PHYSICAL EQUILIBRIA OF HEART AND VESSELS 



9' 



This arbitrary division, by these definitions, does not 

 imply that vascular smooth muscle does not possess 

 elasticity, nor that the force of contraction of smooth 

 muscle may not alter with the degree of stretch of 

 the muscle. There is no doubt tiiat such a dependence 

 on stretch does exist, though the elastic force due to 

 stretch is, from the scanty evidence available, much 

 less than that found in the other elastic elements 

 of the wall, namely, the elastin and the collagen 

 fibers. However, any part of the smooth muscle 

 tension that depends on stretch may, for convenience, 

 i)e included in the clastic tension, T^, leaving the 

 rest, Ta, completely independent of stretch. This 

 arbitrary definition of the symbols used effects a 

 great simplification, without loss of \alidity. 



TABLE I 



8. TOTAL TENSION IN THE WALL OF DIFFERENT VESSELS 



Without separating the total tension into the 

 two components, elastic tension (dependent on 

 stretch) and active tension (dependent on vasomotor 

 tone, not dependent on stretch), we may study the 

 value of the total tension in vessels of different cate- 

 gories by applying the law of Laplace (5). In table 

 I the approximate radii of the categories of vessel, 

 from aorta to vena cava, are those given in any 

 textbook of anatomy or histology; whereas the 

 values of pressure are those accepted for the mean 

 pressure through the vascular system, after the 

 graphs given in any elementary textbook of circula- 

 tory physiology. These are converted from mm Hg 

 to the fundamental units of dynes per square centi- 

 meter, and merely multiplied by the appropriate 

 radius to give the total tension in dynes per centi- 

 meter (i mg = 0.98 dynes). 



The table shows the enormous range of tension 

 in the wall required for equilibrium with the blood 

 pressure, from i 70,000 dynes per cm in the aorta to 

 16 dynes per cm in the capillary. The dominant 

 factor in determining this is the size of the vessels, 

 so that although the pressure decreases from capil- 

 laries to vena cava, the tension very greatly in- 

 creases. Two rather puzzling problems are elucidated. 



First, it has been difficult to see how such a fragile, 

 thin-walled, relatively unsupported structure as the 

 capillary could withstand the pressure within it. 

 Normal capillaries do not burst even under trans- 

 mural pressures of several hundred millimeters of 

 mercury, though impaired capillaries in hemor- 

 rhagic diseases may "burst" at relatively low pres- 

 sures. The law of Laplace shows that because of 



the very small radius of the capillary, very little 

 tension is required. A simple test shows that a single 

 layer of the thinnest tissue paper (Kleenex) in a 

 strip I cm wide will support 50 g (about 50,000 

 dynes/cm) before breaking. This is 3000 tiines as 

 much as a capillary wall needs to withstand, at 

 normal capillary pressure. 



The .second problem is why elastic tissue, i.e., 

 elastin and collagen fibers, which are so prominent a 

 feature of the larger arteries, is almost absent in 

 the capillaries and venules but reappears in the 

 veins (elastic tissue reappears in vessels more than 

 200 microns in diameter) in spite of the fact that 

 the blood pressure is less. Again the size factor ex- 

 plains this. There is a very good correlation of the 

 amount of elastic tissue with the tension in the wall 

 given in table i . 



The tension required to withstand the prevailing 

 blood pressure might be called the "maintenance 

 tension." If this is produced by the elasticity of the 

 wall, rather than by vasomotor tone, it will be much 

 more efficient, in that the maintenance of elastic 

 tension, due to stretch, does not involve any con- 

 tinuous expenditure of energy. 



Whereas the energy expenditure of smooth muscles, 

 particularly of \ascular smooth muscle, has not yet 

 been adequately measured, it seems probable on 

 general physiological principles that the maintenance 

 of active tension by vasomotor tone requires con- 

 tinuous energy expenditure. The provision of main- 

 tenance tension by elasticity, reserving the activity 

 of smooth muscle in the wall for accomplishing 

 changes in the total tension, and thus of the size of 

 the vessels, would appear teleologically to offer 

 optimal efficiency. 



