THE GASES OF THE BLOOD 251 



In a series of observations on the venous blood of dogs the oxygen 

 content ranged from 5-5 to 16-6 c.c. (average 11-9 c.c.), and the carbon 

 dioxide content from 38-8 c.c. to 47-5 (average 44-3 c.c.) per 100 c.c. 

 of blood (Schoffer) . It will be sufficiently accurate to assume that 

 on the average, 



Volumes of 

 67 C0 2 . N 



100 volumes of arterial blood yield 20 40 1-2 



,, ,, mixed venous blood (from 



right heart) yield - 10-12 45-50 1-2 



(reduced to o C. and 760 mm. of mercury). 



Average venous blood contains 7 or 8 per cent, by volume less 

 oxygen, and 7 or 8 per cent, more carbon dioxide, than arterial 

 blood. Thus, in the lungs the blood gains about twice as many 

 volumes of oxygen per cent, as the air loses, and the air gains about 

 half as many volumes of carbon dioxide per cent, as the blood loses. 

 It is easy to see that this must be so, for the volume of air inspired 

 in a given time is about twice as great as that of the blood which 

 passes through the pulmonary circulation (pp. 223, 236). Even 

 arterial blood is not quite saturated with oxygen; it can still take 

 up a variable small amount. The percentage saturation with 

 oxygen of the arterial blood of a normal woman from whom 

 blood was being transfused into a patient was directly determined. 

 The blood proved to be 94 per cent, saturated i.e., it could stili 

 have taken up about one-sixteenth of the quantity contained in 

 it. Nor is venous blood nearly saturated with carbon dioxide; 

 when shaken with the gas it can take up about 150 volumes per cent. 



The total oxygen capacity of the blood in any individual can be 

 determined, if the volume of the blood is known (p. 56), by estimating 

 the amount of oxygen needed to saturate a sample of the blood. 

 This is most conveniently done by the ferricyanide method with 

 Barcroft's apparatus. Suppose, for example, that i c.c. of blood, 

 after being thoroughly shaken up with air or oxygen, gives off 

 0-2 c.c. of oxygen when acted upon by ferricyanide, and that the 

 total volume of the blood has been determined to be 5 litres, then 

 the total oxygen capacity of the blood will be 1,000 c.c. This 

 quantity, it is clear, is a measure of the power of the blood to trans- 

 port oxygen. The oxygen capacity of a sample of blood is propor- 

 tional to the amount of hemoglobin in it, so that the estimation of 

 the percentage amount of haemoglobin by a properly standardized 

 haemoglobinometer is really an estimation of the oxgyen capacity of 

 the quantity of blood used for the determination. The total oxygen 

 capacity of the body cannot, of course, be derived from such an 

 observation, any more than the total quantity of haemoglobin, unless 

 the amount of blood in the body is known. 



When the gases are not removed from blood immediately after 



