IMPORTANT FOODSTUFFS, IMPORTANT TISSUES 137 



change in the reaction (pH) of the blood have been suggested. 

 In vigorous activity adrenalin secretion from the adrenal glands 

 is augmented, resulting in increased vasomotor tone, and break- 

 ing down of liver glycogen (see section on metabolism of carbo- 

 hydrates). 



The production of acid also has an effect on the muscles them- 

 selves, a dilation of the arterioles, and changes in the tone of 

 the capillaries. If the supply of 2 is adequate, lactic acid 

 either is not produced, or disappears very quickly. The earlier 

 view that the disappearance of lactic acid in the period of 

 relaxation was due to its oxidation has now been questioned. 

 An excised muscle which of course is not receiving any repair 

 material may be made to contract many times, and as much 

 lactic acid can be obtained from it as could have been extracted 

 after a few contractions. Also the oxidation of 1 gram lactic 

 acid yields 3.7 calories, whereas 4.5 calories are produced in a 

 muscle while 1 gram of lactic acid disappears. The belief is 

 gaining ground that lactic acid goes back into its original form, 

 while something else, perhaps glucose, is oxidized. 



If a muscle is tetanized, the amount of lactic acid increases, 

 and the muscle does not respond so well to the stimuli. If the 

 circulation is vigorous, however, the muscle regains its activity. 

 Fatigue is thus due to the lactic acid and perhaps other sub- 

 stances which accumulate in the cells during contraction. In 

 smooth muscle contraction little energy is expended, hence 

 smaller amounts of waste products are formed, and the smooth 

 muscles are not susceptible to fatigue. 



Whereas muscle contraction is accompanied by the appear- 

 ance of lactic acid, it is interesting that the greater part of 

 the oxygen consumed is used up after the period of contraction. 

 Also the heat production lasts much longer than the contrac- 

 tion, so it appears that the contraction is not directly caused 

 by the oxidation process. 



During muscular activity, the amount of muscle glycogen is 

 greatly reduced. It is generally considered that it is converted 

 into glucose, which is oxidized with the liberation of energy. 



