172 PHYSIOLOGY FOR DENTAL STUDENTS. 



The pumps or the standpipe correspond to the heart and flu- 

 large arteries, the distributing pipes to the smaller arteries and 

 capillaries. With these ideas in mind let us consider the part the 

 heart and blood vessels play in maintaining the circulation. 



The Part the Heart Plays. At each systole 60 to 90 c. c. of 

 blood are forced into the aorta. Cardiac systole lasts about 0.3 

 of a second, the diastole 0.5 second. Therefore the heart is rest- 

 ing about 60 per cent of the time. By experiment it has been 

 demonstrated that the left ventricle forces the blood out into the 

 aorta with a pressure equivalent to the weight of a column of 

 mercury from 160 to 190 mm. in height. The heart alone, how- 

 ever, actually propels the blood through the arteries for only the 

 time of its systole; during the diastole, as already explained, the 

 blood would cease to flow entirely if it were not for the part 

 which the large arteries play in the maintaining of the circula- 

 tion. 



The Part the Arteries Play. If 100 c. c. of water are forced 

 every 0.8 second into an ordinary metal pipe, in 0.3 second, 100 

 c. c. must flow out from the opposite end in the same period. For 

 0.5 second no water will be flowing in the tube. Let us now re- 

 place the metal tube with an elastic rubber tube, the end of which 

 is fitted with a nozzle filled with glass beads. If now 100 c. c. of 

 water are forced into the tube in 0.3 second, the rubber tube ex- 

 pands because the beads retard the free outflow of water and thus 

 make it impossible for 100 c. c. of water to pass through them in 

 the time alloted. After the water ceases to flow into the tube, the 

 water stored up in the expanded portion continues to flow out 

 through the beads because of the elastic recoil of the rubber. If 

 the resistance offered to the water and the expansile force of the 

 tube be properly adjusted, a constant stream of water may be 

 obtained from the outlet, in spite of the fact that an intermittent 

 force is supplying the water (Fig. 20). 



The intermittent stream of the arteries is changed into the 

 constant stream in the veins by a somewhat similar process. The 

 walls of the arteries are composed in part of a layer of strong 

 elastic tissue, and this expands to a greater or less degree at each 

 heart beat. The resistance which the arteries and the capillaries 



