1 148 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



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fig. 5. A: changes in renal vein flow, F Ry , and venal artery pressure, P RA , with perfusate composi- 

 tion changes. T RV = temperature of renal vein outflow. [From Harvey (104).] B: effect of hypertonic 

 saline solution on blood flow through femoral artery measured with ultrasonic flowmeter. Note the 

 zero calibration check at the beginning of the record and again during the period of increase in How. 

 The fall in flow rate at time of injection is an artifact. [From Marshall & Shepherd (148).] 



is monitored by means of fine cannulae passed into 

 the vascular tree at several points distal to the pump 

 so that a detailed description of the resistance of the 

 various segments of the circulation can be compiled. 

 Haddv (101), too, found that amounts of NaCl, 

 insufficient to alter systemic pressure, produced 

 arteriolar dilatation in the dog forelimb. 



Unfortunately, it is not possible to elevate plasma 

 Na concentration in the perfusing blood without at 

 the same time raising its tonicity. Marshall & 

 Shepherd (148) first noted this as an experimental 

 defect and found that they could obtain a similar 

 degree of vasodilatation by infusing dextrose and 

 urea matched for tonicity to their sodium salts and 

 concluded that the mechanism of vascular relaxation 

 was uncertain. Muirhead et al. (154) had earlier 

 noted this effect of hypertonic infusions. This point 

 led Overbeck & Haddy (156) to restudy the problem. 

 Thev found that hypertonic solutions of NaCl, 

 Na->S0 4 and Na>HP0 4 , which produced the same 

 final serum Na concentration, evoked decreases in 

 limb vascular resistance in parallel with their actual 

 tonicity. Equally hypertonic infusions of Na>S0 4 , 

 and NaCl, irrespective of the amount of Na supplied, 

 evoked equal decreases in small vessel resistance. 

 They concluded that the addition of Na apparently 

 had little or no independent effect apart from that of 

 its tonicity. On the other hand, these same workers 

 (102) have obtained some evidence that a reduction 



of Na in the perfusing medium is slightly vasocon- 

 strictive and decreases the caliber of the small vessels. 

 Harvey (104) and Read et al. (164) have studied the 

 effects of hypertonic solutions on the renal vascular 

 bed and have arrived at the same conclusion regarding 

 the relation of hypertonicity of the infusion and its 

 vasodilators- effect. 



These findings are in substantial agreement with 

 those obtained with vascular and other smooth 

 muscle strips studied in vitro. It will be recalled that 

 following exposure to high Na„ such tissues may show 

 a temporary increase in tension, but very high levels 

 are required for this. On the other hand, after equili- 

 bration in such high Na media, tension is usually lower 

 than normal and responsiveness of these tissues is 

 reduced, and this obtains in vivo as well. Thus, 

 following perfusion of the rat with Na salts we (86) 

 found the blood pressure responses to norepinephrine 

 and to Pitressin sharply reduced. Haddy (ioi), in 

 more precise studies, has shown this to be due to a 

 reduction in the responsiveness of peripheral vessels 

 to both pressor and depressor agents. Since this effect 

 persists, it max not be directly related to the osmotic 

 effect of the hypertonic solution. 



It should be emphasized that an osmotic effect 

 produced by infusing a hypertonic solution cannot, 

 in fact, be dissociated from an ionic effect. The 

 withdrawal of water from cells causes a proportionate 



