170 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



gen and oxygen usually in the same proportion to oxygen as in water (H^O). 

 Carbohydrates furnish a large share of the energy required for the regular 

 needs of living; they provide the energy for such routine processes as respira- 

 tion, circulation, digestion, and excretion — the metabolism of the body. Fats 

 also provide energy but they are chemically less quickly accessible for use. 

 During digestion carbohydrates are broken down into glucose, a simple sugar 

 (CeHi-Oo) which is distributed in the blood to the liver, muscles, and other 

 tissues throughout the body. It is converted into glycogen, commonly called 

 animal starch. This is readily reconverted to glucose for immediate use any- 

 where in the body. Human blood usually contains about 0.1 per cent glucose 

 ready for instant use. 



Fats. Fat accumulates as pure fat, not mixed with water like protein and 

 carbohydrate, and when oxidized has a high heat output. Fat is a long-range 

 supply not ready for quick use like glycogen. Thus, the fat boy gets just as 

 hungry as the thin one. 



Fats (or lipids) are simplified by hydrolysis, that is, by chemically splitting 

 up and taking in water, into glycerol (or glycerin) and fatty acids. The true 

 fats, liquid and solid, are combinations of glycerin and fatty acids; oleic acid 

 in butter (CisH.s40o) is an example. They all contain carbon and hydrogen, 

 with less oxygen than carbohydrates. All fats are greasy and are soluble in 

 organic liquids such as ether or benzene, rarely in water. Certain of them, such 

 as cod-liver oil, are liquid in ordinary temperatures; others, such as lard and 

 tallow, are solid. The wax produced in the human ear and beeswax are sub- 

 stances very like the fats. The sterols are complex waxlike compounds of a 

 different chemical nature. Cholesterol in the bile and calciferol (vitamin Do) 

 are such sterols; the male and female sex hormones and certain cancer-pro- 

 ducing compounds also belong to the steroid group. Compound fats such as 

 lecithin contain nitrogen and phosphorus in addition to the elements regularly 

 contained in fats. Lecithin occurs in almost all living cells; it is a major item 

 in the yolk of eggs. 



Vitamins. Vitamins are compounds that are present in foods in small quan- 

 tities. They play an important part in human nutrition and probably in that of 

 all plants and animals. Vitamins are highly specific; for example, vitamin A 

 affects the cornea of the eye; others affect the hardening of bone (Fig. 1 1.3). 

 Some are soluble in fats, others in water; certain ones are destroyed by heat, 

 others are not. Human diet is apt to be deficient, especially in vitamin A, folic 

 acid, riboflavin, ascorbic acid, calciferol, and thiamine (Table 1 1.1). In nature 

 most of the vitamins are produced by plants. They are abundant in grasses, 

 and cats frequently bite off grass blades, apparently satisfying some kind of 

 hunger. Cats and other carnivores secure vitamins as they lick their fur and by 

 eating the fur and feathers of their prey. The old name "limey" for a British 

 sailor is indirectly connected with vitamins. In the days of sailing ships and 



