198 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



substances that create immunity to disease. Amino acids, the building stones 

 of protein, arc the materials tor building and repairing tissues. 



Blood Sugar. Glucose, the fuel necessary for most cell metabolism, must 

 always be available in proper amounts. It is never excreted by the kidney 

 until the amount in the plasma is excessive, as it is in diabetic conditions 

 when the insulin from the pancreas fails to provide for its complete use in 

 the body. Uncomfortable and serious conditions follow if blood sugar falls 

 to half its normal amount. This drop occurs only in certain illnesses, because 

 normally the glycogen stored in large amounts in the liver is converted into 

 glucose as need arises. Among other organic substances in the plasma are 

 urea, uric acid, and fats absorbed through the walls of the intestine. 



Inorganic Substances. Sodium chloride (table salt), the commonest salt 

 in the blood plasma, is continuously taken in with the usual diet and lost in 

 urine, in sweat, and the lachrymal fluid which keeps the eyes moist and is 

 known to everybody as tears. Salt hunger is persistent in all animals, espe- 

 cially those that live on plant diets, usually low in salt. Much was heard about 

 the salt hunger of East Indians at the time of their rebellion under Gandhi's 

 leadership against the British salt tax. Wild animals will take great risks in 

 order to reach a saltlick. 



Calcium is also an essential substance for metabolism, for deposition in 

 bones, and coagulation of blood. The control of the amount of calcium in 



Table 12.1 

 Important Constituents of Human Blood Plasma 



