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TUP INTPRNAL PNVIRONMPNT OF THE BODY 



Part III 



whip of the heart, the inhibitor (parasympathetic) curbing its speed, the 

 accelerator (sympathetic) whipping it up. 



Blood rrcssiire. Rvcry aspect of blood pressure depends upon the pumping 

 of the heart that works against the friction of the blood vessels, and gravity. 

 The nearer the blood is to the pump, the greater the pressure upon it. Farther 

 away from the pump with more and smaller vessels, the friction increases and 



AV node 



Pace-mahc 



A-V bundle 



Fig. 12.19. The neuromuscular mechanism that establishes the rhythm of the 

 heartbeat. Diagram of a frontal section of the heart. During each beat a wave of 

 contraction begins in the peculiar muscle fibers in the pace-maker (sino-auricular 

 node) and spreads through them in the walls of the auricle (arrows); another 

 wave of contraction begins at the auriculo-ventricular node (A-V), spreads through 

 the A-V bundle of muscle and on throughout the walls of the ventricles. If 

 either of the nodes is damaged, the auricles continue to beat normally but the 

 ventricles stop beating or beat irregularly. (Courtesy, Gerard: The Body Func- 

 tions. New York, J. Wiley and Sons, 1941.) 



the energy of pressure is expended in overcoming it. In the capillaries the 

 friction is enormous and the drop in pressure correspondingly great. In the 

 veins no pressure is regained until just before the blood enters the heart. 



Pressure in the Vessels, Arteries. In the arteries blood travels by 

 spurts since the pressure upon it increases each time the heart contracts 

 (systolic pressure) and decreases each time it relaxes (diastolic pressure). 

 The pulse is an expression of the uneven pressure upon blood in the arteries 

 (Fig. 12.20). It is a wave of the muscular contractions that begins in the left 

 ventricle and spreads throughout the arteries. The contraction of the left 

 ventricle sends a spurt of blood into the aorta that swells out the walls. The 

 spurt of blood is immediately squeezed forward by the rings of muscle behind 

 it and by the action of the elastic tissue in the arteries. By this time the 

 ventricle has contracted again and another lump has started along the aorta. 

 Thousands of these little lumps are constantly moving in processions over 

 the arteries. The rate at which they move past a certain spot is the pulse, 



