated blood is 

 called plasma. It can be isolated from whole blood by removing the 

 formed bodies by centrifugation. Plasma contains a group of proteins 

 including the albumins, globulins, and fibrinogen. If blood is allowed 

 to clot, the fluid portion remaining is termed serum. Serum and 

 plasma are similar in composition except that the fibrinogen is re- 

 moved in the clotting process. Blood plasma contains about 90 per 

 cent water, 6 to 8 per cent proteins, 0.85 per cent inorganic salts, 

 and small amounts of glucose, triglycerides, phosphatides, cholesterol, 

 and the Jiitrogenous constituents urea, uric acid, ammonium salts, 

 creatine, creatinine, and purine bases. Table 13-1 lists the inorganic 



TABLE 13-1. Inorganic Constituents of Human Plasma 



Constituent Per Cent 



constituents of human plasma. The amounts of organic and inorganic 

 constituents are subject to narrow fluctuations, depending on the 

 state of nutrition and the activity of the individual. The non-protein 

 nitrogenous components are largely waste products from cellular 

 metabolism in transit to the kidneys and other organs of excretion. 



Serum albumins and globulins are very similar in physical properties 

 to the albumins and globulins obtained from eggs. Although the 

 albumins and globidins isolated from different species have similar 

 molecular weights and other physical properties, antigenically they 

 appear to be distinct for each species. Serum globulins can be sep- 

 arated into a number of fractions, of which a-, ft-, and y-globulin are 

 the most important. The y-globulin fraction has received very exten- 

 sive study as it contains a large portion of the antibodies (page 114) 

 found in blood. 



Serum proteins serve as buffers helping to regulate the pH of the 



