134 RESPIRATION 



existence in solution in the blood. There is no evidence that nitrogen 

 enters into combination with the blood-corpuscles. It exists simply in 

 solution in the blood, which is capable of absorbing about ten times as 

 much as water. Nothing is known in regard to the relations of the free 

 nitrogen of the blood to the processes of nutrition. 



Using the mercurial gas-pump, it is possible to extract about sixty 

 volumes of gas from one hundred parts of blood. In experiments on 

 the blood of the dog, the average percentage volumes of oxygen, carbon 

 dioxide and nitrogen in arterial and venous blood were as follows 

 (Kirkes): 



ARTERIAL BLOOD VENOUS BLOOD 



Oxygen i'*f.\ . -/. , ... . ... . 20 8 to 12 



Carbon dioxide . . .- .'.> . . . ' 40 46 



Nitrogen . .'""'. . * . .'/''. I to 2 i to 2 



Oxygen of the Blood. When oxygen exists in a condition of simple 

 solution in any liquid, the quantity dissolved is in direct ratio to the 

 pressure ; and when the pressure is increased, the amount held in 

 solution is increased in exact proportion. This law, however, is not 

 operative in the case of the oxygen of the blood. When the condition 

 is one of saturation, the amount does not vary with the pressure. In 

 the case of a mechanical mixture of gases, what is known as the 

 " partial pressure " of each gas is the force which leads to its diffusion, 

 and this may be measured by means of a column of mercury. 



The following application of this law may be made to the oxygen 

 of the inspired air : The partial pressure of oxygen in the atmosphere, 

 expressed in percentages of the pressure of one atmosphere = 20.96 ; 

 the pressure of oxygen in the air contained in the air-cells = 18; the 

 tension of oxygen in the arterial blood =14; in the tissues, the tension 

 of the oxygen is zero (Fredericq). 



This reduction in pressure, from 20.96 to zero, indicates the direc- 

 tion of diffusion of oxygen. The oxygen of the inspired air is diffused 

 into the air-cells because the pressure is 18 instead of 20.96; it is dif- 

 fused into the blood because the tension is here reduced to 14; it passes 

 into the tissues because the pressure in the tissues is zero. The ten- 

 sion of gases held in solution in liquids is the force which holds them 

 in solution, or which prevents their escape by diffusion. The oxygen 

 contained in the blood is " associated " with hemoglobin, a condition of 

 union that has already been considered in treating of the composition 

 of the blood. 



Carbon Dioxide of the Blood. The relations of carbon dioxide to 

 the tissues, the blood and the contents of the air-cells are rather more 

 complex than in the case of oxygen, especially as regards the blood. 



