ARTERIAL BLOOD PRESSURE. 175 



down in one arm and up in the other arm of the U tube. The 

 difference between the levels of the mercury in the two arms mul- 

 tiplied by 13.5, the specific gravity of mercury, gives the pres- 

 sure of the blood in terms of water, or, as is usually done, the 

 blood pressure is expressed as the number of millimetres through 

 which the mercury has been raised. 



Determinations of the pressure existing in different portions 

 of the vascular system show that there is a steady decrease of 

 pressure of the blood from the aorta to the entrance of the vena 

 cava into the right auricle. It thus happens that the blood is al- 

 ways flowing from a place of higher pressure to one of lower 

 pressure. 



Methods which are of much practical importance in the diag- 

 nosis of vascular diseases have been devised to determine the 

 blood pressure in man. The principle of these methods consists 

 in measuring the pressure required to shut off completely the 

 blood supply in an artery. This is accomplished by placing a 

 rubber sac encased in a leather band about the arm, (Fig. 22). 

 By means of tubing this sac is connected with a mercury gauge 

 and an air pump. When the sac is pumped up with air, the ves- 

 sels in the arm are compressed, and when the blood can no longer 

 force its way under the obstruction, the pulse at the wrist disap- 

 pears and at this moment the height of the 'mercury in the gauge 

 is measured. This represents the systolic blood pressure. If de- 

 sired, a similar measurement may be made in the arteries of the 

 leg. 



To measure the diastolic pressure is more difficult. The method 

 depends on the experimentally determined fact that when the 

 pulse wave produced in the arteries by each systole of the heart, 

 is of greatest amplitude, the pressure in the air sac or compress- 

 ing band equals the lowest pressure present in the vessel between 

 the pulses. 



Recently improvements have been made in the method of judg- 

 ing the point of obliteration of the artery, and also the point of 

 maximum pulsation, by listening to the sounds produced at each 

 pulse wave when the artery is being compressed. 



The systolic blood pressure in the artery of the arm in healthy 



