SECTION X 



THE MECHANICS OF TH& CIRCULATION. 



HEMODYNAMICS 



CHAPTER XXX 

 PHYSICAL CONSIDERATION 



The Sources of Pressure. — If considered from the kinetic or dy- 

 namic standpoint, the movements of fluids may be said to be dependent 

 upon the force of pressure, which in turn is derived from three sources, 

 namely from : 



1. An outside factor (hydraulic pressure). 



2. Imparted motion (hydrodynamic pressure). 



3. The weight of the fluid (hydrostatic pressure). 



In a similar manner it may be stated that the flow of the blood finds its 

 cause in the pressure to which it is subjected while traversing the 

 vascular channels. This force, as has just been emphasized, must be 

 regarded as the product of three factors, although it cannot be doubted 

 that in this case the dynamical action of the heart is the most important 

 of the three. 



Hydraulic influences are brought to bear upon a fluid from without. A con- 

 dition of this kind may be produced either by permitting oil or mercury to press 

 upon water or by subjecting the fluid contents of a syringe or of a hydraulic pump 

 to pressure by means of a piston. In all these cases, the fluid must be confined in 

 a closed receptacle, or must be kept under such conditions that its chances of 

 escaping to the outside are so slight that a general displacement of it cannot result. 

 The vascular system fulfills these mechanical requirements very efficiently, because 

 its channels are closed and are sufficiently elastic to yield to pressure. The degree 

 of their distention, however, is not sufficiently great to neutralize the pressure. 

 In this case, the heart takes the place of the piston and the capillary bloodbed, that 

 of the narrow outlet. Hydrodynamic influences are brought into play in so far 

 as every moving fluid is in possession of a certain kinetic energy which tends to 

 drive it onward, even at a time when the external force has ceased to act upon it. 

 At this moment, one component of the fluid presses upon the one ahead of it, and 

 so on, until the end of the column has been reached. Hydrostatic influences are 

 also present, because every fluid possesses weight, and hence, its lower layers are 

 always subjected to the pressure of its overlying strata. 



In determining the degree of pressure exerted by these forces, the following 

 facts should be kept in mind. The pressure of the air resting upon the surface 

 of the earth, amounts to about 1 kg. per square cm. of area. This volume of air 

 weighs 1033 gm. This pressure which is designated as one atmosphere, may be 



347 



