FOOD DIGESTION AND RESPIRATION 89 



to the ribs and spinal column. By these two means air is sucked 

 into the lungs. When the contraction of the muscles ceases, the 

 chest returns to its position of rest, expelling part of the air con- 

 tained in it. Under vigorous respiration, however, there are 

 muscular movements which assist in pressing air out. The whole 

 of the air is, of course, not driven out, so that in the depths of the 

 small air sacs, where the exchange between blood and air is 

 effected, the tension of oxygen is not as high as in the atmosphere, 

 although it is high enough to saturate the haemoglobin ; while 

 the tension of carbon dioxide is not so low as in the outer air, but 

 is lower than in the venous blood. 



The way in which the supply of air by respiratory movements 

 is regulated in accordance with the needs of the organism is by 

 the fact that the nerve centre, which is responsible for sending 

 periodic discharges to the muscles acting on the chest, is extremely 

 sensitive to a slight rise in the hydrogen-ion concentration of the 

 blood. When more oxygen is being consumed by the activity of 

 cells, more carbon dioxide is being given off. This becomes an 

 acid when dissolved in water, and raises the hydrogen-ion concen- 

 tration of the blood to a slight degree. The respiratory centre is 

 stimulated, more oxygen is supplied, and the excess of carbon 

 dioxide removed (E., p. 202). It is important to remember that the 

 production of any acid by the tissues has the same effect. Since 

 acids are produced in the course of the normal metabolism of fats 

 and carbohydrates, and subsequently oxidised, it is clear why they 

 appear in the blood in states where there is deficient supply of 

 oxygen, as in lowered rates of circulation of blood, and so on. A 

 rise in hydrogen-ion concentration of the blood results, owing to 

 the fact that these acids drive off carbon dioxide from the bicar- 

 bonates contained therein. This rise stimulates the respiratory 

 centre, and tends to automatic benefit by a more copious supply of 

 oxygen. It will also be clear that treatment of this so-called 

 "acidosis" by giving alkalies is inappropriate, because the stimu- 

 lation of the respiratory centre is thereby prevented, and 

 spontaneous supply of the necessary increase in oxygen is 

 retarded. 



The Mechanism of Oxidation 



The supply of oxygen to the tissues is provided by the means 

 described. But the mere presence of ordinary oxygen is not 

 sufficient, as will be evident when it is remembered that glucose is 

 one of the foods most largely burned for the supply of energy, and 

 that glucose is not oxidised by the air, or so slowly as to be useless 

 for the purpose in view. What we need is to raise the chemical 

 potential of the oxygen so that it shall attack substances refractory 



