so 



How a Co?nplex Aimnal Uses Food unit iv 



called isotopes (ice'oh-topes). Some iso- 

 topes are radioactive, that is, thev give 

 off powerful rays like those given off by 

 radium. Some can be made radioactive by 

 a machine known as a cyclotron. Among 

 such isotopes is radioactive iodine; an- 

 other example is radioactive phosphorus. 

 By chemical action, radioactive atoms 

 can be put into compounds which can 

 then be fed to people or injected into 

 their bodies. The compounds are carried 

 about the body and made use of by one 

 organ or another. Thus the compound 

 containing radioactive iodine is carried 

 to the thyroid gland which uses iodine 

 in the making of its hormone. While the 

 iodine is there the rays it gives off help 

 to relieve the abnormal condition of the 

 gland. Many cases of overactivity have 

 been successfully treated by this method. 

 Later in the book you will read of an- 

 other isotope useful in the curing of 

 disease. 



Isotopes are also being used in animal 

 experimentation to trace the various 

 chemical substances through the body. 

 If the isotope is radioactive it readily 

 shows its presence in the organs. If not, 

 it may be located in the tissues chemi- 

 cally. Such isotopes are called tracers or 

 tagged atoms. Thus isotopes, whether 

 radioactive or not, are of great impor- 

 tance. By their use it is hoped that much 

 more can be learned about the metabo- 

 lism of the body. 



A hormone regulating sugar metabo- 

 lism. Thyroxin speeds up oxidation of as Roman times diabetes had been given 



presence of a little more or less sugar in 

 the blood may seem unimportant to you 

 but a physician knows that any unbalanc- 

 ing of this sort has far reaching and 

 serious results. Insulin, it is true, also 

 hastens oxidation but let us see how else 

 it affects sugar metabolism. You will re- 

 member that when sugar is not oxidized 

 immediately it is changed into the stor- 

 age product, glycogen. This may happen 

 within muscle cells or in the liver. Now 

 insulin helps in the storage of glycogen. 

 You can see that if the body produces no 

 insulin or too little insulin sugar will ac- 

 cumulate in the blood for two reasons: 

 first, less sugar is used up in oxidation; 

 and second, less sugar is taken out of the 

 blood to be stored as glycogen. 



Insulin helps to keep sugar out of the 

 blood in still another way too compli- 

 cated to explain. In fact, with extreme 

 lack of this hormone tissue proteins be- 

 gin to break down into sugar and other 

 substances, thus adding to the amount 

 of blood sugar. This condition, where 

 insufficient insulin is produced in the 

 body, is known as diabetes. The disease, 

 if unchecked, is dangerous and may lead 

 to death. Diabetes can be recognized by 

 the presence of too much sugar in the 

 blood. From the blood some of the sugar 

 passes into the urine. In fact, until re- 

 cently the testing of urine for sugar was 

 considered sufficient for detecting dia- 

 betes. 



The discovery of insulin. As long ago 



sugar but there is another hormone, 

 called insulin, which is also intimately 

 connected with sugar metabolism. In- 

 sulin is responsible for keeping the nor- 

 mal amount of sugar in the blood. The 



a name and its symptoms described care- 

 fully. For centuries it was believed to be 

 a disease of the kidneys, since sugar ap- 

 pears in the urine. Not until the begin- 

 ning of this century did we learn that 



