EFFICIENCY OF LUNG MECHANISM 811 



total pressure exerted by the walls will be increased four times, 

 i.e. distending force = resistance to distension =pressure of gas 

 multiplied by area of vessel. Moreover with increasing distension, 

 the lung substance will become more attenuated. 



(c) The blood and lymph enmeshed in the pulmonary system 

 has to adjust its position to suit every alteration in the shape of 

 the lungs. These fluids are highly viscous and as such resist 

 distortion roughly in proportion to their pressure and to the area 

 of the cross-section of their vessels. Further, the capillary 

 vessels are so narrow that the corpuscular component of the blood 

 viscosity becomes predominant. 



(d) In addition to these factors which may be deduced from 

 a study of lungs removed from the thorax one must take into 

 consideration the position of the lungs in the thorax. Certain 

 parts of the thoracic wall are stationary and the surfaces of the 

 lungs in contact with these parts cannot directly expand. 



(i) The mediastinal surface is in contact with the pericardium 

 and with the structures of the mediastinum, (ii) The dorsal 

 surface lies close against the vertebral column and spinal portions 

 of the ribs, (iii) The dorsal part of the apical surface is bounded 

 by Sibson's fascia at the root of the neck. 



On the other hand the parts of the lungs in contact with (iv) the 

 diaphragm, (v) the lower ribs (ventro-lateral aspect) and (vi) upper 

 ribs (sternal aspect) undergo direct expansion at each inspiration. 



VI. The efficiency of the lung mechanism. If figures could be 

 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 0-7 c.c. of 

 oxygen per litre of ventilation. Assuming that all the energy 

 used is obtained from glucose, these figures indicate that from 

 0-0017 to 0-005 calories are expended for each litre of air breathed. 

 This amount of energy is liberated from 0-0040-0012 grams of 

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

 most 0-002 calorie obtained from just about 0-0007 of a gram of 

 glucose and 0-28 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, 0-000160-0004 calories are 

 converted into work =1/3 to 1/2 kilogram-metre. 



