686 PHYSIOLOGY 



According to Speck and Lowy, moderate muscular work which is not 

 associated with dyspnoea, although attended by a large increase in the 

 carbon dioxide output and the oxygen intake of the body, does not alter the 

 respiratory quotient. This is probably correct if the respiratory changes 

 are taken over a sufficient length of time, when the respiratory quotient 

 must depend on the food which is furnished to the body however the energy 

 of this body is expended. When, however, as in Benedict's and Cat heart's 

 experiments, quoted in^he Table on p. 683, the observations are of short 

 duration, muscular exercise is almost always found to be associated with a 

 rise in the respiratory quotient, which lasts too long to be accounted for by 

 a mere washing out of the carbon dioxide from the blood and the body 

 tissues. After the cessation of the exercise the respiratory quotient sinks 

 below normal. The respiratory quotient however rarely rises even during 

 exercise to 1, as it would if the muscular work were performed solely at the 

 expense of carbohydrates. 



These results suggest that, while muscular work can be performed at the 

 expense of any of the foodstuffs or of the three classes of constituents of 

 the body, carbohydrate is the immediate or the most readily available source 

 of muscular energy. When the body passes suddenly from a resting to an 

 active condition, the first call is therefore on the carbohydrates of the blood 

 and those stored up in the muscles and liver, whereas after exercise these 

 carbohydrate stores are slowly replenished probably at the expense of the 

 proteins. The fact that the body can draw on its fat stores for the perform- 

 ance of muscular work suggests that this substance may also serve as a 

 source of muscular energy, and in default of evidence that the body is able 

 to convert fats into carbohydrates, we must assume that under certain 

 conditions the fats or their decomposition products may be directly utilised 

 by the muscles. 



The lowering of the respiratory quotient which follows severe exercise 

 suggests either that the metabolic processes of the body are being performed 

 at the expense of proteins and fats while the body is replenishing its limited 

 stores of carbohydrates, or else that carbohydrates e. g. sugar or glycogen 

 are being manufactured out of proteins and possibly fats in order to make 

 good the exhaustion of carbohydrate resulting from the exercise. 



Since the carbonic acid secreted during exercise may result from the 

 oxidation of protein, fat, or carbohydrate, we cannot deduce directly the 

 total energy set free in the body by an estimation of the C0 2 output alone. 

 The respiratory quotient must also be determined in order to throw light 

 on the real amounts of fat and carbohydrate consumed, and in accurate 

 experiments the output of nitrogen in the urine must be also measured. 

 Every gramme of nitrogen in the urine, if resulting from the oxidation of 

 protein, corresponds to the intake of 84 grm. oxygen and to the output 

 of 9-35 grm. carbon dioxide. If the amounts of C0 2 and oxygen corre- 

 sponding to the protein metabolism be deducted from the total respiratory 

 exchanges, the remainder must be due to the metabolism of fat and carbo- 

 hydrate, and the respiratory quotient obtained after deducting the gaseous 



