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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



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fig. 10A : Longitudinal muscles in the intima of the branches 

 of the bronchia] artery. Schematic presentation of their differ- 

 ent arrangements. [Weibel (97).] 



elastic vessels, are concentrated in the elastica interna, 

 which separates the intima from the media, and in 

 the elastica externa, which separates the media from 

 the adventitia. Attached to these membranes are the 

 tension muscles, but they account for but a small per- 

 centage of the total vascular smooth muscles [Ben- 

 ninghoff (10, 11)]. The ring muscles are arranged in 

 the wall in a helical structure [Schultze-Jena (84), 

 Fischer (23)]. 



Arteries which are frequently extended in the 

 longitudinal direction, like the branches of the bron- 

 chial artery of the lung, possess longitudinal muscles 

 in addition to the ring muscles [Weibel (96)]. These 

 longitudinal muscles are situated in the split mem- 

 brana elastica interna and can be arranged either in 

 fairly thick one-sided bundles, or as concentric shells 

 which surround the whole lumen (fig. 10A). 



The mechanical behavior changes in the same way 

 as the anatomical picture, smooth muscle forming 

 the major support of muscular arteries, elastic tissue 

 of elastic arteries. In contrast to the elastic vessels, 

 the muscular arteries can change their radius over a 

 wide range. The smallest vessels, like the arterioles 

 and the precapillary sphincters, can even close their 

 lumens completely. Figure 1 1 shows pressure-diame- 



fig. 1 1 : Pressure-diameter diagrams of a small branch of the 

 mesenteric artery of the horse — a: 1st stretch cycle; b: 2nd 

 stretch cvcle; c: 6th stretch cycle. [After Wezler & Schliiter 

 (100).] 



fig. i 2 : Pressure-volume diagrams of the vessels of the hand : 

 an in vivo experiment, a- Temperature in the plethysmograph : 

 25.5 C (vessels contracted); b: temperature 31.5 C (vessels 

 normal); c: temperature 36.0 C (vessels relaxed). [After Thron 

 et al. (90).] 



ter diagrams of a small branch of the mesenteric 

 artery of the horse [Wezler & Schliiter (100)]. Six 

 extension-release curves are made successively, the 

 first, second, and sixth being shown. During the first 

 stretch cycle the smooth muscles are assumed to be 

 contracted, during the following cycles they are more 

 and more relaxed. The first extension-release curve 

 shows a large hysteresis, where the extension curve is 

 concave to the abscissa and the release curve is con- 

 vex. This indicates a very large visco-elasticity of the 

 vessel wall, and very different behavior from that of 

 elastic-type vessels for which the extension and the 

 release curves have a similar shape. Later extension- 

 release cycles show smaller hysteresis, and the exten- 



