CHAPTER VI 



THE OCCURRENCE OF LIPIDS IN THE 

 ANIMAL AS A WHOLE 



1. Introduction 



Since the levels of the several lipids in the blood mirror the conditions 

 which obtain in the tissues, any abnormality which results in variations in 

 the blood picture is usually to be ascribed to an alteration of the metabolic 

 breakdown in the tissues themselves. Blood is generally regarded as an 

 indifferent fluid insofar as the lipids are concerned ; it functions chiefly in 

 providing a means for transporting the latter from the gastrointestinal 

 tract to the tissues for storage, or from one tissue to another to supply 

 material for oxidation. The blood also helps to regulate the intermediary 

 metabolism of lipids by rendering available, at the site of transformation of 

 the fats or other lipids, any hormones necessary for the chemical trans- 

 formations. 



The intermediary metabolism of each of the several lipids is quite distinct, 

 and involves its own characteristic metabolic transformations. How- 

 ever, the fate of neutral fat and that of phospholipids are sufficiently 

 closely related to render a simultaneous consideration desirable. On the 

 other hand, the metabolism of the steroids and of the vitamins is each of a 

 sufficiently specific nature to warrant separate treatment. 



In the study of the intermediary metabolism of the fats and phos- 

 pholipids, one is first concerned with the nature of the changes occurring in 

 the liver, with their storage in the fat depots, with their conversion to 

 other substances, and finally with their complete oxidation to carbon 

 dioxide and water. The partial breakdown of fatty acids to the ketone 

 bodies, which occurs in diabetes and in other conditions, helps to explain 

 the pathways of transformation involved when a complete oxidation ob- 

 tains. 



The lipids occurring in different tissues vary markedly in nature. In 

 some cases, the lipid component represents a metabolically active storage 

 depot for fat. On the other hand, the lipid may actually comprise the 

 structural tissue. This applies to the cerebrosides in the brain and, to 



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