496 



ORGAN SYSTEMS OF MAN 



50 days 



Fig. 19-6. The phylogenetic development of the verte- 

 brate heart from the shark through the reptile is 

 shovt^n on the left. This is recapitulated in the embryol- 

 ogy of the human heart shown on the right. 



The walls of the arteries need to be 

 stronger than those of the veins because the 

 blood is under considerable pressure when 

 it leaves the heart and this pressure alter- 

 nately rises and falls with each contraction 

 of the ventricles. Since the capillaries need 

 a continuous flow there must be some 

 means of absorbing these high and low 

 pressure levels, and this is taken care of 

 by the elastic arterial walls. When the pres- 

 sure rises suddenly, as it normally does, the 

 walls stretch, absorbing the pressure. Be- 

 tween beats, then, the resilience of the walls 

 forces the blood through the capillaries at 

 an even flow. If this were not true the blood 

 would gush through at high pressure and 

 stop altogether between beats when the 

 heart is at rest. Indeed, something approxi- 

 mating this happens in advancing years 

 when the walls of the vessels grow hard due 

 in part to deposition of salts. When this oc- 

 curs, the pressure of the blood rises so high 

 that the brittle vessels are apt to burst and 

 cause damage by cutting off the blood 

 source to vital organs. 



THE HEART 



The pumping mechanism of animals has 

 had a long history of evolution from the 

 simple pulsating tubes of the annelid to 

 the highly efficient organ of tlie vertebrate. 

 Much of this history has been portrayed in 

 earlier chapters, and with respect to ver- 

 tebrates can be summed up briefly by com- 

 paring the hearts of several adult forms 

 (Fig. 19-6). The history is correlated with 

 the transition from aquatic to land life. The 

 primitive heart of the shark, as well as the 

 more elaborate one of the salmon, are two- 

 chambered pumps that function satisfac- 

 torily where gills are the breathing organs. 

 With the advent of lungs, a second heart 

 was formed over millions of years of evolu- 

 tion. Today we see how this probably hap- 

 pened in the adult forms of the frog, lizard, 

 and crocodile. The story is also nicely re- 



