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A TEXTBOOK OF PHYSIOLOGY 



To sum up 



1. The respiratory centre is normally rhythmically stimulated by 

 the hydrogen ion concentration of the blood. This ion concen- 

 tration is kept constant both by the expiration of carbon dioxide and 

 by the action of the kidneys. The regulation is such that the pressure 

 of carbon dioxide in the alveolar air is kept remarkably constant. 

 Any alteration markedly affects the breathing. An increase of 0-22 

 per cent. (2 to 3 per cent, in inspired air) doubles the ventilation of 

 the resting man; a diminution by that amount causes a temporarj- 

 cessation of breathing (apncea). 



2. The response of the respiratory centre is not modified under 

 normal conditions by the amount of oxygen in the alveolar air. When 

 the breath is held, or when hard muscular work is being performed, 

 lactic acid is produced in the muscles owing to lack of oxygen. The 

 breathing of oxygen lessens this acid production and its effect on 

 the respiratory centre. 



FIG. 163. FIGURE SHOWING THE ELECTRICAL CHANGES IN THE VAGUS NERVE WHICH 



ACCOMPANY THE RESPIRATORY AND HEART MOVEMENTS. (Einthoven.) 



V, Electro vagogram ; p, respiration record (up, inspiration ; down, expiration); c, pxilsc 



record. 



3. In certain animals, such as dogs, which depend upon breathing 

 for the regulation of heat loss, the temperature of the blood affects 

 the action of the respiratory centre. Similarly, in a man immersed 

 in a very hot bath, the breathing becomes rapid. 



4. Afferent fibres run to the respiratory centre from the lungs 

 in the vagus nerve. The function of these fibres is to co-ordinate the 

 action of the respiratory centre with the degree of distension or collapse 

 of the lungs. Of the two sets of fibres, those which normally inhibit 

 inspiration are the most generally active. By the action of these 

 fibres, waste of time and muscular effort is saved in breathing. 

 They play no part in exciting the normal rhythmic activity of the 

 respiratory centre. 



5. The respiratory centre is also affected by nervous impulses 

 from other parts of the body, which induce modifications of the 

 respiratory act. 



