H 4 RESPIRATION 



the arterial blood with oxygen can be only very slightly increased 

 by the lowering of alveolar CO 2 pressure. The oxygen pressure 

 of the venous blood must in consequence be lowered, so that anox- 

 aemia might be produced without any diminution, and even with a 

 slight increase, in the saturation of the haemoglobin of the venous 

 blood. On the other hand if the haemoglobin of the arterial blood, 

 with normal alveolar CO 2 pressure, were only half-saturated, a 

 lowering of the alveolar CO 2 pressure would considerably in- 

 crease the saturation of the haemoglobin in both arterial and 

 venous blood, but without sensible alteration of the venous oxygen 

 pressure. Only in the practically impossible case of the saturation 

 of the arterial haemoglobin being much below half would there 

 be any rise in the venous oxygen pressure. Practically speaking, 

 therefore, the Bohr effect, the increased oxygen content in blood, 

 due to lowering of alveolar CO 2 pressure, is never of service in 

 increasing the real oxygen supply to the tissues, and is sometimes 

 of great disservice, although it always tends to make the venous 

 blood less blue, and so diminishes cyanosis. On the other hand the 

 corresponding effect due to raising of alveolar CO 2 pressure will 

 practically never diminish the oxygen supply to the tissues, and 

 will usually increase it, though the venous blood will always be 

 more blue. 



With forced breathing of normal air there is, as mentioned in 

 Chapter I, a slight increase in the oxygen present in the arterial 

 blood. This is due, partly to the Bohr effect and partly to the effect 

 of the increased alveolar oxygen pressure. Hence the saturation 

 of the haemoglobin is increased from about 95 to 100 per cent. 

 There is also a small increase in the free oxygen dissolved in the 

 arterial blood. On the other hand the amount of CO 2 and its 

 partial pressure are enormously reduced in the arterial blood, and 

 to a less extent the venous blood, since the circulation rate, as 

 will be shown in Chapter X, is much diminished. The net result 

 must be a considerable fall in the oxygen pressure in the tis- 

 sues. Now it is well known that forced breathing produces a 

 train of symptoms which, if the forced breathing is pushed, 

 tend towards unconsciousness, so that forced breathing has 

 even been used by dentists as a means of producing partial 

 anaesthesia. In many respects these symptoms are similar to those 

 of anoxaemia, except for the absence of spontaneous increased 

 breathing. It was discovered by Hill and Flack 6 that when the 

 forced breathing is with oxygen instead of with air the symptoms 



8 Hill and Flack, Journ. of Physiol., XL, p. 347, 1910. 



