THE CIRCULATION OF THE BLOOD 



359 



this the pressure begins to fall, but endures until the close of the systole, 

 indicated in this curve by the notch preceding the elevation c. 



Diastolic Pressure. Diastolic pressure may be denned as the lowest 

 pressure in the artery during the diastole of the heart and occurs at the 

 end of the diastolic period just before the time of the succeeding systole. 

 The diastolic pressure gradually falls from the time of closure of the semi- 

 lunar valves to the end of the diastolic period. 



Of these two pressures the more important to determine from the 

 clinical point of view is the diastolic, for the reason that it represents the 

 load which the walls of the arteries sustain from moment to moment. 

 Moreover it represents the pressure supported by the aortic valves, as 

 well as the resistance which the heart must overcome at the beginning 

 of the systole in order to open the aortic valves. 



The diastolic pressure is the resultant of the rate of the heart and the 

 peripheral resistance. If the latter is increased the former will be dimin- 



FIG. 162. A SPHYGMOGRAM SHOWING THE VARIATIONS IN THE ARTERIAL PRESSURE 

 DURING THE CARDIAC CYCLE, a, b, c, represents the rise and fall of the arterial pressure dur- 

 ing the systole; c, d, the continued fall of the pressure during the diastole; c, will represent 

 approximately the time of closure of the semi-lunar valves. 



ished; if the peripheral resistance is diminished the rate of the heart will 

 be increased (see page 356). Should the diastolic pressure be found to be 

 high and the heart rate diminished, the indications are that the resistance 

 is increased. Should the diastolic pressure be found to be low and the 

 heart rate increased the indications are that the peripheral resistance is 

 decreased. The higher and the more persistent the diastolic pressure, the 

 greater is the strain on the arteries and heart. The systolic pressure, 

 though of extremely short duration is important to determine as it repre- 

 sents approximately the intra-ventricular pressure and the cardiac energy. 



The Sphygmomanometer. The instruments by which these pressures 

 are determined are called sphygmomanometers. Some of the many 

 forms of this instrument are adapted for obtaining the systolic pressure 

 only, while others are adapted for obtaining either the systolic or the 

 diastolic pressure, or both. 



The principle involved in the first group is the application of a hydrostatic 

 pressure to an artery, e.g., the temporal, radial, etc., until the lumen is com- 

 pletely obliterated as indicated by the disappearance of the pulse beyond the 

 point of compression, and at the same time the registration of the pressure 

 applied, by means of a mercurial or spring manometer. The pressure just 

 sufficient to obliterate the pulse or to allow it to reappear after obliteration, 

 is taken as the systolic pressure. 



The principle involved in the second group is based on a suggestion of 

 Marey, that the maximum pulsation of the artery or the maximum distention 

 and recoil following a heart-beat would be most likely to take place when 



