54 



THE VITAL PROCESSES 



small arteries, becomes comparatively slight in the capillaries, and falls 

 practically to nothing in the veins. Near the heart in the superior and 

 inferior vena cavae, the pressure at intervals is said to be negative. 

 This means that the blood from these veins is actually drawn into the 

 right auricle by the expansion of the chest walls in breathing. 1 



The velocity of the blood is greatest in the arteries, less in the veins, 

 and much less in the capillaries than in either the arteries or the veins. 

 The slower flow of the blood through the capillaries is accounted for by 

 the fact that their united area is many times greater than that of the 

 arteries which supply, or the veins which relieve, them. This allows 

 the same quantity of blood, flowing through them in a given time, a 

 wider channel and causes it to move more slowly. The time required 

 lor a complete circulation is less than one minute. 



Summary of Causes of Circulation. The chief factor 

 in the circulation of the blood is, of course, the heart. The 

 ventricles keep a pressure on the blood which is sufficient 

 to force it through all the blood tubes and back to the 

 auricles. The heart is aided in its work by the elasticity 

 of the arteries, which keeps the blood under pressure while 

 the ventricles are in a state of relaxation. It is also aided 

 by the muscles and elastic tissue in all of the blood vessels. 

 These, by keeping the blood vessels in a state of " tone," or 

 so contracted that their capacity just equals the volume of 

 the blood, enable pressure from the heart to be transmitted 

 to all parts of the blood stream. A further aid to the circu- 

 lation is found in the valves in the veins, which enable mus- 

 cular contraction within the body, and variable pressure 

 upon its surface, to drive the blood toward the heart. The 

 heart is also aided to some extent by the movements of the 

 chest walls in breathing. The organs of circulation are 

 under the control of the nervous system (Chapter XVIII). 



about one third as great. In maintaining this pressure the heart does a work equal 

 to about one two-hundredth of a horse power. 



1 The location of the heart in the thoracic cavity causes movements of the chest 

 walls to draw blood into the right auricle for the same reason that they " draw " air 

 into the lungs. 



