352 physiological, eegulations 



§ 125. Arterial blood pressure 



The arrangements in mammalian organisms whereby constancy 

 of arterial pressure is maintained, have been studied in consider- 

 able detail and are widely known. Certain ' ' insensitivities ' ' in the 

 piezostatic arrangements are required to prevent the variations 

 of pressure during each cardiac cycle from inducing large modifica- 

 tions of mean tension of blood vessels at each diastole. Auto- 

 matically a decrease of pressure lasting more than one heart beat 

 raises the frequency of muscular contractions in blood vessel walls ; 

 and conversely. 



I select as conditions for recovery from excess of systolic 

 arterial pressure (as commonly measured) the sequelae of physical 

 exercise (fig. 176), and for recovery from deficit the sequelae of 

 pressure upon the neck and carotid sinus (Mies, '32; "Weiss and 

 Baker, '33). The pressure is changing at a net rate, and tells noth- 

 ing as to whether the heart and the various arterioles are working 

 at ''cross purposes" or not. 



The velocity quotients of recovery are in figure 176 about 

 24/hour, and in two tests of Mies about 34/hour ; utilizing in both 

 series of data the first 0.01 hour after the ''stimulus" ceased. 

 Recovery of systolic arterial pressures in man from certain other 

 types of load, also appears to be at about this rate. 



Diastolic arterial pressures show opposite changes to those of 

 systolic in the same exercise (fig. 176). Their recoveries are of 

 approximately the same velocity quotient, however. The same 

 holds true in the recoveries of arterial pressures that follow change 

 of posture (Ogden et al., '38). 



A few data for recovery of mean arterial pressure following 

 nerve stimulations in anesthetized cats (Bayliss, '15, p. 691) indi- 

 cate velocity quotients, both in excesses and deficits, of 250 to 

 700/hour. Following hemorrhage, the recovery of mean arterial 

 pressure in similar animals is less rapid (Brooks, '35). 



Since the intra-arterial pressure varies cyclically with each 

 heart beat, it is possible to study also the rate of pressure change 

 within a single cycle. From analyses of rapid recordings it is shown 

 (Hamilton and Woodbury, '37) that during diastole the fall of pres- 

 sure is approximately exponential with time. The same curve, with 

 equal velocity quotient, is obtained upon suddenly occluding an 

 artery and observing the continued fall of pressure within its 

 peripheral segment (Dow and Hamilton, '39). It may be said that 



