370 THE MECHANICS OF THE CIRCULATION, HEMODYNAMICS 



structed in which a metal spring ^ is employed instead of the mercury- 

 manometer. These so-called sphygmotonometers possess the advan- 

 tage of being convenient to handle, although they must be calibrated re- 

 peatedly to make sure that the tension of the spring has not changed. 

 The Factors Influencing the Arterial Pressure. — As far as the 

 influence of age is concerned, it has been well substantiated that the 

 arterial blood pressure increases constantly until the normal mean is 

 reached in adult life. In later years and old age, it again increases 

 owing to the fact that the elasticity of the vascular tissue diminishes 

 steadily at this time in consequence of retrogressive changes. These 

 facts are fully illustrated by the succeeding table in which values fur- 

 nished by Cook and Briggs,^ Shaw,^ McCurdy and Thayer* have 

 been included : 



First few months 70-75 mm. Hg 



1-2 years 80-90 mm. Hg 



2-3 years 90-100 mm. Hg 



3-10 years 95-115 mm. Hg 



10-15 years 100-115 mm. Hg 



15-20 years 105-128 mm. Hg 



20-30 years 135 mm. Hg 



30-40 years 140 mm. Hg 



40-50 years; 142 mm. Hg 



50-60 years 154 mm. Hg 



60-70 years 180 mm. Hg 



Janeway^ considers 150 mm. Hg as the upper limit in normal adults, 

 while a systolic pressure of 60-75 mm. Hg is generally regarded as 

 dangerously low, although a pressure of 30-40 mm. Hg is sometimes 

 observed during operations. The average normal systoUc pressure 

 amounts to 135-145 mm. Hg; women generally showing a somewhat 

 lower pressure than men. Persons with sedentary habits usually 

 exhibit a pressure between 120 and 125 mm. Hg. The diastohc pres- 

 sure most frequently retains a value about 35-40 mm. Hg below that 

 of the systolic.® If the pressure persists for longer periods of time 

 at 180-200 mm. Hg, and over, a condition of hypertension is said to 

 exist. Quite similarly, persistent low pressures indicate a state of 

 hypotension. Both conditions generally possess pathological causes. 

 The pressure is lowest during the first hours of sleep, and rises 

 gradually until the time of awakening, when it increases rather sud- 

 denly to a level somewhat higher than that retained before retiring.^ 

 During the day the blood pressure shows considerable variations which 



1 von Recklinghausen, Archiv fiir exp. Pathol., Iv, 1906, 375. 



2 Johns Hopkins Univ. Report, xi, 1903, 451. 



3 Albany Med. Jour., xxi, 1900, 88. 



^ Am. Jour. Med. Sciences, cxxvii, 1904, 391. 

 6 Clm. Study of Blood Pressure, New York, 1904. 



^ Hirschfelder, Diseases of the Heart and Aorta, Lippincott, Philadelphia, 1913, 

 and Faught, Blood Pressure, Saunders Co., 1916. 



^ Brush and Fayerweather, Am. Jour, of Physiol., v, 1901, 199. 



