RESPIRATION 631 



slightly higher tension in carbon dioxide. At once the respiratory centre is 

 stimulated, and more copious ventilation rapidly washes away the excess of 

 carbon dioxide from the .alveoli, and thus from the venous blood. 



That it is to changes in the hydrogen ion concentration that the respiratory 

 centre reacts is, perhaps, most definitely shown by the experiments of Hasselbalch 

 (1912), although previous workers had found that the centre responds to acids 

 other than carbon dioxide. Reference to these results will be found in the 

 paper by Hasselbalch. Those of Winterstein (1911, p. 179) maybe mentioned. 

 He found that respiratory movements could be induced in rabbits, four days 

 old, which were perfused with oxygenated Ringer's solution from the aorta, 

 when O'OOl molar hydrochloric acid was added to the solution, although no 

 carbon dioxide was present. The nature of one kind of proof brought by 

 Hasselbalch will be clear from the following consideration. Since a particular 

 carbon dioxide tension in the alveoli corresponds to a definite ventilation, 

 when other things are unaltered, it follows that, if we find this same ventilation 

 along with a lower carbon dioxide alveolar tension, some other cause must 

 be adding its influence on the centre. Hasselbalch found that, by altering 

 the diet, he could alter the hydrogen ion concentration of the urine, hence 

 that of the blood, which he also measured by the hydrogen electrode described / 

 above (page 192). The carbon dioxide of the alveolar air always varied inversely! 

 with this hydrogen ion concentration; hence the lung ventilation is always 1 

 adjusted in such a way as to maintain the hydrogen ion concentration of 

 the blood constant. Incidentally, we may note also that since, normally, 

 the alveolar carbon dioxide tension varies only in very narrow limits, the 

 sensibility of the kidney to acid in the blood must be such as to keep the 

 concentration of hydrogen ion in the blood, other than that due to carbon 

 dioxide, at a constant level. 



It may, perhaps, seem surprising that it is to carbon dioxide rather than 

 to oxygen tension that the respiratory centre is adjusted. Carbon dioxide 

 is not very harmful to tissue processes, and it is a supply of oxygen that is 

 the chief requirement. This consideration has led various observers to seek 

 for a sensibility of the centre to a fall of oxygen tension ; the conclusion arrived 

 at in the most accurate experiments has been that, until the oxygen tension 

 falls very low, and the products of tissue activity are not completely oxidised, 

 no increase of ventilation takes place, provided that increase of carbon dioxide 

 tension is prevented. Although the centre is not sensitive to fall in oxygen 

 tension in the sense of being excited by it, it is possible that its excitability 

 might be raised so that the same carbon dioxide tension which excited it 

 normally under normal oxygen tension might, under reduced oxygen tension, 

 excite greater activity. Careful experiments by Campbell, Douglas, Haldani 

 and Hobson (1913) showed that the alveolar oxygen pressure can be varieA 

 within wide limits without sensibly affecting the excitability of the respiratorya 

 centre to carbon dioxide. 



This want of response to lowered oxygen tension may, under certain conditions, 

 lead to serious consequences, such as mountain sickness, which will be referred to 

 in a later paragraph. It is the increase in carbon dioxide tension that produces 

 the increase of pulmonary ventilation in asphyxia, so that, if increase in carbon 

 dioxide be prevented, as by respiration of pure nitrogen, a man may become 

 unconscious before experiencing any unpleasant symptoms. Similarly, by 

 forced breathing, the carbon dioxide tension may be reduced to such an extent, 

 that so long a period of absence of stimulus to the respiratory centre may ensue, 

 that severe signs of want of oxygen show themselves. Krogh points out that, 

 if the carbon dioxide is excreted by the activity of the alveolar epithelium, this 

 mechanism ought to be inhibited when the organism is threatened with death 

 because the blood has parted with too much carbon dioxide. 



It was mentioned above that, when the oxygen supply to the tissues is 

 considerably below their requirements, acid products are formed, especially in the 

 muscles ; these products naturally play a part in the stimulation of respiration 

 by their hydrogen ion content. Thus Ryffel (1909) has shown that lactic acid is 



