174 BIOLOGICAL CHEMISTRY 



The sequence of events is most probably some sort of 

 chemical alteration (such as production of lactic acid from 

 glucose) which causes an increase in oxidation by furnishing 

 an increase in amount of oxidisable substance. Increased 

 oxidation causes a fall in oxygen tension, hence increased 

 absorption of oxygen. Increased oxidation is followed by 

 liberation of the products of oxidation such as carbon dioxide 

 and water. 



It has been stated that oxidation may furnish a larger 

 supply of energy than can be accounted for by the heat of 

 combustion of the substance oxidised. In some chemical 

 reactions this has been realised because the chemical affinity 

 is sufficient to cause combination with a fall in temperature, 

 the extra heat value being absorbed from the surroundings. 



In living cells this is not possible as the temperature of the 

 cell is above that of its surroundings, and to say that one mole- 

 cule of glucose gives more energy than its heat of combustion 

 is to demand that something else, for instance another molecule 

 of glucose, must be oxidised at the same time. It is more 

 logical to regard the energy produced as the result of the 

 combined oxidation because both are necessary for the normal 

 activity of the tissue. 



We thus see that respiration requires a respiratory organ 

 in which gas exchange takes place between the external 

 medium and the circulating blood. This exchange is due to dif- 

 fusion, but active oxygen secretion may occur in some instances. 



The transport of gases in the blood is aided by chemical 

 combination of oxygen with haemoglobin and of carbon dioxide 

 with sodium and possibly also with proteins. 



The oxidation in the tissues liberates energy for various 

 processes, and the amount of oxygen taken in can be used to 

 calculate the energy production. The output of carbon 

 dioxide also furnishes an indication of the energy production, 

 but one must bear in mind certain fallacies in the latter mode 

 of estimation. These fallacies depend on the nature of the 

 substance oxidised and on the possibility of removing excess 

 of carbon dioxide by forced respirations. Assuming an 

 average respiratory quotient of 0-85 the estimation of carbon 

 dioxide output is a method which can be used in circumstances 

 under which the more elaborate respiratory measurements are 

 not possible.* 



GENERAL REFERENCE 



J. BARCROFT : The Respiratory Function of the Blood, 1914. Cambridge 

 University Press. 



* A. D. Waller, Journ. PhysioL, 1919, vol. 52, proc., p. Ixxii. 



