VI 

 RESPIRATION IN AIR 



The general properties of air are much more favourable to 

 the acquisition of oxygen than those of water. In saturated 

 water at 20° 1 ml oxygen is contained in about 200 g water, 

 while 1 ml oxygen is present in 5 ml air, weighing only 7 mg, 

 and at the same time the rate of diffusion of oxygen in air is 

 about 300,000 times as rapid as in water. Diffusion will 

 therefore be able to supply large amounts of oxygen over 

 short distances, and the mechanical ventilation with air does 

 not normally require the expenditure of much energy 

 (see p. 68). 



For the majority of organisms breathing air the restriction 

 of loss of water by evaporation from the respiratory surfaces 

 is a major problem, and accordingly the main respiratory 

 organ is the lung, formed by folding the respiratory surfaces 

 into a hollow organ in which the exchange of air is limited 

 and can very often be regulated. 



When in a lung a certain amount of oxygen diffuses into 

 the organism it will normally be replaced by a similar volume 

 of carbon dioxide diffusing out. The C0 2 pressure within the 

 organism must be at any time slightly higher than correspond- 

 ing to the concentration in the lung to allow diffusion to take 

 place, and as an increase in C0 2 pressure can be tolerated only 

 up to a certain point, this sets a limit to the utilization of 

 oxygen. The general effect of C0 2 is somewhat similar to 

 that of a narcotic, stimulating within certain narrow limits 

 and above that depressing. 



In a large number of air-breathing animals the C0 2 effect 

 is utilized to build up a mechanism regulating the ventilation 

 of the lungs (or tracheal system). This will be described in 

 some detail below. 



52 



