METABOLISM i?3 



half the blood, produces no change in the extent of meta- 

 bolism, for the lack of oxygen is completely covered by 

 increased respiration and heart activity, so that oxyhaemo- 

 globin is used to better advantage than normally. But if 

 real lack of oxygen occurs, e.g. in dyspnoea or excessive 

 muscle work, the metabolism does not decrease, but, on the 

 contrary, there is an increase in the decomposition of pro- 

 teids. But the combustion is, in this case, incomplete; 

 hence considerable quantities of lactic acid are excreted 

 with the urine (see page 42). 



Increase or decrease in atmospheric pressure has, within 

 certain wide limits, no effect on the amount of combustion 

 in the body. 



II. The effect of work and loss of animal heat upon 

 metabolism. 



(a) Effect of muscular activity. By muscular work the 

 respiratory metabolism and all the processes of combustion 

 are increased. The combustion process may be increased 

 four or five times the normal amount. During moderate 

 work the respiratory quotient is the same as during rest. 

 But during excessive work the increase in the excretion of 

 carbon dioxide may be greater than that of the oxygen 

 taken up, so that the respiratory quotient is increased. 



The consumption of proteids is generally not increased by 

 work, hence the increase in combustion must be at the ex- 

 pense of non-nitrogenous substances, fat, or carbohydrates. 

 Often, however, the excretion of nitrogen is also increased 

 by muscular activity; this is always the case in exclusive 

 or predominating proteid diet. But if sufficient fats or 

 carbohydrates are present in the mixed diet, there is no in- 

 crease in nitrogenous excretions. During muscular activity, 

 the body generally consumes non-nitrogenous material. 

 But if the work is very excessive, an increase in the excretion 

 of nitrogen may result even with a mixed diet containing 

 much fats and carbohydrates. This is perhaps due to the fact 

 that excessive work injures the muscle tissue. To maintain 

 the body at its weight during work it must be supplied with 



