PRINCIPLES Of PHYSIOLOGY g^ 



marily in liver and muscle. Glycogen synthesis occurs to a lesser extent 

 in other tissues. Between meals the stored glycogen is broken down for 

 use. Liver glycogen can be converted enzymatically into glucose and 

 secreted into the blood stream. One of the prime functions of the 

 vertebrate liver is the maintenance of a constant level of glucose in 

 the blood. It does this by absorbing glucose from the blood coming 

 from the intestine just after a meal, when the blood has a high concen- 

 tration of glucose, and by secreting glucose into the blood stream be- 

 tween meals. The glycogen in muscle and other tissues cannot be con- 

 verted to glucose (one of the enzymes required is absent) and hence must 

 be utilized locally. Carbohydrates are rapidly converted to fats if more 

 are taken in than can be used directly. These, plus the fats taken in as 

 food, are stored for use between meals. 



Nutritive Requirements. In addition to proteins, fats and carbohy- 

 drates, animals require water, minerals and vitamins to maintain health 

 and to grow. Minerals are constantly lost from the body in urine, feces 

 and sweat, and an equivalent amount must be taken in with the food. 

 Most foods contain adequate supplies of minerals, and mineral de- 

 ficiencies are comparatively rare. Certain htmian deficiency diseases may 

 be traced to a lack of iron, copper, iodine, calcium or phosphorus. A 

 disease which was resulting in the death of whole herds of sheep in 

 Australia was finally shown to be due to a deficiency of cobalt. The soil 

 in that region, and hence the grass eaten by the sheep, was very poor in 

 this metal which is required as a trace element for normal metabolism. 



Water is required by every animal. Aquatic animals have no prob- 

 lem about obtaining water; indeed, their problem is to prevent the 

 osmotic inflow of water and the consequent bursting of their cells. Many 

 land animals drink water, but others, certain desert animals for example, 

 obtain all they require from the food eaten, and from the water formed 

 when the food molecides are metabolized. 



Vitamins are organic substances required in small amounts in the 

 diet. They differ widely in their chemical structure but are similar in 

 that they cannot be synthesized by the animal and hence must be present 

 in the diet. What is a vitamin for one animal is not necessarily one for 

 anot/ier anitnal. That is, some species can synthesize certain of these 

 required substances and hence do not need them in their food. It is 

 probable that all plants and animals require these vitamin molecules for 

 similar metabolic functions; organisms differ, however, in their ability 

 to synthesize them. Only man, monkeys and guinea pigs, for example, 

 require vitamin C in the diet; other animals can make it from some 

 other substance. The vitamins whose role in metabolism is known— 

 niacin, thiamine, riboflavin, pyridoxine, pantothenic acid, biotin, folic 

 acid and cobalamin (vitamin Bio)— have proved to be constituent parts 

 of one or more coenzyme molecules. Vitamin A is a part of the light- 

 sensitive pigment of the retina of the eye (p. 580). A lack of any one of 

 these vitamins produces a particular deficiency disease with characteristic 

 symptoms, e.g., scurvy (lack of vitamin C), beriberi (lack of thiamine), 

 rickets (lack of vitamin D) and pellagra (lack of niacin). 



