40 PHYSIOLOGY FOR NURSES 



with the pressure applied. As the essential function of 

 the respiratory act is to convey one gas to the tissues 

 and another away; and as experiments prove that some 

 of the gas is in solution, but most of it in combination, 

 there must be pressure changes, or differences, in the 

 lungs and in the tissues to facilitate the exchange. 



If the tidal air is analyzed we shall find that inspired 

 rich in oxygen and that expired rich in carbon dioxide. 

 The reserved air must, therefore, contain a larger 

 amount of C0 2 than the complemental. Now as tidal 

 air enters the alveoli and finds them filled with air con- 

 taining a high percentage of C0 2 , there must be a dif- 

 fusion or mixing of air in the alveoli. This process of 

 mixing goes on at all times. 



When arterial blood passes to the tissues it finds them 

 filled with C0 2 existing under a pressure which causes 

 it to enter into chemical combination as well as solution. 

 The oxygen in the arterial blood, chiefly in combination 

 with hemoglobin, finds the pressure lowered as soon as it 

 reaches the tissues. Pressure on the oxygen being low, it 

 escapes into the tissues ; that on C0 2 being high, it com- 

 bines with the blood. Upon reaching the lungs the re- 

 verse of this action takes place, venous blood giving up 

 its C0 2 and taking oxygen instead because the air in the 

 alveoli is rich in oxygen and poor in C0 2 , while the 

 blood is rich in C0 2 and poor in oxygen, the tendency 

 being for oxygen to escape from the alveoli and C0 2 to 

 escape from the blood into the alveoli until the pressure 

 on each gas is equalized. 



Oxygen and carbon dioxide are the chief, but not the 

 only, gases inhaled and exhaled. Atmospheric air con- 

 tains, in a hundred parts, approximately 79 parts of ni- 

 trogen, 20.96 parts of oxygen, .04 parts of carbon diox- 

 ide and minute quantities of other gases with about 1 



