THE GASES OF THE BLOOD 29 1 



blood is 10 to 12 per cent, and the carbon dioxide 45 per cent. The blood 

 in different veins of the body varies within wide limits as regards its gas 

 content. 



Oxygen. 



Carbon 

 dioxide. 



Nitrogen. 



zoo c.c. arterial blood 22.6 c.c. 34 c.c. 



100 c.c. venous blood 12 . c.c. 45 c.c. 



1.7 c.c. 

 1.7 c.c. 



The large quantity of oxygen found in arterial and in venous blood is the 

 more striking when the facts of absorption of gases by liquids are reviewed. 

 A liquid such as water will, when exposed to a gas, take up the gas by absorp- 

 tion according to definite physical laws. Under constant temperature the 

 amount of gas absorbed, oxygen for example, varies directly as the pressure of 

 the gas, or partial pressure if the gas is in a mixture. The oxygen absorbed 

 by water from pure air as compared with expired air is in direct proportion 

 to the partial pressure of oxygen in the two airs, which is as 159 to 122. 



The amount of gas absorbed for a unit of fluid under standard tempera- 

 ture and pressure (one atmosphere at o C.), called its absorption coefficient, 

 is about the same for blood plasma as for water. Before one can determine 

 the actual amount of oxygen in the plasma, the tension or absorption 

 pressure must be determined. 



The tension of the oxygen in arterial blood is found by an instrument 

 which enables one to measure the pressure at which oxygen is neither ab- 

 sorbed nor given off. The instrument commonly used is called an aerotonom- 

 eter. The principle of the instrument depends upon the fact that blood 

 exposed to mixtures of the gases in air tends to give up or absorb gases from 

 the air until complete equilibrium is established. 



By this means observers have measured the tensions of the blood gases. 

 The results have been not very constant, but the oxygen tension has been 

 found to be from 4 (Strassburg) to 10 (Herter) per cent, of an atmosphere. 

 Many determinations have been given of both lower and higher percentages, 

 but, accepting the above limits for a working average, the oxygen tension 

 in arterial blood would be from 30.4 to 76 mm. of mercury. 



Blood plasma exposed to an air with a partial pressure of 30 . 4 to 76 mm. 

 of mercury would absorb only from o. i to 0.3 (o. 26 c.c. Pfluger) of a cubic 

 centimeter of oxygen for 100 c.c. of blood. As a matter of fact, 100 c.c. of 

 whole blood contains from 20 to 22 . 6 c.c. of oxygen. It is evident that blood 

 cannot hold the oxygen in simple solution, but must retain it in chemical 

 combination. The red blood corpuscles have been shown to carry the 

 excess of oxygen by virtue of the special respiratory pigment, hemoglobin. 



