PRINCIPLES OF PHYSIOLOGY 87 



28. Respiration 



The energy requirements of cells are met by the release of energy, 

 generally by oxidative processes, from foodstuff molecules. These cellu- 

 lar oxidative processes, which include the removal of hydrogen and 

 carbon dioxide from certain molecules and the combination of the 

 hydrogen with oxygen to form water, are the fundamental reactions of 

 respiration at the cellular level. We may define cellular respiration as 

 the sum of the processes in which oxygen is utilized and carbon dioxide 

 is produced. For these processes to continue, the supply of oxygen must 

 be renewed constantly and the carbon dioxide produced must be re- 

 moved. 



Animals differ tremendously in their general levels of activity and 

 hence in their requirements for energy and for oxygen. As a corollary 

 of this, animals differ in their susceptibility to oxygen deprivation. A 

 mouse, which uses 2,500 cu. mm. of oxygen per gram per hour when 

 resting, and as much as 20,000 cu. mm. per gram per hour when active, 

 rapidly dies of suffocation when deprived of oxygen or when poisoned 

 with carbon monoxide. But an earthworm, which uses 60 cu. mm., or a 

 sea anemone, which uses only 13 cu. mm. of oxygen per gram per hour, 

 has a much lower rate of metabolism and does not readily suffocate. 

 "Life" goes on in these lower animals at a much lower rate, in general, 

 than it does in birds and mammals. There are exceptions to this generali- 

 zation, and some animals with low rates of oxygen consumption are 

 very sensitive to oxygen deprivation. 



The transfer of gases across the cell membrane to the surrounding 

 body fluid— or pond or sea water— is also part of the respiratory process. 

 In the larger and more complex animals, further exchange of gases must 

 occur between the body fluids— blood and interstitial fluid— and the out- 

 side environment, an exchange which usually involves some specialized 

 respiratory surface, such as lungs or gills. The molecules of oxygen or 

 carbon dioxide, whether in man or anieba, move simply by diffusion, 

 from a region of high concentration to a region of lower concentration. 

 The diffusion gradients are maintained, for oxygen is constantly utilized 

 and carbon dioxide is produced within the cell. Physiologists use the 

 terms partial pressure and tension of a gas to describe these diffusion 

 gradients quantitatively. 



The partial pressure of a gas is simply the pressure due to that one 

 gas in a mixture of gases. It is calculated by multiplying the total pres- 

 sure of the mixture of gases by the percentage of that gas in the mixture. 

 Air, for example, normally has a pressure of about 760 mm. Hg and is 

 one-fifth oxygen. The partial pressure of oxygen in air is 760 X 0.20 

 or 152 mm. Gas molecules dissolved in a liquid have a certain tendency 

 to escape, to leave the liquid and enter the gaseous phase. This escaping 

 tendency can be measured by the pressure of that gas in the gaseous 

 phase in contact with the liquid which is required to prevent any net 

 loss of the gas, i.e., to maintain equilibrium. When a liquid and gas 

 are in contact, an equilibrium is reached when the rate at which mole- 

 cules pass from the liquid to the gas equals the rate at which they pass 



