CHAPTER XVI 

 MUSCULAR ACTIVITY AND FATIGUE 



MUSCULAR ACTIVITY 



We are now in a position to piece together the changes 

 which occur in the different organs when the body passes 

 from a state of rest to a state of muscular activity. Upon 

 the proper co-ordination of these changes depends the 

 efficiency of the animal as a machine. 



The repeated contraction of the muscles produces three 

 changes in the blood flowing through them : (1) a 

 mechanical effect, the pumping of the blood at a greater 

 rate through the capillaries ; (2) the production of meta- 

 bohtes, such as CO2 and lactic acid, which have a direct 

 vaso-dilator action upon the arterioles and capillaries ; (3) 

 a rise in the temperature of the blood. 



The increase in the venous flow and the raised tem- 

 perature of the blood have a direct effect upon the output 

 of the heart, the former increasing the diastohc filhng and 

 therefore the output per beat, the latter increasing the 

 rate of the beat. Further, the increased venous pressure 

 causes a quickening of the beat reflexly through the vagus. 

 The combined result is therefore a greatly increased cardiac 

 output. 



The increased hydrogen ion concentration of the arterial 



blood stimulates the medullary nuclei — the respiratory 



and vaso-motor centres. By the enhanced activity of the 



.former the pulmonary ventilation is increased. In this 



way the increase in ventilation of the lungs keeps pace 



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