12 THE HUMAN BODY 



substances, the first steps in the digestive process give rise to com- 

 pounds which differ from the simple proteins by a slight degree 

 only. These are the derived proteins. The members of the group 

 which occur most commonly in the Body are the proteases and pep- 

 tones. These are present in the stomach during protein digestion. 

 They are characterized by greater solubility than simple proteins 

 possess. 



Nitrogenous Extractives. Under this head are grouped various 

 nitrogen-containing substances most of which represent materials 

 that have done their work in the Body and are about to be gotten 

 rid of. Nitrogen is present in the living tissues of the Body chiefly 

 as a part of their proteins. The vital activities of the tissues in- 

 volve the breaking down of these complex proteins into simpler 

 substances. Part of their carbon combines with oxygen and passes 

 out through the lungs as carbon dioxid ; their hydrogen is similarly 

 in large part combined with oxygen and passed out as water; 

 while their nitrogen, with some carbon and hydrogen and oxygen, 

 is passed out in the form of crystalline extractives. 



Urea is the most important substance of this class; fully nine- 

 tenths of all the nitrogen that is eliminated from the Body is in 

 this form. It is a diamide of carbonic acid, having the formula 

 NH 



; the relationship of urea to carbonic acid is clear when 



OH 

 the formula for the latter is written thus: CO<Q^. Fully 30 



grams of urea are eliminated daily from the Body of an adult man. 



Creatinine (C 4 H 7 N 3 O) is an interesting member of the group of 

 extractives because the amount of it that is eliminated from the 

 body daily is very constant, regardless of changes in the amount 

 of food or exercise taken, and seems to depend closely upon the 

 amount of muscle tissue present in the Body; persons of great 

 muscular development have a larger daily creatinine output than 

 those of smaller build. 



Creatine (C 4 H 9 N 3 O 2 ) is closely related chemically to creatinine, 

 but appears to play a very different part in the Body. Creatinine 

 is undoubtedly a waste product of protein decomposition, being 

 merely an incidental product of the vital processes which go on 

 within the organism. Creatine, on the other hand, seems to be of 

 use to the muscles, although just what purpose it serves is not clear. 

 About one per cent of the solid substance of muscle is creatine. 



