RESISTANCE AND CAPACITANCE PHENOMENA IN VASCULAR BEDS 



955 



volume. In response to intra-arterial epinephrine the 

 vascular volume in the dog's paw tends to decrease as 

 the resistance to flow increases, i.e., as flow decreases. 



During constant-flow perfusion of the isolated dog's 

 hind quarters intra-arterial perfusion of levarterenol 

 caused a) a marked rise in the perfusion artery 

 pressure (increased resistance to inflow through the 

 arterioles); b) a decreased venous outflow; and c) a 

 rise in limb weight (due to distention of the arteries 

 proximal to the site of constriction). When the rising 

 arterial pressure overcame the arteriolar resistance 

 outflow increased above inflow and limb weight fell 

 despite persistence of the elevated arterial pressure. 

 The authors concluded that the latter effect was due 

 to translocation of blood by constriction of the veins. 

 Because of the small changes of T-1824 and P 32 tag 

 concentrations, they concluded also that only a very 

 small percentage of the leg weight change was due 

 to transcapillary movement of either plasma or 

 interstitial water (104). 



Reductions in flow and volume were noted in cat's 

 hind legs in response to levarterenol; this finding was 

 interpreted as being due to constriction of both the 

 "resistance" and the "capacitance section" of the 

 vascular bed (31, 80). In this preparation intra- 

 arterial epinephrine caused an initial increase in out- 

 flow simultaneously with a decrease in volume, 

 whereas isoproterenol and acetylcholine increased 

 both flow and volume. Since levarterenol seemed to 

 be more potent than angiotensin in reducing the 

 volume of blood contained in the capacitance section 

 of the hind leg (muscle and skin) of cats, Folkow 

 et al. (33) claimed that a greater constrictor effect on 



the capacitance vessel (larger veins) is exerted by 

 levarterenol than by angiotensin. As we visualize it, 

 the data should be interpreted oppositely, i.e., that 

 angiotensin has a more potent effect than does 

 levarterenol on the postcapillary, or a less potent 

 effect on the precapillary vessels (see below). 



The vascular volume of the spleen decreased more 

 rapidly and abruptly than the inflow in response to 

 intra-arterial epinephrine, levarterenol, or sympa- 

 thetic nerve stimulation. After a period of time the 

 inflow began to increase rapidly exceeding outflow 

 and resulting in a progressive augmentation of 

 spleen weight (47) (fig. 25). 



The usual accompaniment of an increase in vascular 

 resistance of an organ is a decrease in its volume as 

 measured plethysmographically or by weight changes. 

 This change is the result of a diminution of the volume 

 of the resistance vessels (arteriolar constriction) and 

 perhaps more importantly the lessening of the 

 distention of the postarteriolar vessels and even the 

 closure of some of the capillaries and venules (fig. 26). 

 Only rarely is an increase in resistance accompanied 

 by an increase in organ volume. When this occurs, it 

 is due probably to a preponderance in the resistance 

 increase of postcapillary or even venular vessels over 

 that occurring in the precapillary vessels, thus causing 

 an increase in upstream (capillary) pressure which 

 may lead to an augmentation of organ volume by 

 distending the capillaries (fig. 26) and by increasing 

 the filtration of fluid into extravascular spaces. The 

 kidney is the only organ in which a well-documented 

 increase in volume accompanies an increase in 

 resistance. That this is due in part to an increase in 



22 6 



196 



fig. 25. Records of arterial inflow, venous 

 outflow, and relative weight changes of the 

 dog's spleen in response to stimulation of 

 sympathetic nerve supply and of the dog's 

 paw in response to slow intra-arterial infusion 

 of 1 Mg/min of epinephrine. [Modified after 

 Green el al. (47).] 



