CHAPTER XVIII. 

 THE CIRCULATORY SYSTEM. 



Introduction. The circulatory system provides for the trans- 

 portation of blood through the tissues, thus enabling each indi- 

 vidual cell to obtain nourishment and to rid itself of the waste 

 products of its activity. The system includes the heart, the 

 blood vessels, and the lymphatics. 



From a mechanical standpoint, we may say that the heart con- 

 sists of a pair of pumps ; each pump consisting of two parts, an 

 upper chamber, the auricle, and the lower one, the ventricle. 

 Thin, membranous valves, called auriculo-ventricular, separate 

 the upper and lower chambers and prevent the blood from flow- 

 ing back into the auricle when the ventricle contracts. Connect- 

 ed with the ventricles are the arteries, which conduct the blood 

 away from the heart, to which it is returned by the great veins 

 leading into the auricles. At the point where the arteries emerge 

 from the heart are cup-shaped valves, called semilunar, which 

 prevent the passage of blood from the arteries into the ventricles, 

 while the latter are relaxing. 



As will be seen from the accompanying diagram (Fig. 14) the 

 blood pumped from the two sides of the heart circulates through 

 two distinct and separate systems of blood vessels. From the 

 right ventricle the blood goes through the pulmonary artery to 

 the lungs and is returned to the left auricle by the pulmonary 

 veins, then to the left ventricle, whence it is sent over the body 

 through the aorta and its branches, to the capillaries imbedded 

 in the tissues. From these it is returned through the veins to the 

 venae cavae, which discharge it into the right auricle. We may 

 say, therefore, that the circulatory system consists of two circles 

 of tubing interposed in which are two force pumps, the valves 

 of which are so disposed as to allow the blood to flow in one direc- 

 tion only. 



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