132 



324 J. A. macwilliam 



Again the strikingly exaggerated pressure changes which may rapidly 

 occur in response to nervous disturbances, emotional causes, etc., as 

 noted by numerous observers in many cases of high blood pressure, 

 bear testimony to the presence of disturbed innervation involving de- 

 fective regulation as an important factor in the condition. Thus 

 causes of slight elevations of pressure in the normal state may have 

 abnormally great effects in causing rapid and extensive variations in 

 many subjects of high pressure. 



Low hlood pressure. The mechanism of the acute condition of 

 excessively low pressures seen in circulatory shock, etc. — due to the 

 altered capacity factor dependent on capillary relaxation and later on 

 diminished blood volume — has been elucidated by various investiga- 

 tions, especially by the work of Cannon (24) on traumatic shock and 

 that of Dale with Laidlaw (32) and Richards (33) on the action of the 

 histamine and histamine-like bodies. Similarly the pressure falls in 

 acute infections like cholera, etc., are rendered intelligible. But in 

 persistent low pressures attendant on exhausting diseases or occurring 

 without obvious cause (essential hypotension) the available data are, 

 as in the case of persistent high pressure, inadequate for a satisfactory 

 explanation of the mechanism involved — whether a defective periph- 

 eral resistance or defective cardiac output depending, apart from 

 cardiac enfeeblement, or lessened return of blood to the heart as a 

 result of undue expansion of the capacity of the vascular system from 

 capillary or venous relaxation, contraction of venules, diminished 

 volume of blood in circulation, etc. It is also unknown how far such 

 conditions are mediated through the nervous system and how far due 

 to the direct injQuence of chemical agents — depressor bodies, lack of 

 pressor substances, etc. 



As to what constitutes "low pressure" the level below which a pres- 

 sure is to be regarded as low or abnormal is not sharply defined and 

 no doubt varies considerably, as in the case of high pressure, for the 

 individual and the conditions present. Roughly anything decidedly 

 below 100 systolic or 60 diastolic may be suspected of being "subnor- 

 mal." Some athletes in good training have such pressures as systolic 

 105 and diastolic 65. 



Subnormal blood pressures naturally exercise a generally depressing 

 influence on the active tissues and tend to establish a vicious circle. 

 There is significance in the observation of Markwalder and Starling 

 (95) that for the mammalian heart (in the heart-lung preparation) 

 an average (innominate artery) pressure of at least 90 mm. Hg is neces- 



