Ain'EUIAL TENSION OR BLOOD PIIESSUIIE. 273 



tliese are stretched, the greater is the pressure they exert on the 

 "blood they contain ; and the amount of this is termed the 

 arterial tension or Uood-prcssure. Tliese two terms mean the 

 same thing, and we use one or other just as the iancy strikes us. 

 At each systole, the fresh supply of blood pumped in by tlie 

 ]ieart stretches them more ; that is, the arterial tension rises. 

 During each diastole, the blood escapes into the wide and 

 dilatable veins, and the arteries become relaxed; that is, the 

 iirterial tension falls. 



Besides the o.'^illations which take place in the blood-pres- 

 sure at each beat of the heart, a rise and fall in the form of a 

 long wave occurs at each respiration. The wave begins to rise 

 just after inspiration has begun, reaches its maximum just after 

 the beginning of expiration, and then begins to fall again till a 

 new wave succeeds it. The heart-beats are generally quicker 

 ■during inspiration, and slower during expiration. 



The blood-pressure thus oscillates up and down at each heart- 

 beat and rises and falls with each respiration, and the average 

 between the highest and lowest points is called the mean 

 .arterial tension or mean blood-pressure. 



Causes of Variation in the Blood- Pi^essurc. — The pressure of 

 blood in the arteries depends on two circumstances : first, the 

 •amount of blood pumped into them in a given time ; and 

 second, the amount that flows out of them into the veins in the 

 .same time. It' more be pumped in, or if less flow out, it will 

 xise ; if less be pumped in, or if more flow out, it will fall. 

 It may, therefore, be raised — 1. By the heart beating more 

 ■quickly; 2. By a larger amount of blood being sent into the 

 .aorta at each beat ; 3. By contraction of the small vessels. It 

 may be lowered — 1. By the heart beating more slowly ; 2. By 

 •the heart sending out less blood at each beat ; 3. By dilatation 

 of the small vessels, allowing the blood to flow^ more quickly 

 into the veins ; 4. By contraction of the pulmonary vessels, or 

 obstruction to the passage of blood through them. 



The influence on the pressure exerted by the amount of blood 

 sent out by the heart at each beat, and by the number of beats, 

 to a certain extent, though by no means completely, counteract 

 •each other; for. when the heart is going quickly, it has not 



T 



