CHAPTER LXXX 



FAT METABOLISM (Cont'd) 



THE FAT OF BLOOD 



Methods of Determination. Normally the blood contains only a small percentage 

 of fat, but after a fatty meal it may contain so large an amount that the fat actually 

 rises to the surface of the blood like a cream. By means of the ultramicroscope, ex- 

 amination of the blood in the dark field after a fat-rich meal reveals the presence of 

 glancing particles, the so-called "fat dust." These particles are most abundant about 

 six hours after the meal has been taken, and they gradually disappear by the twelfth 

 hour. They do not appear after a meal when the thoracic duct is ligated. They dis- 

 appear when oxygen is bubbled through the blood. 



Fat dust has also been found abundantly present in the blood of embryo guinea pigs 

 at full time, but not in the mother 's blood. This would indicate that the placenta must 

 have the power of taking the constituents of fat from the mother's blood and building 

 them into fat, which then passes into the blood of the fetus. The placenta under these 

 conditions acts like the mammary gland. In this connection it is of interest that there is 

 also much fat present in the blood of pregnant women. The fat content of the placenta 

 is, however, greater in the early stages of pregnancy than later. 



Although these facts have been known for some time, it has been impossible, either on 

 account of the large quantities of blood required for a chemical examination or because 

 of the difficulty in estimating the amount of fat from the density of the "fat dust," 

 to follow with any great degree of accuracy the exact chemical changes that take place 

 in the fat of the blood. Recently, however, Bloor has succeeded in elaborating methods 

 by which the fat content of the blood can be determined with satisfactory accuracy in 

 small quantities of blood, so that a continuous series of observations can be made over a 

 considerable period. 



The fat is extracted from the blood by an alcohol-ether mixture with moderate heat. 

 An aliquot portion of the filtrate is evaporated in the presence of sodium ethylate , which 

 saponifies the fat. The residue, consisting of soap, is well washed and then treated 

 with hydrochloric acid so as to precipitate the fatty acid. The density of the precipitate 

 thus produced is compared in an optical apparatus, called a nephelometer, with a 

 standard solution of two milligrams of oleic acid treated in the same way. The fatty 

 acids in human blood are mainly oleic and palmitic. 



The lecithin and cholesterol may also be estimated in the same blood extract. For 

 lecithin the above extract of blood, after the removal of the alcohol and ether, is digested 

 by heating with concentrated HNO 3 and H 2 SO 4 . This decomposes the lecithin, liberating 

 the phosphorus, a solution of the resulting ash being rendered faintly alkaline to phenol- 

 phthalein and then slowly added to a silver nitrate solution. The density of the pre- 

 cipitate thus produced is compared in the nephelometer with that of a precipitate pro- 

 duced in the same amount of silver nitrate by adding to it a standard phosphoric acid 

 solution. 



For cholesterol an aliquot portion of the above extract is saponified with sodium 

 ethylate and then saturated with chloroform ; the chloroform extract is mixed with acetic 



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