938 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



200 



O ■ PRUf VS C 



• ■ PRUp vs c 



A . PRU F vs n 



A. . PRUp vs n 



x - c vs n 



i i i i i i 

 6 8 I 



i i i i ii i 

 4 6 8 1 



4 6 8 1 



i I i 

 4 6 



XIO" 



X 10" 



X 10" 



X 10" 



fig. 4. Log-log plots of the interrelations of the parameters from fig. 3. Open circles — plot of the 

 relationship of the resistance at a How of 5 ml/min to the constant c; solid circles — plots of the rela- 

 tionship of the resistance at a perfusion of 100 mm Hg to the constant c; open triangles — plots of the 

 resistance at a flow of 5 ml/min to the exponent n; solid triangles — plots of the relationship of the re- 

 sistance at a pressure of 100 mm Hg to the exponent n; X — plot of the relationship of the constant c 

 to the exponent «; points labeled A, B, and C refer to curves A, B, and C, respectively, in tig. 3. Ordi- 

 nate scale applies to resistance at a flow of 5 ml/min (PRUf), to resistance at a perfusion pressure of 

 100 mm Hg (PRUp), and to n. Abscissal scales apply to n (left graph) and to the constant c (right 

 graph). See Table 1 for further identification of symbols. 



in the above experiments (46) was also approximately 

 linear in a log-log plot (fig. 4). 



There are at least two possible explanations for a 

 value of n greater than 1 . a) The apparent viscosity 

 of whole blood decreases as the pressure difference 

 across a length of rigid capillary tubing 0.3 mm or 

 less in diameter is increased, i.e., as flow increases 

 (40) ; this is due probably to the red cells being 

 clumped progressively closer together as a solid core 

 in the middle of the tube as the rate of flow increases, 

 leaving a sleeve of plasma adjacent to the intima of 

 the blood vessel and thus reducing viscous drag, b) 

 Other factors being constant, flow through a conduit 

 is proportional to the fourth power of the diameter. 

 If, with increasing internal pressure, a slight but 

 progressive increase in the diameter of the resistance 

 vessels occurs, then flow through vascular beds con- 

 taining such distensible structure will increase in 

 proportion to some power of P greater than 1 [Green 

 el al. (46); Folkow (27-29, 32)]. From the data in 

 figure 4 it appears that these effects become aug- 

 mented with increase in "tone" of the resistance 

 vessels. Computations, based on data compiled by 

 Burton (7), indicate that an increase of internal 



pressure from o to 102 mm Hg in an arteriole might 

 increase the cross-sectional area sufficiently that the 

 relative conductance would be 146 per cent of that 

 at zero pressure, i.e., at the unstretched diameter 

 (fig. 5). However, Baez & Lamport (2) report that 

 arterioles of 34 to 42 /u diameter showed essentially no 

 change in diameter under considerable pressure 

 variation. They did note selective closing of pre- 

 capillary sphincters at positive pressures. 



The relationship of resistance to flow, PRU 

 (peripheral resistance unit = P/F = mm Hg (ml/ 

 min)), to either flow or A-V pressure difference was 

 also a power function in the above studies; for each 

 level of tone the resistance to flow varied inversely 

 with either flow or perfusion pressure (table 1). 

 Since both the constant and the exponent varied as 

 vasomotor tone changed, the relationship of resist- 

 ance at one level of tone to that at another (B A; 

 C/A in table 1 ) was also a power function of either 

 flow or pressure; this ratio, which was inversely re- 

 lated to pressure and flow, can be expressed as a 

 number if the same pressure (i.e., 100 mm Hg = 

 PRU Pl00 ) or flow (i.e., 5 ml/min = PRU F .) is used 

 for each curve (table 1). 



