552 Pulmonary Ventilation and Oxygen in Venous Blood of Man. 



The theoretical oxygen capacity determined from the haemoglobin value 

 was about 17 per cent. These authors say : — 



" From the experiments it is a fair conclusion that during its passage 

 through the pulmonary capillaries the blood is rarely fully saturated with 

 oxygen even when oxygen is inhaled. For an explanation, it is probable that 

 parts of the lung, for example the apices, are imperfectly ventilated, and 

 also, since the circulation time in the lung is only about five or six seconds, 

 that complete equilibrium is not attained between the blood and alveolar air."* 



We know that anything over 75 per cent, of an atmosphere of oxygen when 

 continuously breathed produces pneumonia, and that exposure to two or three 

 atmospheres of oxygen causes convulsions. A high partial pressure and 

 concentration of oxygen on the blood acts as a poison. It may be that there 

 is at work some mechanism which prevents, within certain limits of oxygen 

 partial pressure, the over-concentration of free oxygen in the blood, and 

 therefore we find no more oxygen in the venous blood on forcibly breathing 

 air than on forcibly breathing oxygen. 



Our figures show that forcible breathing of air, or oxygen, equally and 

 notably increases the oxygen in the venous blood above the average result 

 obtained when breathing air quietly. We cannot ascribe this result to vaso- 

 dilatation and accelerated flow through the arm produced by the forced 

 breathing, for G. 1ST. Stewart has shown that forcible breathing diminishes the 

 velocity of flow in the hand by about 40 per cent.f Forcible breathing 

 mechanically interferes with the circulation and the hand tends to become pale 

 and cold when such is continued. 



We conclude that the arterial blood is not always saturated with oxygen 

 during the passage through the lungs when the breathing is quiet. Some 

 parts of the lung may remain unexpanded, and the blood passing through 

 these parts is not oxygenated. Forcible breathing ensures the expansion of 

 all parts and the better saturation of the arterial blood. In one case 

 Caske and BarcroftJ obtained a sample of arterial blood and found it 94 per 

 cent, saturated with oxygen. The sample was obtained from a young woman 

 acting as donor in a direct transfusion of blood. Her artery was opened 

 under local anaesthesia. The emotional conditions probably ensured in her a 

 good pulmonary ventilation. 



If it be true that the person engaged in sedentary occupation does not 

 expand the lungs sufficiently to arterialise the blood in all their parts, this 



* 'Roy. Soc. Proc.,' B, vol. 85, p. 56 (1912). 



+ 1 Amer. Journ. Physiol.,' vol. 28, p. 190 (1911). 



% 'Proc. Physiol. Soc.,' 'Journ. Physiol.,' vol. 47, p. xxxv (1914). 



