220 ANATOMY FOR NURSES .r (Cha^. XII 



5. The resistance oflFered by the arterioles. C-'^'V} \l 



6. The increased surface over which the blood flows in the capil- 

 laries. 



When the blood leaves the left ventricle the high pressure which it 

 exerts against the wall of the aorta may be regarded as a measure 

 of energy. This energy is transformed into heat in overcoming 

 the friction encountered in the vessels. When the blood reaches 

 the arterioles, the surface is multiplied and the friction increased. 

 This offers an impediment to the flow, and the result is a decided 

 drop in the pressure. 



Arterial pressure is not uniform, but varies (1) with the systole 

 and diastole of the heart, being greater during the systole ; (2) it 

 is less in youth, and increases as we grow older, because the arteries 

 are less elastic ; (3) conditions of health may affect the normal 

 muscular tone of the arteries and heart. When the arteries lose 

 their tone, or the heart-beat loses its force, the blood pressure is 

 low. When the arteries are over-constricted, or the heart over- 

 stimulated, the blood pressure is high. 



Capillary pressure. — The pressure of blood in the capillaries 

 is much lower than in the arteries, but is high considering the thick- 

 ness of the capillary walls. It depends upon the condition of the 

 arterioles : if they dilate, the capillary pressure rises ; if they con- 

 strict, the capillary pressure falls. The phenomena produced by 

 these local variations in the blood supply of certain parts are very 

 familiar to us : the redness of the skin produced by an irritating 

 application, the blushing or paling of the face from mental emotion, 

 the increased flow of blood to the mucous membranes during 

 digestion, being all instances of this kind. 



Venous pressure. — When a vein is severed, the flow of blood, 

 which is chiefly from the distal end (that away from the heart), 

 is not intermittent, but continuous ; the blood comes out with 

 comparatively little force, and " wells up " rather than " spurts 

 out." This indicates that the blood pressure in the veins is low. 

 It is influenced by : — 



1 . The presence of valves which prevent a backward flow. 



2. Respiration. The effect of inspiration is to suck venous 

 blood into the thorax, empty the large veins, and cause a fall in 

 the blood pressure. The effect of expiration is to offer a slight 

 resistance to the flow of blood into the thorax and thus raise the 

 blood pressure in the large veins. 



