RESPIRATION 



123 



there is at first an increase of the frequency, and apparently also 

 in the strength, of the heartbeats. This indicates an increase in the 

 circulation rate. But just as in the case of the respirations, the 

 frequency and vigor of the pulse soon fall again, though the fre- 

 quency remains above normal, just as does the frequency of res- 

 piration. Thus the pulse may rise to about 120 at first, and then 

 fall after a few minutes to about 90, and remain steady. With 

 greater anoxaemia the increase in rate is more marked. The great 

 temporary increase in blood pressure with acute anoxaemia in 

 animals is also a well-known phenomenon. 



At first sight it might seem that a great increase in both res- 

 pirations and circulation would be the natural physiological: 

 response to anoxaemia, since the increased respiration will raise 

 the alveolar oxygen pressure and the increased circulation rate 

 will increase the amount of oxygen left in the red corpuscles of 

 the blood passing through the capillaries. But, as already seen, 

 ^the increased respiration lowers the pressure of CO2 in the respira- 

 tory center and tissues, and this lowering rapidly reduces the in- 

 creased breathing to within relatively narrow limits. A similar 

 lowering of CO2 pressure in the tissues must also be produced by 

 increased circulation rate ; and the falling-off in the initial increase 

 of pulse rate is probably at bottom due to the same cause as the 

 falling-off in the initial depth and frequency of breathing. With 

 further increase in the anoxaemia the heartbeats, like the respira- 

 tions, become more and more feeble. A fuller discussion of the 

 relatively little that is at present known definitely as to the physio- 

 logical regulation of the circulation will be found in Chapter X. 

 It is of course evident that the physiology (not the mere physics) 

 of the circulation is intimately related to that of the breathing. 



As a sign of anoxaemia, the appearance of the lips, tongue, and 

 face is of much importance, but requires careful interpretation. 

 The bluish color or cyanosis seen in the lips and skin during 

 ordinary anoxaemia is, of course, due to the fact that in the blood 

 passing through the capillaries the proportion of oxyhaemoglobin 

 to haemoglobin is abnormally low. A somewhat similar color may 

 be produced by the action of poisons which produce methaemo- 

 globin and other colored decomposition products in the blood; 

 and this condition, which is of course quite exceptional, and can 

 quite easily be distinguished, will be referred to in Chapter VII. 

 Cyanosis may either be due to general or local slowing of the 

 circulation, or to the fact that the arterial blood is imperfectly 

 oxygenated, and the latter cause, as will be shown in Chapter VII, 



