PHYSICAL EQUILIBRIA OF HEART AND VESSELS 



101 



seems to be 5 to lo mm Hg (20). The active tension 

 causing this residual CCP is evidently physico- 

 chemical in nature, an "interfacial tension'" be- 

 tween blood or perfusion solutions and the blood 

 vessel wall. The evidence for this v'iew is that only 

 by the use of surface-tension lowering agents, e.g., 

 bile salts, Tween 80, can this residual CCP be re- 

 moved. An interesting confirmation of this physico- 

 chemical factor in the cerebrospinal fluid-vascular 

 system has been found by Welch & Friedman (27), 

 who studied the flow-pressure relations through the 

 "valves" between the cerebrospinal fluid (CSF) 

 and blood systems. There was no flow if the driving 

 pressure went below 5 cm H2O (4 mm Hg), unless 

 Tween 80 was added, after which there was flow 

 down to zero pressure. Yamada (30) found the CCP 

 in the hind limbs of rats reached a minimum value 

 of 10 to 15 mm Hg, but this was reduced by adding 

 bile salts to the perfusate. The force causing closure 

 in these cases would be like that (surface tension) 

 which pulls together two cover slips with water 

 between them, or that which holds a cylinder of 

 water, flowing from a tap, from scattering. There is 

 a good deal of suggestive evidence from other sources 

 that an interfacial tension exists between blood and 

 the endothelium of normal blood vessels. For ex- 

 ample, the angle of contact of the meniscus of a 

 bubble of air in a living vein is quite high, and de- 

 creases as the vessel wall deteriorates. Also there is a 

 correlation between clotting time of blood and degree 

 of wettability of surfaces with which it is in contact 

 (17), yet clotting in the live vessel does not take place 

 for many hours, even with stasis. Many other refer- 

 ences to the role of interfacial tension are given by 

 Nichol (18). A very small degree of "unwettability," 

 a very low interfacial tension, would explain the 

 residual CCP. When the arterioles lack vasomotor 

 tone, the critical vessels are most likely to ije the 

 capillaries, because they have the smallest radius. 

 Using equation 22 with a radius ro = 5 X io~^ cm 

 (5 microns), a residual CCP of 10 mm Hg would 

 correspond to an interfacial tension of 5 or 6 dynes 

 per cm. The surface tension of plasma vs. air is 

 about 70 dynes per cm. 



22. PHYSIOLOGICAL RANGE OF CRITICAL 

 CLOSING PRESSURES 



While the minimum CCP is the above residual 

 CCP of 5 to 10 mm Hg, the \alues found for the 

 various vascular beds studied under electrical stimu- 



lation of the sympathetic vasoconstrictor nerves 

 range from a minimum of about 10 mm Hg to a 

 maximum value, at a frequency of stimuli of 20 per 

 sec, of about 60 mm Hg. In the human forearm (4) 

 the values range from 15 mm Hg, when the subject 

 is warm, and has a vasodilation, to 60 mm Hg when 

 cold and in intense vasoconstriction. The values for 

 the finger have a very similar range (29). In patients 

 with essential hypertension (30) the values are much 

 higher, up to 95 mm Hg. Similarly, the values in 

 the hind limbs of normotensive rats ranged from 10 

 to 40 mm Hg, but in rats made hypertensive by 

 Compound F the range was from 25 to 55 mm Hg 

 (30). In the case of the rats and the hypertensive 

 patients there was a very good correlation (coeflicient 

 greater than 0.9) between the CCP in standard 

 conditions, and the level of sustained hypertension. 

 In conditions of disturbed physiology, as with 

 secretion of adrenaline into the blood stream, or 

 very violent sympathetic discharge, there is no doubt 

 that CCP can reach very high values, leading to 

 complete shutting off of vascular beds from the 

 circulation. 



23. APPLICATION OF THE LAW OF LAPLACE 

 TO THE HEART 



The application of the law of Laplace to the heart 

 was made by Woods (28) before the turn of the cen- 

 tury. He studied the radii of curvature and the 

 thickness of the ventricular wall in autopsy hearts, 

 fixed in alcohol. The measurement of curvature was 

 made at a number of different points on the surface 

 of the heart, by sticking in a pin normal to the surface 

 and fitting arcs of circles to the surface at that point 

 (fig. 4). The results are shown in table 2. 



Woods argued that the tension in the wall was 

 likely to be proportional to the thickness, /. If, then. 



^^"(i + i) 



and 



T = kt, then P 





(23) 



and t(i/Ri -\- i/R-i) should be a constant, for the 

 left ventricle where P, in life, was about 1 20 mm Hg, 

 and a different constant, about one-fifth of the value, 

 for the right ventricle where P was about 25 mm. The 

 table shows that his a.ssumption was correct, for the 

 figures in the last column are remarkably constant, 

 in view of the wide range of values of the radii of 

 curvature. 



