CHEMICAL CONTROL OF RESPIRATION. 221 



increase in the hydrogen ion concentration in the blood is suffi- 

 cient to stimulate the respiratory center and augment the rate 

 and depth of respiration in order to expel the carbon dioxide 

 and thus reduce the acidity of the blood. All acids which yield 

 hydrogen ions in solution have this effect on respiration when 

 they are injected into the blood. Lactic acid, which is formed 

 when the oxygen supply to the tissues is diminished or inade- 

 quate, is perhaps the most important factor coming into play in 

 the stimulation of the respiratory center which occurs during 

 exercise. The carbon dioxide tension of the blood during exer- 

 cise may be actually decreased owing to the increased ventilation 

 of the lungs as a result of the presence of lactic acid in the blood. 



The increase in breathing due to lack of oxygen is not nearly 

 so easily elicited as that caused by excess of acids. In fact, the 

 percentage of oxygen may be diminished to about one-half of 

 that found in the atmosphere before breathing is markedly af- 

 fected. 



In disturbances of the gaseous exchange of the lungs, the re- 

 spiratory center attempts to compensate for the change by in- 

 creasing the number and the depth of the respirations. If the 

 gas exchange be markedly insufficient, the breathing becomes 

 very much exaggerated, and practically all possible respiratory 

 muscles are called into play. This is the case during an attack 

 of asthma, in which the muscles of the arms and abdomen are 

 used by the patient in his efforts to obtain enough air. Difficult 

 breathing of this kind is known as dyspnoea. If the gas exchange 

 is very insufficient, the phenomenon of asphyxia sets in. 



The control of the respiration, therefore, may be said to be a 

 double one, one dependent on the nerve supply of the respira- 

 tory center from the afferent sensory and cerebral nerves, and the 

 other on the chemical constitution of the blood, which stimulates 

 the center directly. Both play an important part in the control 

 of the respiratory movements. 



The 'bronchial muscles are supplied through the vagi with 

 nerve fibers which produce dilatation and constriction of the 

 bronchi. Just what the normal conditions are which call for the 

 action of these nerves is not known. It is generally thought that 



