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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



arterial perfusion pressure was decreased and evidence 

 was presented that this reaction depends upon sympa- 

 thetic innervation of the postcapillary blood vessels 

 (142a). 



One more series of studies must be mentioned 

 briefly because the possible role of the arteriovenous 

 anastomoses on small vein pressures has not been 

 mentioned so far. These relatively large vessels run 

 parallel to the capillaries and are numerous in the 

 skin, particularly of the digits. Schroeder (324), 

 using a pressure plethysmograph similar to that used 

 in man by McLennan et al. (234), studied the effects 

 of acetylcholine, epinephrine, histamine (325), 

 hypoxia (327), calcium, and rutin (326) on vascular 

 volumes and pressures in the dog's foreleg. External 

 pressure was set arbitrarily at 35 mm Hg to measure 

 "changes of capillary pressure" and at 15 mm Hg 

 to measure "changes of venous pressure." No abso- 

 lute pressure readings could be obtained by this 

 method. The curves were variable and often difficult 

 to interpret. However, Schroeder placed considerable 

 emphasis upon independent reactions of the arterio- 

 venous anastomoses and their secondary effects upon 

 venous and capillary pressures. It is conceivable that 

 some of the variability in the observed small vein 

 pressures in the skin of the extremities may be reduced, 

 or at least explained in part, if body temperature and 

 environmental temperatures are adjusted to maintain 

 the arteriovenous anastomoses in as constant a state 

 as possible. In any case it is clear that pressures and 

 resistances in large veins and in small veins, together 

 with any factors which modify them, must be taken 

 into account when describing the mechanisms which 

 determine changes of capillary blood pressure. 



3. OSMOTIC PRESSURE OF THE PLASMA PROTEINS, Upl 



A . Methods of .Measurement 



Starling's conception of a balance between capillary- 

 hydrostatic pressure and protein osmotic pressure 

 was supported by actual measurement of the pressure 

 required to maintain fluid balance across a semi- 

 permeable membrane separating blood serum from 

 serum ultrafiltrate. Starling's osmometer consisted 

 of a small glass bell, provided at the top with two side 

 arms. A piece of peritoneal membrane, soaked in 

 10 per cent gelatin, was tied over the mouth of the 

 bell and prevented from bulging by a perforated 

 silver plate. One sidearm was connected to a vertical 

 tube and the other side arm was used to introduce 

 serum into the bell. The lower end of the bell, in- 



cluding the membrane, was then dipped into serum 

 ultrafiltrate or other protein-free salt solution. Within 

 a few hours osmotic flow of fluid from the salt solu- 

 tion through the membrane was made evident by a 

 rise of fluid in the vertical tube. Equilibrium was 

 established in 2 to 6 days; at this time the pressure on 

 the membrane, exerted by the fluid column in the 

 vertical tube, was considered equal and opposite to 

 the osmotic pressure of the serum proteins. In a typical 

 measurement Starling (345) found that serum con- 

 taining 7.56 per cent "proteids" caused fluid to rise 

 in the vertical tube to a height of 53 cm (~4i mm 

 Hg). This value is considerably higher than modern 

 estimates shown in figure 3.1, probably as a result of 

 bacterial degradation of protein during the long 

 period required to reach equilibrium. In later work 

 (1899) Starling (347) obtained values which were 

 generally lower than his first estimate and well 

 within the range expected for capillary hydrostatic 

 pressures. 



Starling's measurements were of great interest to 

 colloid chemists as well as to physiologists. Accord- 

 ing to van't Hoff's analogy, in 1887, between ideal 

 solutions and gases (157), the osmotic pressure, II, 

 should be given by 



IT 'CRT ( 30 



where c is expressed in moles per liter. 



fig. 3.1. Osmotic pressure-concentration curves for whole 

 plasma and selected plasma proteins. Based on data from 

 references (268, 270, 312, 313, 343), original measurements 

 corrected to 37 G. Experimental points for 7-globulin are 

 included to indicate magnitude of experimental error. 



