242 HUMAN BIOLOGY 



blood. Unlike carbonic acid it cannot be breathed away. 

 It is eliminated by being discharged, along with excess 

 of NaCI, by way of the kidneys. If large amounts of non- 

 respirable acid appear in the blood, ammonia, which is 

 alkaline and which is ordinarily changed to a neutral prod- 

 uct, urea, is utilized to render the acid harmless and to 

 carry it away in the urine. 



A modification of these processes occurs when the blood 

 tends to become alkaline. Let us suppose that a sharp pain 

 has caused unusually deep breathing. The carbon dioxide 

 percentage in the lungs is thereby reduced and in consequence 

 it is reduced also in the blood. The ratio of H2CO3 to NaHCOs 

 is lowered, i.e. the reaction shifts towards the alkaline side 

 of neutrality. Under these conditions respiration may cease 

 altogether for a time. In the absence of breathing the carbon 

 dioxide, which is continuously being produced by the 

 beating heart and other persistent activities, accumulates 

 in the blood until the normal ratio of H2CO3 to NaHCOa 

 returns, whereupon the rhythmic ventilation of the lungs 

 begins again. And if the reaction of the blood is for some 

 time shifted towards alkalinity, alkaline sodium phosphate 

 is excreted by the kidneys until neutrality is assured. 



In the main the delicate balance between a dangerous 

 acid and an almost equally dangerous alkaline reaction is 

 maintained by the extraordinary sensitiveness of the respira- 

 tory center in the brain and of the kidneys to. even slight 

 alterations in the blood. We may think of these sentinels as 

 being continuously on the alert, ready at the first indications 

 of a change to act in such a way as to prevent a harmful 

 swing away from the normal steady state of neutrality. 



Other Integrative Services oj the Circulating Blood. We have 

 been considering the blood and lymph as the fluid matrix 

 of the body and noting the various devices which work 

 towards the maintenance of constancy of the supplies 

 and of the working conditions which it provides for the 

 living cells. Among the devices for regulating the storage 

 and mobilization of sugar, it will be recalled, the adrenal 

 medulla and the islands of the pancreas were mentioned. 

 These are examples of glands of internal secretion, or endo- 

 crine glands, organs which elaborate special substances 



