954 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



Flo. 24. Effects of perfusion pressure per se (circles) and of an infusion of acetylcholine at each 

 level of perfusion pressure (triangles) on flow, mean transit time, volume, and conductance in the 

 dog's paw. Same experiment as that in fig. 23. .-1 : plot of perfusion pressure (arteriovenous difference 

 of pressure) vs. flow. [In a larger series of experiments the pressure-flow relationship in the vascular 

 bed of the paw was best represented by a curve with convexity towards the pressure axis, when plotted 

 in linear coordinates, similar to that observed in the skin (fig. 3)]. B: plot of perfusion pressure vs. 

 mean transit time. C: plot of perfusion pressure vs. vascular volume (computed from product of flow 

 X mean transit time). D : plot of conductance vs. vascular volume. Figures in D represent the per- 

 fusion pressures used for each pair of determinations. 



infused intra-arterially in this experiment had similar 

 effects at all levels of perfusion pressure. This agent 

 increased flow and, thereby, lowered the amplitude of 

 the indicator concentration curve (fig. 23). However, 

 mean transit time did not change appreciably and 

 therefore vascular volume and conductance were both 

 increased, the latter proportionately more than the 

 former (fig. 24). The straight-line relationship between 

 flow and pressure may indicate a considerable 

 though not maximal dilation of the arterioles (p. 



937)- 



When using indicators it is necessary that a stable 



state exist since, if the volume is changing during the 



determination, a false reading will be obtained. When 



this occurs, the flow computed by the indicator 



method may differ significantly from that recorded 



simultaneously by a flowmeter. 



ESTIMATION OF CHANGE OF VASCULAR VOLUME 

 DUE TO EXTRINSIC INFLUENCES 



Changes in vascular volume in an organ may be 

 measured by recording continuously the organ's 

 weight or its volume with a plethysmograph. These 

 methods give the relative change of vascular volume 

 but not the actual volume of blood in the bed at any 

 moment. Changes in vascular volume measured by 

 these methods may be used to confirm changes in 

 volume obtained with the indicator method. Com- 

 parison of change of vascular volume with simul- 

 taneously recorded blood flow gives an indication of 

 the interplay of resistance and capacitance vessels 

 (31,80). 



Different vascular beds vary significantly in their 

 response to factors causing a change of vascular 



