268 ESSENTIALS OF PHYSIOLOGY. 



to the condition known as acapnia, in which the. blood pressure is very 

 low and the tone of the muscular walls of the digestive tract and of 

 other unstriped muscles is lost ; the symptoms closely resemble those 

 of surgical shock (p. 235). 



The activity of the respiratory centre is excited, not only by a 

 rise in the tension of carbonic acid, but also by the addition to the blood 

 of other acids, such as lactic acid. When O02 to 0*04 per cent, lactic acid 

 is added to the blood, a considerable increase is produced in the depth 

 of the respiratory movements even though the tension of carbonic acid 

 in the blood is unaltered. It has been shown, indeed, that carbonic 

 acid stimulates the respiratory centre, not in a specific mariner, but 

 because it is an acid and increases the concentration of H ions in the 

 blood. Any other acid, by acting in the same manner, produces the same 

 effect, and when lactic acid is added to the blood the centre is stimulated 

 by the combined effect of this acid and of the carbonic acid already 

 present in the blood. Since the respiratory movements are increased 

 without any corresponding rise in the production of carbonic acid, the 

 tension of carbonic acid in the blood and alveolar air falls. 



(2) The Tension of Oxygen in the Blood. The normal tension of 

 oxygen in alveolar air is 105 to 110 mm. Hg, At this tension the blood 

 leaving the lungs is almost fully saturated with oxygen, and the dis- 

 sociation curve of blood (p. 258) shows that, even when the oxygen tension 

 is reduced to 70 mm. Hg, the blood still contains 90 per cent, of its haemo- 

 globin as o-xyheemoglobin. Experiment shows, in fact, that atmospheric 

 air containing only 12 to 13 per cent, oxygen, which corresponds with 

 about 8 per cent, oxygen in the alveolar air, can be breathed without dis- 

 comfort and without any alteration in the respiratory movements. When 

 the percentage of oxygen in alveolar air falls below this figure, the 

 breathing becomes deeper and considerable hyperpnoea may be produced. 

 At the same time, the individual becomes cyanosed and may feel giddy, 

 or may even lose consciousness. 



These symptoms are the result of the imperfect oxygenation of the 

 blood. The mere lack of oxygen in itself does not act as a stimulus 

 to the respiratory centre, and the hyperpnoea is caused indirectly by 

 the passage of lactic acid into the blood. Lactic acid is normally formed 

 in the body, and in the presence of an adequate supply of oxygen it is 

 subsequently oxidised to carbonic acid and water. When the supply 

 of oxygen is insufficient, the lactic acid passes into the blood, and, by 

 increasing its H ion concentration, stimulates the respiratory centre ; 

 the respiratory movements become deeper, and more air enters the 

 lungs at each breath. The deeper breathing raises the tension of 

 oxygen in the alveolar air, and thus increases the amount of oxygen 



