THE CIRCULATION OF THE BLOOD 355 



A 



contracts and a smaller volume of blood flows into the veins, the reverse 

 change of pressure obtains. 



The Determination of the Arterial Blood-pressure in Man. Inas- 

 much as the blood-pressure undergoes considerable variation in both physio- 

 logic and pathologic conditions as well as in response to the action of drugs, 

 it seemed desirable to possess some means by which an accurate knowledge 

 of the pressure under a variety of conditions could be obtained both for 

 diagnostic and therapeutic purposes. The foregoing method of obtaining 

 the blood-pressure not being of general application to human beings for 

 obvious reasons, special instruments have been devised by which the pres- 

 sures may be determined at least approximately without resorting to any- 

 surgical procedure. 



By reason of the fact that both the systolic and diastolic pressures are' 

 regarded as important factors in clinical conditions, and their determination 

 of value for diagnostic and therapeutic purposes in diseases of the circulatory 

 apparatus, it is desirable to have clear ideas of what is meant by these terms. 

 If the changes of pressure are registered by a sphygmograph, a curve re- 

 sembling that shown in Fig. 173, page 367, will be recorded, which shows 

 more or less accurately the qualitative, if not the quantitative variations, of 

 the pressure in the arteries during a cardiac cycle. 



Systolic pressure may be denned as the highest pressure developed in 

 the artery during the systole of the heart and occurs in the first half of the 

 systole and is therefore of very short duration; after 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 may be defined 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 semilunar valves to the end 

 of the diastolic period. 



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 o 

 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 

 an elastic pressure applied to the outside of an artery is just sufficient 

 equalize the diastolic pressure within. Inasmuch as these pulsation 

 be transmitted to, taken up and reproduced by a mercurial column ir 

 tion with the pressure appliances, it becomes possible, when the maximum 

 oscillation of the mercurial column is attained, to read off the diast 

 pressure. 



