420 



PHYSIOLOGY OF STRESS 



A. Physiological Conditions 

 1. Oxygen and carbon dioxide 



We shall not review the problem of habit- 

 ability, O2, COo, humidity, temperature, and 

 related problems in detail, since these have 

 been treated in other chapters. Our con- 

 cern is primarily with the physiological mech- 

 anisms of adjustment — i.e., reactions by 

 which the organism maintains the internal 

 miheu. It is these mechanisms which 

 determine the manner of adjustment to 

 the important conditions of stress. The re- 

 actions of these mechanisms are of import- 

 ance not merely because they provide home- 

 ostatic control in maintaining the internal 

 environment. A knowledge of their opera- 

 tion may also throw hght upon some of 

 the coincidental or "side effects" that may 

 occur secondarily to the main adjustment. 



The oxygen and carbon dioxide in the 

 respired air does not present the same prob- 

 lem to the submariner that it does to the 

 aviator or mountain climber (85, 86, 98). 

 Not only is the oxygen deficiency of high 

 altitudes not associated with an increase of 

 respired carbon dioxide as in the submarine, 

 but in conditions of flight the increased rate 

 and depth of breathing in response to defi- 

 cient oxygen results in further depletion of 

 CO2 with consequences physiologically (29) 

 which may be quite different from those in 

 the submarine where the carbon dioxide of 

 the respired air may go as high as 3% and 

 oxygen may be reduced to 17% in a 20- to 

 30-hour period under the surface. In con- 

 sequence, much of the valuable research 

 which has built up an extensive laiowledge 

 regarding the reactions of the organism to 

 oxygen deficiency is not applicable in a sub- 

 marine, where depletion of oxygen is ordi- 

 narily attended by accumulation of CO2. 

 (Cf. Section V, 1.) 



The submariner is relatively fortunate in 

 that conditions giving rise to decreased O2 

 lead to an increase of CO2. An increase in 

 CO2 to the level of 2-4% in the inspired air 

 has dramatic effects, postponing or prevent- 



ing syncope and helping to maintain blood 

 pressure even during hyperventilation. For 

 example, breathing of 8% oxygen in a stand- j 

 ing posture will produce a marked hyper- 

 ventilation associated with a fall in blood 

 pressure, increase of heart rate, and syncope 

 in the average male subject. Addition of 

 3% CO2 to the mixture will in many cases 

 prevent the fall in blood pressure and other 

 consequences (48, 50). 



CO2 also helps to maintain venous pressure 

 via effects on the sjonpathetic system, and 

 this provides a better return of blood to the 

 heart with consequent support of blood pres- 

 sure and maintenance of supply of blood to 

 the brain. CO2 decreases the alkalosis, shift- 

 ing the O2 dissociation curve and enhancing 

 the ease with which the increased blood 

 supply can unload O2 in the body tissues 

 (96). Particularly important is the effect 

 of CO2 on brain circulation. The powerful 

 vasodilation effects of CO2 on cerebral blood 

 vessels markedly increases the supply of oxy- 

 genated blood to the brain (47, 50, 51, 

 54,92,97). 



Many studies could be cited deahng with 

 the details of these complex reactions con- 

 stituting the sequence of events occurring in 

 the body's adjustment to the stress of oxy- 

 gen lack. In one Navy study (37, 39) low 

 O2 was used as a stress situation and changes 

 in the electroencephalogram (EEG), inter- 

 action chronogram, adrenal cortical hor- 

 mones, lymphocytes, pulmonary ventilation, 

 heart rate, blood pressure, and certain psy- 

 chological changes were studied on normal 

 controls and on subjects with psychoneurosis 

 and combat fatigue. Whereas 8 to 10% 

 O2 resulted in a clear shift of the distribu- 

 tion curve of occipital alpha frequency to 

 the slow side, even sHght amounts of CO2 

 (0.3-0.5%) definitely reduced the slowing, 

 whether measured by hand or with the 1 

 Walter analyzer (4). 



Eariier work (Gellhorn, 48, 50, Gibbs, 53, 

 and others) using more severe stress (lower 

 O2) emphasized the importance of CO2 for 

 maintenance of normal brain function. As 



