536 NUTRITION OF FARM ANIMALS 



are rapidly removed and an abundant supply of oxygen is en- 

 sured. In fact, it is usually true that during work which is not 

 excessive the venous blood contains less carbon dioxid and 

 more oxygen than during rest. 



Since the heart is a muscular organ, it is obvious that this 

 increase in the circulatory activity must add materially to its 

 metabolism. In the performance of work, therefore, there is 

 an expenditure of matter and energy, not only for the work of 

 the skeletal muscles, but likewise for the additional work of the 

 heart. Zuntz and Hagemann in their experiments upon the 

 horse just mentioned compute that during moderate work the 

 katabolism due to the work of the heart amounts to 3.8 per 

 cent of the total katabolism of the body. 



634. Respiration. The greater activity of the circulation 

 consequent upon muscular exertion would be futile were not 

 provision made for more efficient aeration of the blood in the 

 lungs through an increased activity of the respiration. Under 

 the stimulus of the carbon dioxid and other katabolic products 

 of muscular activity which enter the blood, the respiratory move- 

 ments are increased in frequency or depth or both, as described 

 in Chapter IV (194), thus making possible a more rapid gaseous 

 exchange between the blood and the air in the lungs. This 

 action is usually so efficient that the expired air during work 

 contains a smaller proportion of carbon dioxid than it does 

 during rest, notwithstanding the fact that the total quantity 

 eliminated is much greater. Since respiration, like circu- 

 lation, is maintained by muscular action, it is true in the former 

 case as in the latter that a greater activity of the function neces- 

 sitates a greater metabolism for that purpose. 



Effect of work upon protein katabolism 



As already indicated, knowledge of the details of muscular 

 katabolism is still meager. The student of nutrition, however, 

 is less directly interested in these details than he is in knowing 

 the aggregate effect of the performance of work upon the ex- 

 penditure of matter and energy by the body under varying 

 conditions, since it is this latter which must be made good by 

 the feed supply. Much effort has therefore been devoted to 

 studies of the influence of muscular exertion upon the kind and 



