250 RESPIRATION 



by absorption with a solution of pj-rogallic acid and potassium hydrox- 

 ide, or an alkaline solution of sodium hydrosulphite, which is more 

 cleanly. The remainder of the original mixture of blood-gases, after 

 deduction of the carbon dioxide and oxygen, is put down as nitrogen 

 (with, no doubt, a small proportion of argon). For the sake of easy 

 comparison, the observed volume of gas is always stated in terms of its 

 equivalent at a standard pressure and temperature (760 mm., or some- 

 times on the Continent i metre of mercury, and o C.). 



It is also possible in various ways to estimate the amount of oxygen 

 in blood without the use of the pump. Thus, since a definite volume of 

 oxygen (i -338 c.c. at o C. and 760 mm. pressure) combines with it 

 gramme of haemoglobin, we can calculate the total volume of oxygen 

 present if we know how much of the blood-pigment is in the form of 

 oxyhaemoglobin ; and this can be determined by means of the spectro- 

 photometer. Or potassium ferricyanide may be added to the blood. 

 This expels the oxygen from its combination with the haemoglobin, 

 which then unites with an exactly equal amount of oxygen obtained 

 from the ferricyanide to form methaemoglobin (Haldane) (p. 75). 



In the hands of Barcroft and his pupils this method has been highly 

 developed, so that accurate results can be obtained with small quantities 

 of blood (i c.c., and with the smaller apparatus even o-i c.c.). Bar- 

 croft's differential apparatus consists essentially of two small bottles, 

 as nearly alike as possible, connected by a manometer filled with oil (of 

 cloves). The amount of oxygen liberated in one of the bottles by 

 potassium ferricyanide from a measured amount of blood can be esti- 

 mated from the displacement of the liquid in the manometer. The 

 function of the second bottle is to automatically eliminate effects due 

 to changes in the temperature of the bath in which the apparatus is 

 immersed, etc., since both bottles are affected alike. 



The Quantity of the Blood-Gases. In arterial and in venous blood 

 oxygen, carbon dioxide, nitrogen, and argon are constantly found. 

 Both the oxygen and the carbon dioxide vary considerably in 

 amount in the arterial blood, even of individuals of the same animal 

 group, and, of course, much more in the venous blood, as might 

 naturally be expected, since even to the eye it varies greatly accord- 

 ing to the vein it is obtained from, the rapidity of the circulation, 

 and the activity of the tissues which it has just left. In one 

 observation on blood obtained directly from a human artery, 2i6c.c. 

 of oxygen, 40*3 c.c. of carbon dioxide, and 1-6 c.c. of nitrogen were 

 found in 100 c.c. of blood. The quantity of oxygen taken up outside 

 of the body by specimens of human blood drawn from six normal 

 persons, when shaken up with atmospheric air, varied from 17-6 c.c. 

 to 22-5 c.c. per 100 c.c. of blood, the variations depending mainly 

 on the haemoglobin content. The arterial blood as it actually left 

 the lungs of those persons must have contained somewhat less oxygen 

 (about i c.c. less per looc.c. of blood), since the partial pressure of 

 oxygen in the alveolar air is decidedly below that in atmospheric 

 air. In dogs the amount of carbon dioxide in arterial blood has 

 been found to vary from 35 to 45 c.c. per 100 c.c. of blood, the 

 differences being due to variations in the extent of the pulmonary 

 ventilation and to other factors. 



