36 RESPIRATION. 



What causes unite tp compel the return of the blood through 

 the veins? 



(1) Vis a tergo, or the pressure remaining from the force of the 

 left ventricular systole and arterial elasticity. 



(2) Muscular action in pressing upon the veins which have valves. 



(3) Suction Power of the Heart. During diastole the opening of 

 the heart-cavity is sufficiently strong to exert a rather strong " neg- 

 ative pressure " or suction power. 



(4) Aspiration (or suction) power of the chest in respiration. 



(5) The slight rhythmical contractions of the veins. 



What is meant by vital capillary force ? 



The capillaries are not unchanging blood-channels, but may be 

 seen to dilate and contract under the influence of stimuli. We 

 have seen that each capillary has its accompanying vaso-motor 

 sympathetic plexus ; and this it is which controls this force, seen 

 in blushing, pallor, etc. 



What is arterial tension? 



The walls of the arteries being very elastic, and the blood being 

 forced into them at considerable pressure, permits them to keep the 

 blood under elastic compression within the arteries, so that when 

 an artery is cut the blood spurts from it in a jet. The capillaries, 

 though collectively of much greater area than the arteries, by rea- 

 son of the friction they offer to the blood-stream maintain a less 

 degree of tension. 



^ What causes modify the arterial tension ? 



(1) The rate of the heart-beats, by keeping the arteries fuller or 

 less full, will modify the blood-pressure in the arteries. 



(2) Vaso-motor changes, by increasing or decreasing the friction 

 offered the arterial blood, vary the tension in the arteries. 



(3) The amount of blood in the system must to a great extent 

 determine the limits of arterial pressure. In great exsanguination 

 the arterial pressure is quite low. 



(4) Motion of the thoracic walls in breathing necessarily changes 

 the arterial tension by the pumping force exerted by this motion. 



RESPIRATION. 

 What constitutes respiration? 



By respiration we mean the process by which oxygen is intro- 

 duced to the system and by which carbonic oxide is excreted. 



