284 PHYSIOLOGY. 



CIRCULATION IN THE BLOOD-VESSELS. 



This field of physiology presents problems of a physical nature,, 

 in that the flow of the liquid, blood, is through tubes. But it must 

 be remembered that the tubes employed in the circulation are living, 

 more or less elastic ones, and that physical laws are correspondingly 

 altered. 



The analogy between the nervous system and the telegraphic 

 system is a very striking one, and is much used by physiologists and 

 others. Even more forceful is the analogy between the circulatory 

 system and the system of water-supply of a town or city, except that 

 there is no return of the latter's fluid to the starting-place. The 

 water starts upon its flow from the elevated reservoir to pass through 

 large mains at first and is distributed through branches that become 

 smaller and smaller as they subdivide on their way to different 

 houses. Likewise, the blood starts from the centrally located, pump- 

 ing heart, passes through large trunks at first, to be distributed 

 through branches that become smaller and smaller as they subdivide 

 on their way to different tissues. In short, the physical laws of the 

 circulation are the modified physical laws of the flow of liquids 

 through tubes. From this it will be readily deduced that a com- 

 petent knowledge of the laws of circulation must be preceded by some 

 knowledge of physical laws. These will be referred to from time to 

 time in the treatment of the present subject: 



The flow of liquid is caused by a difference of pressure between 

 the different parts of a body of liquid. The attraction of the earth 

 (gravitation) provides a continuous pressure which will produce a 

 flow of liquid along channels or through tubes, provided the source 

 be elevated and the outlet low. 



The circulation through the heart-vessels is also caused by a 

 difference in pressure due to the primary propelling force of the 

 heart-action. That is, the pressure in it exceeds that of the arteries; 

 the latter's pressure, kept high by the heart's force and peripheral 

 resistance, is greater than that in the capillaries. Though that 

 exerted in the capillaries is small, it is yet in excess of that existing 

 in the veins. The lowest pressure is found in the blood about to 

 enter the heart after having first made its circuit through the body- 

 tissues. The direction of the flow of any liquid is always from the 

 higher pressure toward the lower ; therefore the flow of blood within 

 the body is from the heart around through the body back to the heart 

 again ; that is, it circulates. 



