CHAPTER V 

 BLOOD LIPIDS 



1. Introduction 



All types of lipids which are present in the tissues of animals occur in 

 greater or lesser amounts in the blood. Although the lymphatics provide 

 the chief initial pathway for the transport of the fats and other lipids from 

 the intestine, the blood is the ultimate avenue for the further distribution 

 of these substances to the liver and to the several fat depots. Moreover, 

 aside from the gastrointestinal phase, the blood serves as the medium of 

 transfer of the lipid components from one organ to another. 



The composition of the blood is considerably influenced by the ingestion 

 of food. Although the maximum level of the glucose or amino acids in the 

 blood is reached two to four hours after carbohydrates or proteins are in- 

 gested, the highest level of lipids is usually not attained in the blood earlier 

 than six hours after a heavy fat meal. Sometimes as long as nine or ten 

 hours may elapse before the effect of the food fat has disappeared from the 

 blood and the preprandial values are obtained. The lipids remain fairly 

 constant over a fasting period of several days. However, as the glycogen 

 stores are used up, an increased fat metabolism obtains, with the result 

 that a moderate hyperlipemia (increased blood fat level) may occur. 



Although the blood contains considerable amounts of the several lipids, 

 it is generally accepted that the oxidation of these components occurs only 

 in the tissues. There is no proof that any active metabolic changes can 

 take place in the blood stream. Not only are the lipids transported in the 

 blood stream largely in droplets which are in suspension in the plasma, but 

 the heterogeneity of the whole blood renders impossible any uniform dis- 

 tribution throughout this fluid. 



The blood lipids, which are carried from the intestinal tract in the chyle, 

 are mixed with the blood when the thoracic duct empties its contents into 

 the blood stream. The fine droplets, which are composed largely of neutral 

 fat, originally present in the chyle, are retained in suspension in the blood. 



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