424 THE HUMAN BODY 



favors the dissociation of the chemical compounds formed during 

 the passage of the blood through the tissue capillaries; thus the 

 carbon dioxid taken up in the systemic capillaries is gotten rid of 

 in the lung capillaries. 



The Hormone Action of Carbon Dioxid. We have already 

 learned (Chap. XXIII) that carbon dioxid has an important 

 action in connection with maintaining the activity of the respira- 

 tory center. Recent work has shown that it has other functions 

 as well. The carbon dioxid tension of alveolar air is ordinarily 

 about 35 mm. of mercury. The carbon dioxid tension of the blood 

 does not, of course, fall below that of the alveoli, so that arterial 

 blood under normal conditions contains a considerable amount of 

 carbon dioxid. Under exceptional circumstances, as at high 

 altitudes, where the atmospheric pressure as a whole is less than 

 at the earth's surface, the tension of carbon dioxid in the alveoli 

 may be considerably less than 35 mm., and that of the blood 

 correspondingly diminished. There is a condition known as 

 mountain sickness, characterized by nausea and other distressing 

 symptoms, which is due to this diminution of the carbon dioxid 

 content of the blood. Any one, by taking a number of deep 

 breaths in rapid succession, can lower the carbon dioxid tension 

 of his alveolar air, and consequently of his blood, to a point where 

 very disagreeable sensations are felt. Just how the carbon dioxid 

 of the blood prevents these symptoms is not clear. That it has 

 the power to do so is, however, well demonstrated. 



The normal breathing mechanism is an adaptation by which the 

 blood is continuously provided with all the oxygen it is able to 

 carry, and by which also its carbon dioxid content, while never 

 allowed to become excessive, is kept high enough for the proper 

 performance of its hormone function. Deep breathing is there- 

 fore of no particular value from the standpoint of respiration. As 

 an exercise for the chest muscles; as a means of insuring ventila- 

 tion of the remotest alveoli; and most of all as an aid to the flow 

 of venous blood and lymph, through the aspiration of the thorax, 

 (p. 370) the practice has great value. We should remember, how- 

 ever, that shallow breathing is the normal mode, and that only 

 while we are thinking about it can we breathe deeply. As soon as 

 our attention is diverted to other matters we recur at once to the 

 automatic shallow type. 



