RESPIRATION 283 



How very powerfully a local stimulus may act on local blood 

 circulation is strikingly shown by a recent experiment of Meakins 

 and Davies.^^^ They found that when the arm was immersed in 

 cold water the returning venous blood was completely deprived 

 of oxygen. On the other hand, when the arm was kept in hot water 

 the haemoglobin of the venous blood was 94 per cent saturated 

 with oxygen, as compared with 96 per cent for the arterial blood. 

 The oxygen consumption was doubtless much greater in the warm 

 than in the cold skin, so the difference in circulation rate must have 

 been enormous. 



If the regulation of blood distribution in the body were simply 

 a matter of opening the proper sluice gates according to local re- 

 quirements, the matter would be much more simple than it is. 

 Actually, however, the contraction and dilatation of various ar- 

 teries, veins, and capillary tracts must tend to have the effect of 

 varying the total capacity of the blood vessels, with the result that 

 the venous blood pressure at the heart inlet varies, and either too 

 little, or too much, blood is supplied to the heart. As a conse- 

 quence, the arterial blood pressure would either tend to fall too 

 much to secure an adequate supply of blood to the brain and other 

 parts, or else to rise too high. 



There appears to be an elaborate nervous defense against such 

 disturbances. Excessive rise of arterial blood pressure is guarded 

 against, not only by the reflex vagus inhibition already referred 

 to, but also by reflex vasomotor inhibition through the "depres- 

 sor" branch from the cardiac vagus. Excitation of the depressor 

 fibers causes inhibition of the vasomotor center in the medulla and 

 consequent dilatation of arteries and probably veins in the splanch- 

 nic and other areas. Depressor action is brought about (whether 

 directly or indirectly) by excessive arterial blood pressure, so 

 that the pressure is relieved. Deficiency in arterial and venous 

 pressure is guarded against by an opposite "pressor" action re- 

 sulting in excitation of the vasomotor center and consequent rise in 

 blood pressure. A normal stimulus to pressor action of the center 

 is quite evidently deficiency of oxygen combined with excess of 

 carbonic or other acids in the blood supplying the brain. Thus the 

 arterial and venous blood pressures rise very markedly in response 

 to deficiency of oxygen combined with excess of carbonic acid, 

 whether produced by deficient aeration of the blood or circulatory 

 failure. A very important effect of this rise of blood pressure is 



*°^ Meakins and Davies, Journ. of Path. and. Bact., XXIII, p. 460, 1920. 



