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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



defined (152) as vessels which have a single layer of 

 smooth muscle and very little supporting connective 

 tissue. The "metarteriole," a term introduced by 

 Chambers & Zweifach (20), is defined by Zweifach 

 (148) as a primary structural unit which serves as a 

 framework for the distribution of capillaries. 



The term "precapillary sphincter" was first used 

 by Chambers & Zweifach (20) to designate the 

 muscular investment at the origin of the outflowing 

 branches of the preferential channel (distal con- 

 tinuations of arterial vessels that go directly to the 

 venous side). These outflowing branches lead into 

 true capillaries. In more general usage, a precapillary 

 sphincter is the last smooth muscle cell along any 

 branch of a terminal arteriole (130). 



A capillary may most simply be described as an 

 endothelial tube devoid of smooth muscle and having 

 a minimal amount of supporting elements (48). In 

 descriptions of vascular patterns and flow it is not 

 necessary to add any qualifications as to size, direc- 

 tion of flow, or function. 



Venules originate at the appearance of the first 

 smooth muscle cell on a postcapillary vessel. Venules 

 merge into veins which have a double coat of circular 

 and longitudinal muscle cells. 



Vasoconstriction is the contraction of the smooth 

 muscle of the vessel wall, vasodilation is relaxation of 

 the smooth muscle. Vasomotion refers to any active 

 change in the diameter of blood vessels (81, 89). 



There are numerous other designations for vascular 

 structures, unique to specific organs or tissues, which 

 will be discussed where they appear in the descriptions 

 of arteriovenous pathways in various sites. 



TECHNIQUES FOR MICROSCOPIC OBSERVATION 

 OF SMALL BLOOD VESSELS 



In discussing the techniques used for microscopic 

 observation of small blood vessels, the most com- 

 monly used sites and methods have been included. 

 There are numerous adaptations of the basic tech- 

 niques for specific approaches, and also many highly 

 specialized adaptations for specific areas that will be 

 described in sections of this chapter where they are 

 pertinent. 



In general, there are four basic methods: /) obser- 

 vation of tissues and organs in situ illuminated by the 

 fused quartz rod, 2) exteriorization of internal tissues 

 or organs which can be spread out as a thin layer for 

 examination, 3) preparation of tissues using trans- 

 parent chambers, 4) utilization of superficial structures 

 which can be seen with direct or transmitted light. 



The brevity of the descriptive material should not 

 mislead the reader as to the difficulty of mastering 

 the technical problems associated with each method, 

 nor should he overlook the necessity of being com- 

 pletely familiar with the characteristics of the site 

 selected for observation. A survey of the methods 

 should make it clear that some sites or structures are 

 more suitable than others for any specific investiga- 

 tion and should be evaluated on that basis. 



Hamster Cheek Pnuch 



A method for observation of peripheral circulation 

 at the microscopic level in the membranous cheek 

 pouch of the hamster has been developed by Fulton, 

 Jackson, and Lutz (51, 52, 80). The cheek pouch of 

 the anesthetized hamster is everted and, when 

 properly exposed for viewing, forms a flat double- 

 layered preparation suitable for low power magni- 

 fication. The pouch is bathed in a 37 C Ringer's 

 solution. If higher magnifications (200 X to 1200 X) 

 are used, it is necessary to cut through the upper 

 layer to form a flap of a single layer. The originators 

 of the method believe that the cheek pouch is ideally- 

 suited for investigations on small blood vessels be- 

 cause the thin membrane presents a normal physio- 

 logical surface with blood vessels in their usual tissue 

 environment. A valuable feature of the pouch is also 

 that the same natural vascular bed can be studied 

 over long periods and thus changes in circulation or 

 other characteristics, such as growth of vessels, can be 

 followed. The pouch is more vascular than the 

 mesentery of rats or membranes in transparent 

 chambers. Its vascularity makes tumor transplanta- 

 tion extremely successful. Other investigations made 

 on the hamster cheek pouch include the study of 

 blood pressure, inflammation, hemostasis, petechial 

 formation, thromboembolism, bacterial and parasitic 

 infections, drugs, and the vascularization of tumor 

 transplants. 



The disadvantages of the preparation are that the 

 animal is anesthetized, the exposed tissue must be 

 irrigated and kept at body temperature, and for 

 high magnifications the integrity of the vascular bed 

 is disrupted by surgery needed to obtain a single- 

 layered membrane. The membranous surface con- 

 tinuously exudes mucus, which reduces visibility of 

 the underlying structures. In the hands of its origi- 

 nators, judging by their excellent films, the method is 

 very satisfactory for the investigations in which it has 

 been used. 



