390 



ORGAN SYSTEMS OF MAN 



down to form phosphoric acid and adeno- 

 sine diphosphate, releasing energy in a sud- 

 den but controlled manner. There is no 

 oxygen involved in this reaction, which ac- 

 counts for the fact that a man can run a 

 hundred yards without taking a breath. 

 When the adenosine triphosphate has been 

 expended, no further contraction can occur 

 without its recovery. Such an exhausted 

 person must remain quiet undergoing rapid 

 respiration to supply sufficient oxygen to 

 allow the next reactions to proceed. This 

 involves glycogen breakdown to lactic acid 

 (a rearrangement of the molecules) and 

 the subsequent oxidation of the latter sub- 

 stance to CO2 and HoO. This last step re- 

 quires large quantities of oxygen, hence 

 the deep breathing after severe exercise ( or 

 during, if prolonged). The energy released 

 from this reaction is utilized in three ways: 

 part of it is utilized in restoring adenosine 

 triphosphate, part of it to convert 80 per 

 cent of the lactic acid back into glycogen, 

 and the remaining part is converted into 

 heat that keeps the body warm. It will be 

 observed that the entire chain reaction re- 

 sults in the most economical method of 

 obtaining the greatest possible energy from 

 the stored food products. It means that the 

 animal body is unusually efficient, about 40 

 per cent of the available energy being re- 

 leased in the form of work, 60 per cent as 

 heat. This is a very satisfactory figure when 

 one considers that the best internal com- 

 bustion engines rarely exceed 25 per cent. 

 Returning to the runner, the reason why 

 he could run the entire hundred vards with- 

 out taking a breath was that his ATP 

 was being used up, but when he terminated 

 the run he was forced to remain quiet and 

 breathe deeply for some time. During the 

 run he was building up an oxygen debt, 

 which he "paid back" during the heavy 

 breathing period at the termination of the 

 race. The obvious advantage of such a 

 mechanism is that a muscle is ready to con- 

 tract with all of its force on a moment's 

 notice. It can contract until its reservoir of 



high-energy phosphate is exhausted; then it 

 must stop and wait until the blood brings 

 sufficient oxygen to restore its glycogen and 

 ATP to the original unspent condition. This 

 can be compared to a toy gun which oper- 

 ates with a spring; once it is shot the spring 

 must be tightened before it will shoot 

 again. 



One of the great complaints of human 

 beings is fatigue; mankind would never for- 

 get the scientist who could discover a way 

 of preventing its regular and persistent 

 occurrence. From the foregoing discussion 

 it is quite obvious that fatigue involves 

 the accumulation of lactic acid and the ex- 

 haustion of glycogen and ATP in the mus- 

 cles, although this is not the entire story 

 because in an intact animal fatigue is pro- 

 nounced before there is an appreciable 

 amount of lactic acid present in the mus- 

 cles. Experiments have demonstrated that 

 the site most susceptible to fatigue is the 

 junction between the muscle and nerve, 

 and not these organs themselves. 



The limitations for work are set by the 

 ability of the body to restore exhausted 

 organic compounds in the muscles; this de- 

 pends indirectly on the functioning of the 

 respiratory, circulatory, and excretory sys- 

 tems which, in turn, the muscles must de- 

 pend on to receive their quota of burning 

 material (sugar and oxygen) and to carry 

 away their accumulated wastes (urea and 

 CO2 ). There are great individual differences 

 among human beings for this capacity. 

 Some get along well with very little sleep 

 — Edison was such an example — whereas 

 others require eight hours or more per day. 



Muscles can be developed to consider- 

 able size and strength if they are constantly 

 put to difficult tasks. By lifting heavy 

 weights each day tlie muscles of the entire 

 body will grow disproportionately large 

 and will function very well in lifting heavy 

 objects. If this is to be the life work of the 

 individual it is wise to have such a set of 

 muscles, just as it is wise for the man who 

 handles a shovel all day long to have thick 



