OP BLOOD-PRESSURE 59 



Hill). The normal capillary pressure varies with 

 the locality, being 10 to 20 millimetres of water in 

 the arm, and 90 millimetres in the leg. 



THE FACTORS 

 CONTROLLING THE BLOOD-PRESSURE. 



The factors normally at work in maintaining the 

 blood-pressure are more complicated than was at 

 one time apparent. Of course it varies with the 

 activity of the heart. It is further controlled by 

 the calibre of the smaller arteries, which in their 

 turn are regulated by the vasomotor centre in the 

 medulla of the brain. Vasoconstriction naturally 

 raises the blood-pressure, and vice versa. Then, 

 again, the respiratory movements have an influence. 

 In experimenting with animals under an anaesthetic 

 this influence is marked and constant ; the blood- 

 pressure rises with each inspiration, and falls with 

 each expiration. In the normal human subject this 

 does not obtain with the same constancy. Lewis 

 has demonstrated that it is true when the respiration 

 is abdominal in type, but that in the costal type of 

 breathing the blood-pressure falls with inspiration, 

 and rises with expiration. 



Gravity also affects the blood-pressure. It is well 

 known that many people faint on rising too suddenly 

 from a hot bath, because the vasomotor control is 

 not sufficiently powerful to constrict all the vessels 

 in an instant, so as to keep enough blood circulating 

 through the brain. Leonard Hill's well-known 

 experiment on the hutch rabbit and the wild rabbit 

 illustrates the same condition. The hutch rabbit 



