CONDITIONS OF ARTERIAL TENSION. 45 



they became fuller, however, their elastic force or the arterial 

 tension would increase, sending more and more blood through 

 the capillaries, till the quantity escaping through between each 

 systole exactly equalled the amount driven in each time. The 

 arterial tension would then remain nearly fixed, or at least 

 steadily oscillating round one point, becoming somewhat in- 

 creased as each wave of blood was sent in from the heart, 

 (each wave being equal, not only to the blood escaping by the 

 capillaries during its continuance, but to that escaping during 

 the interval, in addition; this additional amount of blood 

 distends the arteries, and increases their tension), and becoming 

 somewhat diminished as it escaped from the capillaries. The 

 arterial tension may be altered in two w^ays : (1) If, ivliile the 

 cardiac pulsations remain the same (a) the capillaries be some- 

 what dilated, the blood will escape more quickly, and the 

 tension will be diminished, (b) If the capillaries be contracted, 

 less blood will escape, and the tension will be increased. 



(2) The capillaries remaining the same, {a) if the beats of the 

 heart be reduced in number, less blood is sent in, and the ten- 

 sion falls ; if they be increased, the tension rises, {h) If the 

 wave of blood sent in at each pulsation be increased in size, the 

 tension rises ; if it be reduced, it falls. The arterial tension or 

 force with which they tend to drive the blood through the capil- 

 laries, is easily measured by the hgemadynamometer, and its 

 amount expressed in inches of mercury. It is obvious, then, 

 that the arterial tension is the product of two factors, viz. — 1. 

 The amount of blood pumped into the arteries by the heart in 

 a given time. 2. The amount of blood escaping through the 

 capillaries in the same time. This was clearly enunciated by 

 Blake,* and more fully worked out by Marey. The first factor 

 depends on the amount of blood in each w^ave, multiplied by 

 the number of waves in the given time. The second, only on 

 the size of the capillaries, and the rapidity of the flow of blood 

 through them, and this latter, again, depends on the arterial 

 tension. 



Influence of Eespiration on Arterial Tension. — If inspiration 

 and expiration be made with difficulty, as, for instance, by 



* Edin. Med. Journ., 1839. 



