166 THE HUMAN MECHANISM 



oxygen with the proteins, the carbohydrates, and the fats 

 of the food. 



The cell draws oxygen from the surrounding lymph very 

 much as a burning match draws oxygen from the surround- 

 ing air. Consequently the amount of oxygen dissolved in 

 the lymph is generally comparatively small and would be 

 removed altogether were it not constantly renewed from 

 the blood. 



For similar reasons the lymph must be relatively rich in 

 carbon dioxide, since it is this fluid which directly receives 

 the gas (in solution) from its source of manufacture, the 

 working cell. 1 



3. The quantity of oxygen and of carbon dioxide in arterial 

 blood. It is through the lungs that the body as a whole 

 receives its oxygen and discharges its excess of carbon 

 dioxide. Consequently arterial blood contains more oxygen 

 and less carbon dioxide than venous blood. The actual 

 figures are as follows: 



OXYGEN CARBON DIOXIDE NITROGEN 



100 cc. of arterial blood contain 20 cc. 38 cc. 1-2 cc. 



100 cc. of venous blood contain 8-12 cc. 45-50 cc. 1-2 cc. 



These figures apply to the whole blood, that is, to plasma 

 and corpuscles; but what is true of the whole blood is true 

 in a general way also of the circulating plasma, which con- 

 sequently enters the capillaries 2 relatively rich in oxygen 

 and poor in carbon dioxide, thus presenting exactly the 

 reverse composition, in respect to these gases, of that found 

 in the lymph surrounding the living cells. 



1 The gases oxygen and carbon dioxide are, of course, dissolved in the 

 liquid lymph and blood plasma. A liquid exposed to a gas absorbs or dis- 

 solves the gas. Thus 100 cc. of water when exposed to atmospheric air at 

 0C. dissolves 4 cc. of oxygen and 2 cc. of nitrogen. 



2 The total time consumed by the blood in passing from the capillaries of 

 the lungs through the heart to those of the rest of the body seldom exceeds 

 five or six seconds. Hence the amount of the gases in the blood entering 

 the capillaries, for example, of a muscle is practically the same as in the 

 blood leaving the lungs. 



