Cellular Respiration 



We have traced some of the many adaptations found among 

 vertebrates for extracting oxygen from their environment and 

 transporting it in the circulatory system to their tissues. Adapta- 

 tions have also been described which exist for the removal of 

 carbon dioxide derived from metabolic activities within the 

 cells. The nature of these cellular processes and some aspects of 

 the overall performance of the respiratory mechanism will now 

 be discussed. 



Some indication of the total performance is gained from deter- 

 minations of the volumes of these two gases. 



. volume of CO 2 given out . „ , , 



The ratio — -. — ^tt^ — -. -. — r is called the respiratory 



volume of O 2 absorbed 



quotient (RQ). The overall reaction for the combustion of a 



carbohydrate such as glucose is given below: 



CsHisOg + 6O2 ^6C02 + 6H2O + 690,000 cals. 



From this equation it can be calculated that 1 gram of glucose 

 reacts with 0-75 litres of oxygen to form an equal volume of 

 CO 2. The RQ when carbohydrates are used as the respiratory 

 substrate is therefore approximately 1. For fats the RQ 

 = 0-7-0-71 and for proteins 0-80-0-82. As a general guide to 

 the type of respiratory substrate being used these figures have a 

 certain value, but they cannot be relied upon invariably. The 

 relative volumes of O2 and CO 2 exchanged at the respiratory 

 organ may be affected by many factors, such as acidosis, hyper- 

 ventilation, and other respiratory irregularities. Inter-conversion 

 of foodstuffs, and other metabolic activities may also affect the 

 RQ. 



