9°4 



HANDBOOK OF PHYSIOI.n<;V 



CIRCX'LATION II 



to a number of capillaries that ran parallel to each 

 other in the same direction as their parent vessel. 



A postcapillary venule was formed by the union of 

 capillaries which came from opposite directions along 

 the muscle fibers. The postcapillary venule increased 

 in size as it was joined by similar tributaries. These 

 tributaries formed venules, which were identified by 

 their muscular walls. 



Provenza & Scherlis (99) studied sections made 

 from dog hearts and placed great emphasis on the 

 appearance of "muscle sphincters" in various small 

 vessels. Although the authors have used the terminol- 

 ogy of Chambers & Zweifach (20), it has not in every 

 instance been properly applied, and comparison 

 with other terminal vascular beds is difficult. A highly 

 imaginative diagram indicates the presence of arterio- 

 venous anastomoses, metarterioles, thoroughfare 

 channels, and precapillaries. 



Microcirculation in Skin 



Zweifach (149) has presented a description of the 

 cutaneous circulation in a flap of skin of the rat from 

 which the connective tissue had been cleaned off. 



A network of arterial vessels in the connective tissue 

 between the skin and underlying muscle gives rise to 

 small arteries which enter the dermis. These small 

 arteries, as well as the ones from which they originate, 

 form a regular pattern of interconnecting links or 

 arcades. The capillary bed of the dermis is composed 

 of a secondary network lying between the inter- 

 arcading arterioles. This secondary network is formed 

 by precapillary and capillary vessels that are branches 

 of the interarcading arterioles. 



Blood flows away from the capillary bed in wide 

 vessels, which join to form collecting venules. The 

 collecting venules form an interconnecting plexus that 

 is similar to, but more extensive than, the arterial 

 plexus. Many short arteriolar branches are seen to go 

 directly into the vessels of the venous plexus. Also 

 seen are direct connections between arterial and 

 venous arcades that allow blood to go from arterial 

 to venous side without going through a capillary net- 

 work. The venous vessels form the major portion of 

 the cutaneous vascular beds. 



Zweifach believes that the branches which leave 

 the arterial arcades are structurally similar to metar- 

 terioles in that they have a thin layer of smooth 

 muscle and a comparatively straight course. The 

 vessel finally becomes part of the capillary bed after 

 giving off branches along its course. These offshoots, 

 or side branches, are precapillary vessels, having 



spirally arranged muscle cells in the immediate junc- 

 tional region. The precapillary vessels show character- 

 istic spontaneous vasomotion. Yasomotion is also seen 

 in the deeper lying arterioles. The small venules of 

 the cutaneous bed show a continuous almost rhyth- 

 mic pattern of spontaneous activity that is unrelated 

 to the vasomotion of the deeper lying vessels. 



The arteriolar arcades were found to be very re- 

 sponsive to constrictor and dilator agents. The venous 

 arcades showed a 20-fold increase in responsiveness to 

 epinephrine when the temperature was made to fall 1 

 or 2 degrees, indicating that they are greatly influ- 

 enced by temperature change. Zweifach considers the 

 venous network in the skin to be unique in this regard. 



From his studies, Zweifach concluded that the 

 structural pattern of the cutaneous circulation was 

 atypical, since it was composed predominantly of 

 highly reactive venous vessels. The circulation in the 

 skin appeared to be regulated locally by tissue 

 mediators. 



The description of the cutaneous vascular pattern 

 and its vasomotion conforms in most respects to that 

 of other terminal vascular beds that have been studied, 

 with the possible exception of the mesentery and the 

 omentum. The interconnecting arcades of both arte- 

 rial and venous vessels with a secondary network 

 forming the capillary bed are prominent features of 

 the pattern of small blood vessels in the hamster 

 cheek pouch and the bat wing. Such an arrangement 

 seems to be a common denominator in vascular 

 patterns of the microcirculation. 



Microcirculation in Stomach and Intestine 



Until a recent paper by Baez (6), descriptions 

 dealing with the vascular patterns of small blood 

 vessels in the stomach and intestine have been based 

 on injected and fixed material, ft is extremely difficult 

 to establish the paths of blood flow in a tissue 

 without observing the flow in living material. This 

 would apply especially in such a vast network of 

 venules and arterioles which intercommunicate so 

 freely by a system of arcades as is present in the muscu- 

 lature of the gut. Although the early investigations 

 briefly discussed here are not concerned with the 

 smallest vessels, they will serve as a background for a 

 more detailed description of the terminal vascular 

 beds of the alimentary canal. 



Xoer (90) studied the vascular patterns in the 

 jejunum and ileum of specimens prepared by liquid 

 latex injections. The descriptions are of the mesenteric 

 circulation and the superficial vessels of the gut wall 



