400 EXTERNAL RESPIRATION 



obtained denoting the work done by the respiratory mechanism 

 and its efficiency, they would be invaluable. One may arrive at 

 an approximate value by measuring the oxygen consumed by an 

 animal under standard conditions with normal and with increased 

 respiration. With man, it was found that during muscular rest, 

 1 to 3 per cent, of the total basal oxygen intake is utilised by the 

 respiratory mechanism. This amounts to from 0-3 to 0-9 c.c. of 

 oxygen per litre of ventilation. Assuming that all the energy 

 used is obtained from glucose, these figures indicate that from 

 0-0015 to 0-005 Calorie is expended for each litre of air breathed. 

 This amount of energy is liberated from 0-0004 — 0-0012 gram of 

 glucose. During quiet breathing each breath (400 c.c.) costs at 

 most 0-002 Calorie obtained from just about 0-0005 of a gram of 

 glucose and 0-36 of a cubic centimetre of oxygen. If it is assumed 

 that the lung mechanism is at least 20 per cent, efficient, then 

 at each quiet complete respiration, 0-00014 — 0-0004 Calorie is 

 converted into work = 0*06 to 0*17 kilogram-metre. 



This work is almost entirely performed by the diaphragm. 

 The other muscles concerned, whether synergic or antagonistic, 

 seem to play an almost passive part. This may be inferred from 

 the fact that although they are skeletal in structure yet they 

 undergo constant slow contraction without showing fatigue. 

 When the respirations are forced the subsidiary musculature has 

 to perform work and the CO.^ output increases. The effort 

 sooner or later brings on fatigue. Forced respirations are carried 

 out uneconomically, i.e. at a relatively higher cost per litre than 

 ordinary quiet ventilation (see Voice, Chap. XXVIII.). 



Regulation of Respiratory Rate. The activity of the respiratory 

 centre, which lies in the medulla near the root of the vagus, is 

 normally governed by the tension of the COg in the blood going 

 to it. The rate of breathing is increased by any increase in 

 the CO2 tension ; and, conversely, diminution of the CO2 tension 

 leads to a decreased respiratory rhythm. 



The CO2 tension of the blood and the partial pressure of the 

 COo in the alveolar air, as we have explained (Chap. XXIV.), are 

 always in dynamic equilibrimn, and, therefore, any change in the 

 one will lead to corresponding changes in the other. It has been 

 found that an increase of 0-2 per cent, in the CO2 of the alveolar 

 air of man, i.e. a rise of tension of from 40 to 41-6 mm. Hg, is 

 sufficient to double the alveolar ventilation. The increased 

 ventilation leads to a " washing out " of CO2 from the blood and 

 from the lung, thus rapidly restoring a normal condition. The 

 power of adjustment is so extraordinarily effective that under wide 

 variations of metabolic and atmospheric conditions, the tension 



