BLOOD VESSELS 



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a medium vein, and in a large vein near the heart be 

 measured, it will be found — 



1st. That the venous pressure is less than the lowest 

 arterial pressure. 



2nd. That it is highest in the small veins, and becomes 

 lower in the larger veins. In the great veins entering the 

 heart during inspiration it is lower than the atmospheric 

 pressure. 



3. Capillaries. — The pressure in the capillaries must 



I60 



A I 



Fig. 183. — Diagram of the Distribution of Mean Blood Pressure throughout 

 the Blood Vessels. Ar., the arteries ; C, the capillaries ; T'., the veins. 



obviously be intermediate between that in the arteries and 

 in the veins. 



The pressure in any part of a system of tubes depends 

 upon two factors : — 



1st. The force propelling fluid into that part of the 

 system, 



2nd. The resistance to the outflow of fluid from that 

 part of the system,. 



The pressure in the arteries is high, (1) because with each 

 beat of the heart the contents of the ventricle are thrown 

 with the whole contractile force of the heart into the corre- 

 sponding artery ; and (2) because the resistance offered to 

 the outflow of blood from the arteries into the capillaries 

 and veins is enormous, since the blood, as it passes into 

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