94 



THE INDIVIDUAL ORGANISM 



stream. The capillary resistance to flow is so great that nearly all the 

 pressure created by the beating of the heart is utilized in overcoming it; 

 only a small residue of this pressure remains when the blood flows from 

 the capillaries into the veins. The return of the blood to the heart is 

 caused in part by this residual pressure but is greatly aided by other 

 factors. We have already noted that the veins, like the lymphatics, are 

 well provided with valves opening in the direction of the heart. Muscular 



blood capillaries^- — ^A lymph capillaries 



Fig. 6.6. Diagram of the relationship between the lymphatic and blood circulatory systems. 

 (Adapted from Carlson and Johnson, The Machinery of the Body, by permission University of 

 Chicago Press.) 



movements of the body, including movements of the viscera and those 

 of breathing, aid in the return flow by squeezing the veins and forcing the 

 blood in the direction determined by the valves. The breathing move- 

 ments also assist the process in another manner. The same movements 

 of the diaphragm and ribs that reduce the pressure within the 

 thoracic cavity, causing air to flow into the lungs, also reduce the 

 pressure in the auricles and the veins that lie within this cavity and 

 tend to "pull" blood into the auricles and adjacent portions of the 

 venae cavae. 



