CHAPTER VI 



CIRCULATION OF THE BLOOD 



IF the blood were to stand still in any particular vascular region, it would 

 become impoverished in nutrient substances, especially oxygen, would become 

 overladen with products of tissue activity, and so would be rendered unfit 

 to fulfill its physiological purposes. But by the fact that the blood is con- 

 tinually in motion, this is prevented, for as it moves it both replenishes its 

 store of nutrient substances taken from different parts of the body and gets 

 rid of the products which are useless or harmful to the body. 



This continual movement is maintained by the activity of the heart. The 

 heart represents the motive power which drives the blood through the vessels. 

 The latter however are not mere passive tubes, but in various ways they 

 actively participate in the distribution of the blood throughout the body. 



FIRST SECTION 



GENERAL SURVEY OF THE BLOOD'S MOVEMENTS 



The heart of warm-blooded animals is divided by means of a septum 

 running from above downward, into two completely separated halves, a right 

 and a left (Vesalius, 1542). Each half consists of two communicating cham- 

 bers, an upper, the auricle., and a lower, the ventricle. The opening between 

 auricle and ventricle can be closed in both halves of the heart by means of 

 valves. 



Blood vessels connect with both auricles and ventricles. In those leading 

 from the ventricles blood flows from the heart, and they are called arteries. 

 In the vessels communicating with the auricles blood is conveyed to the heart, 

 and they are called veins. 



The arteries communicate with the veins by means of the capillaries, so 

 that the heart and the vessels form a connected system of tubes entirely shut 

 off from the outside. 



In this system, as was first established by Harvey (1628), the blood moves 

 in the following manner (Fig. 50). It is poured by the two venae cavae into 

 the right auricle, and is driven by the latter into the right ventricle. By the 

 contraction of the ventricle it is pressed out into the pulmonary arteries pro- 

 ceeding therefrom, and flows through the vessels of the lungs and the pul- 

 monary veins into the left auricle. This part of the circulation is called the 

 lesser circulation, and was first described by Servet (1553) and Colombo 

 (1559). From the left auricle the blood is driven into the left ventricle and 



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