RESPIRATION 331 



only went into the emphysematous spaces where there was little 

 or no circulation, leaving the rest of the lung imperfectly venti- 

 lated, with an abnormal excess of CO 2 in the alveoli which were 

 permeable to blood, and consequently an abnormal excess of 

 breathing. 



Considering the depth of the cyanosis it was somewhat re- 

 markable that consciousness was not more impaired; but the ex- 

 cess of CO 2 which accompanied the cyanosis would of course 

 facilitate the dissociation of oxyhaemoglobin in the tissue capil- 

 laries, and thus diminish the real anoxaemia. The distention of 

 superficial veins was an indication of the veno-pressor effect of 

 excess of CO 2 combined with failure on the part of the heart to 

 respond normally to the large amount of blood returning to it 

 from the tissues. This failure was evidently due to the anoxaemic 

 condition of the blood supplied to the heart. The failure was pre- 

 sumably most marked in the left ventricle, which has far the most 

 work to do, and the consequence would be a rise of blood pressure, 

 not only in the veins, but also in the right side of the heart and 

 the whole pulmonary circulation. The rise in pulmonary blood 

 pressure would of course tend to aggravate greatly the oedema of 

 the lungs, and would thus in itself be a very serious source of 

 danger. The ease with which oedema of the lungs follows on in- 

 creased venous blood pressure, even when there is no injury to the 

 lungs, has been shown experimentally by Knowlton and Star- 

 ling. 26 



The cause of the greatly increased flow of blood was simply 

 the fact that the arterial blood was in a venous condition, with 

 both a lowered oxygen pressure and raised CO 2 pressure. The 

 perfectly normal effect of this, as pointed out in Chapter X, is to 

 cause dilation of capillaries and increased blood flow through the 

 tissues. Owing, however, to the pressor reaction of the vasomotor 

 center, the arterioles and probably also the venules in most parts 

 of the body except the central nervous system were contracted, 

 and in this way the blood pressure was maintained, so that the 

 pulse was of good strength. 



It was first observed by Macaulay and Irvine of Johannesburg 

 that in the treatment of cases of oedema of the lungs from 

 poisoning by nitrous fumes in mines, great benefit is often ob- 

 tained by free bleeding to the extent of about half a liter. From 

 the foregoing account it is clear that bleeding will reduce the 

 venous and pulmonary blood pressure, and thus also- reduce the 



M Knowlton and Starling, Journ. of Physiol., XLIV, p. 206. 



