RESPIRATION 189 



269. Tissue Respiration. The lungs were once regarded 

 as the seat of the combustion of the body as the fur- 

 nace to which the waste of the body was brought to be 

 burned up. It is now known that the tissues themselves 

 are the seat of combustion, or oxidation. The oxygen 

 given off to the tissues from the arterial blood in the cap- 

 illaries is not necessarily used at once in new chemical 

 combinations. In muscular tissue, and probably in other 

 varieties of tissue, it is stored for future use. This is 

 shown by the fact that severe muscular action, by which 

 the substance of the muscle fibers is broken down in the 

 production of energy, results in the elimination of carbon 

 dioxide which contains more oxygen than the whole 

 amount taken up by the lungs during the time of action. 



It appears that the tissues are constantly taking oxygen 

 from the blood (or, strictly speaking, from the lymph) 

 and combining it in forms which are easily decomposed, 

 and thus the oxygen is ready when it is needed for the 

 liberation of energy. A man gives off from his lungs 

 more oxygen in the form of carbon dioxide during the 

 day, when his muscles, brain, and digestive organs are 

 at work, than the lungs take up during the same time. 

 The excess had been stored during his periods of rest. 

 Although oxygen is that element in the air which sup- 

 ports life, it has been established by experiments that an 

 animal uses no more oxygen in a given time when it 

 breathes the gas pure than when it breathes ordinary air, 

 that is, the amount of work done by the tissues is not de- 

 termined by the amount of oxygen supplied to them, but 

 the quantity of oxygen used is determined by the amount 

 of work done. An excess of oxygen above that amount 

 needful to prevent suffocation will not make the organs 

 work any more than extra food will make a man work. 



MACY'S PHYS. 12 



