68 



RESPIRATORY MECHANISMS 



The relation between ventilation and 2 uptake is shown in 

 Fig. 40. It is a straight line up to a ventilation of about 50 

 1/min; above that the ventilation increases more than the 2 

 uptake with the result that the 2 concentration in the alveoli 

 rises. The maximum ventilation observed is about 120 1/min. 

 The increase in metabolism due to the work of ventilating 

 the lungs was carefully studied by Liljestrand (1918) and later 



by M. Nielsen (1936), who 

 extended the determinations 

 to high rates of ventilation. 

 Up to a ventilation of about 

 20 1/min the cost is about 1/2 

 mlOa/l, or 1.3% of the rest- 

 ing metabolism, and not very 

 different in different indi- 

 viduals. Larger ventilations 

 are obtained as a rule by 

 first increasing the depth up 

 to about half maximal, which 

 seems to be the most eco- 

 nomical, and further by in- 

 creasing the rate. At these 

 larger ventilations the size 

 of the lungs makes itself felt so that the cost is larger for 

 smaller lungs, and the cost per liter rises rapidly with the ven- 

 tilation, reaching not less than 5% of the total metabolism or 

 a cost of 2 ml/1 in a person doing 1,900 kgm/min with a ven- 

 tilation of 116 1/min. 



The regulation of the ventilation during work is more com- 

 plicated than during rest but, as shown by Fig. 40, just as 

 precise. That an increase in C0 2 tension is insufficient to 

 bring about the maximum increase in ventilation is shown by 

 the fact that by addition of C0 2 to the inspired air during rest 

 the ventilation cannot be increased beyond about 60 1/min, 

 and, as a matter of fact, the C0 2 percentage in the alveolar 

 air during work is generally not increased at all. The chief 

 mechanism appears to be an increase in sensitivity towards 



Fig. 40. Relation between pulmo- 

 nary ventilation in liters per min. and 

 oxygen absorption in a human subject. 

 (M. Nielsen.) 



