THE CIRCULATION AND CIRCULATION RESEARCH IN PERSPECTIVE 



electron microscopy, reveal no evidence of any sub- 

 stances between the endothelial cells [Kisch (38)]. 

 At present opinions are di\'ided as to whether the 

 exchange of substances occurs througii sulsmicroscopic 

 intercellular fenestrations or through ultramicroscopic 

 perforations in the endothelial cells [Bennett el al. (3), 

 Kisch (38), Pappenheimer (47)]. 



The passage of solutes and solvents through the 

 endothelial walls is largely regulated by capillary 

 pressure and flow [Landis (40)]. During the early 

 part of the present century, capillaries were believed 

 to be straight subdivisions of arterioles. Clapillary 

 pressure and flow were considered to be regulated 

 solely by the intensity of arteriolar and \'enular con- 

 striction. The concept that capillaries could change 

 their caliber actively began with observations of 

 Dale & Richards (13) that histamine apparently 

 constricts arterioles and dilates capillaries. It re- 

 ceived major support from the microcirculatory 

 studies of Krogh and his colleagues (39) and from 

 the human studies of Lewis (42). Krogh's observa- 

 tions, made chiefly on amphibia, were not generally 

 confirmed by microcirculatory studies of mammalian 

 blood vessels [Clark & Clark (10)], and his inference 

 that acti\e changes in capillaries are caused by a 

 squeezing action of pericytes (Rouget cells) attached 

 to their walls, is currently out of favor [Burton (6)]. 

 A new concept of the finer regulation of capillary 

 flow than is provided by arteriolar vasomotion was 

 introduced by Chambers & Zweifach (7). Their 

 microcirculatory studies of the tongue and mesentery 

 revealed that arterioles give rise first to thin-walled, 

 contractile metarterioles or precapillaries which 

 lead into arteriovenous channels 12 to 16 ^ in diame- 

 ter. As schematically shown in figure 3, these 

 thoroughfare channels give oft~ side branches that 

 form an anastomotic network of true capillaries. 

 The metarterioles and precapillary sphincters undergo 

 periodic contractions at intervals of 15 sec to 3 min. 

 It is therefore the present consensus that whereas the 

 arterioles dominate resistance to flow from arteries 

 to capillaries and hence the pressure gradient, the 

 metarterioles and their sphincters control capillary 

 filling and patency to a finer degree. 



The smallest postcapillary venules (pre\enules) 

 should probably be included in the category of 

 "minute vessels" as defined by Lewis (42) and are 

 likewise concerned in the interchange of solvents 

 and solutes. According to Hooker (35), small venules 

 are distinguishable by their somewhat larger size 



ARTERY 



FIG. 3. Diagram of true capillary network and A-V capillary 

 bridges. [After Chambers & Zweifach (7).] 



than capillaries and by the presence of a slight invest- 

 ment of connective tissue. In some regions smooth 

 muscle cells are seen in venules 20 to 30 n in diameter; 

 in other regions they are absent in small veins visible 

 to the eye [Franklin (20)]. 



Collecting Veins and Venous Return 



Blood from the venules is collected by mei-ging 

 veins of increasing size. Their thin walls are composed 

 of many collagenous and scanty elastic fibers in which 

 muscle cells are dispersed in circular, spiral, or longi- 

 tudinal directions [Franklin (20)]. Veins collapse 

 when incompletely filled and, when distended to a 

 cylindrical shape, display only limited distensibility 

 [Clark (11)]. Up to internal pressures of 5 to 10 cm 

 of water, the increment of pressure per increment of 

 volume is quite small, but above such pressures 

 dP/dV rapidly increases until the veins become 

 relativelv indistensible. 



