FLOW OF BLOOD IN MESENTERIC VESSELS 



1451 



showed that epinephrine and norepinephrine had a 

 similar effect, with the former being more potent. 

 Phenoxybenzamine reversed the inflow reduction, and 

 reduced the increase in venous outflow and the volume 

 change. Acetylcholine and methacholine increased 

 arterial and venous flow and slightly increased organ 

 volume, these effects being blocked by atropine. 

 This observation is in agreement with that of Hunt 

 (83), but at variance with those of Ferguson et al. 

 (45) and of Fleming & Parpart (46) who observed 

 arteriolar constriction in the mouse spleen with 

 topical application of acetylcholine as well as epineph- 

 rine, norepinephrine, and histamine. An extensive 

 investigation by Grindlay and co-workers (6g) with 

 thermostromuhrs in unanesthctized dogs showed that 

 a loud noise resulted in a temporary increase in 

 venous outflow while having no effect on arterial 

 inflow, thus accounting for the usual volume reduc- 

 tion of the organ. They also found that both arterial 

 and venous flow rose after feeding and fell after 

 hemorrhage in agreement with volume changes. 

 1 Hiring exercise both flows increased. Since splenic 

 volume decreases during exercise, this provides a 

 good example of the danger inherent in assuming 

 that the direction of volume change of an organ 

 indicates the direction of flow change. 



Mesenteric Circulation as a Whole 



The influence of nervous stimulation or drug 

 administration on the mesenteric circulation as a 

 whole must for the most part be inferred from a 

 synthesis of the effects of these factors on the separate 

 organs. Most studies in the intact animal have been 

 on the total splanchnic flow with no separation of 

 this into its hepatic arterial and portal venous com- 

 ponents. Even where the portal flow is determined 

 separately care must be exercised in the interpreta- 

 tion of the results, since the factor under study may 

 alter the portal flow by affecting hepatic resistance 

 and have no effect on mesenteric resistance. Only 

 when measurement of the portal flow is accompanied 

 by determination of the mesenteric arterial-venous 

 pressure difference is it possible to infer the effects 

 of the factor on the mesenteric circulation, and even 

 then the effect may not be direct; for example, a 

 passive dilation of mesenteric vessels due to a rise in 

 portal venous pressure as a consequence of a hepatic 

 resistance increase or the contrary myogenic vaso- 

 constriction studied by Selkurt and Johnson. One 

 such study in which pressures were recorded, al- 



though mesenteric resistances were not calculated, is 

 that of Katz & Rodbard (89). Another pertinent 

 investigation is that of McMichael (99). The results 

 of these workers are considered below with a sum- 

 mary of what seems to be the best evidence to the 

 present time on the factors affecting blood flow 

 through the separate mesenteric organs. 



There is general agreement that splanchnic stim- 

 ulation increases the resistance to blood flow through 

 the mesenteric circuit. Most results indicate that the 

 effect of norepinephrine and the primary effect of 

 epinephrine are similar, with norepinephrine the 

 more potent of the two except in the spleen. In gen- 

 eral, epinephrine has a secondary dilator effect which 

 is the only consequence of its administration following 

 treatment with various sympathetic blocking agents. 

 Katz and Rodbard, and McMichael found that 

 epinephrine first increased then decreased mesenteric 

 resistance. It might be noted that adrenergic stim- 

 ulation may result in a temporary increase in portal 

 venous flow despite the primary rise in resistance, 

 because such stimulation evokes splenic contraction 

 and the discharge of its stored blood. 



Vagal stimulation probably has little if any sig- 

 nificant influence on the mesenteric blood flow, 

 except insofar as flow is changed secondary to an 

 increase in motility in the stomach and gut. Because 

 the results of studies with acetylcholine are contra- 

 dictory with all organs except the intestine, and there 

 their validity may be questioned, much the same 

 conclusion must be drawn for this factor for the 

 present. 



Pitressin seems to have a constrictor effect in most 

 of the mesenteric organs. Again the data of Katz and 

 Rodbard, and of McMichael confirm this for the 

 mesenteric circuit as a whole. Since serotonin seems 

 to be a constrictor in the intestine, and since the 

 major part of the mesenteric flow passes through this 

 organ, the effect of this hormone on the circulation 

 as a whole is probably the same. The influence of 

 histamine on the gastric circulation seems to be 

 dilatory; however, its effect on the other organs is not 

 so clearly established. Katz and Rodbard's data 

 indicate little change in the over-all flow resistance in 

 the mesenteric organs under the influence of this 

 compound; the dilation in the stomach may be 

 balanced by constriction elsewhere. Finally, one 

 physiological maneuver, exercise, seems to cause 

 vasoconstriction in all the mesenteric circulation 

 except in the spleen. 



