RESPIRATION: THE GASEOUS INTERCHANGES 383 



carbon dioxid. It is present in lymph, therefore, at all times in 

 considerable quantity, probably amounting to 70 mm. of mercury 

 carbon dioxid tension. 



2. The blood entering the capillaries contains carbon dioxid 

 under much less tension than this (about 35 mm.), there is there- 

 fore a movement of carbon dioxid from lymph to blood. This 

 movement, by raising the tension of carbon dioxid in the blood 

 brings about conditions under which chemical combination may 

 take place, forming, perhaps, sodium bicarbonate. 



3. The venous blood as it enters the lungs contains carbon 

 dioxid under a higher tension than that of alveolar air, 70 mm. 

 for venous blood, 35 mm. for the alveoli; there is therefore a 

 movement of carbon dioxid from the blood to the alveoli. This 

 movement, by lowering the carbon dioxid tension of the blood, 

 favors the dissociation of the chemical compound 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 

 hormone action in connection with maintaining the activity of 

 the respiratory 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 con- 

 siderable amount of carbon dioxid. Under exceptional circum- 

 stances, as at high altitudes, where the atmospheric pressure as 

 a whole is less than at the earth's surface, the tension of car- 

 bon 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 car- 

 bon 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 



