giO HANDBOOK OF PHYSIOLOGY ^ CIRCULATION II 



c ] mm 



fig. 14. Prominent arteriovenous anastomosis lying below the superficial layer of the conjunctiva. 

 [From Grafflin & Corddry (56).] 



arterioles are most readily identified by noting the 

 direction of blood flow. The direction of flow in these 

 vessels is toward progressively smaller vessels, and 

 the smallest of them empty into the capillary bed. 

 Bloch states that when there is a low blood volume, 

 only one capillary may be seen connecting the 

 arterial and venous systems. The arterial segment of 

 this single vessel has a more rapid rate of flow through 

 it (does not say more rapid than what), differs in 

 regard to the direction of taper of its walls (pre- 

 sumably larger at the venous than the arterial end), 

 and is less permeable than the venous segment (no 

 basis given for this statement). 



Bloch feels that the difficulty in determining which 

 vessel is an arteriole has arisen partly because of 

 sudden changes in the direction of flow. According 

 to Bloch, changes in direction occur when an arterio- 

 venous anastomosis opens, causing flow in the peri- 

 pheral segment of the arteriole to stop while flow in 

 the central portion of the venule speeds up. He states 

 that there is no difficulty in recognizing the cause of 

 this directional change if the arteriovenous anasto- 

 mosis opens while the observer is watching and when 

 the site of the arteriovenous anastomosis can be 

 found. He further states that usually the site of the 

 arteriovenous anastomosis is not identified. 



Linear velocity of arterioles is greater than venules 

 of corresponding diameter. The course of the arte- 

 rioles is straight compared to the relatively sinuous 

 course of the accompanying venules. Arteriolar 

 branches arise gradually from their parent vessels, 

 while venules branch more nearlv at right angles. 

 Arterioles are deeper in the tissue than corresponding 

 venules, their flow rates being so rapid that individual 

 cells cannot be recognized. 



Capillaries are described as cylinders, in contra- 

 distinction to arterioles and venules which are cones. 



Some difficulty arises in comparing the descrip- 

 tions of the vascular structure as given by Bloch with 

 that of other investigators due to the absence of 

 detailed diagrams. True arteriolar branches are not 

 represented in other vascular beds as branching 



gradually, although this type of branching occurs in 

 vascular nets forming arcuate systems. 



Microcirculation in the Spleen 



Differences of opinion regarding the manner in 

 which blood is conveyed from terminal arterioles to 

 collecting venules in the spleen still exist in spite of 

 the continued efforts of numerous investigators to 

 resolve the controversy. Current histology textbooks 

 (9, 112) present three views. The theory of closed 

 circulation is that blood in the spleen flows through 

 completely endothelium-lined pathways from its 

 entrance into the spleen through the splenic artery 

 to its exit from the spleen through the splenic vein. 

 The theory of open circulation proposes that arterial 

 terminations in the spleen pour blood flowing through 

 them into the interstices of the reticulum of the red 

 pulp. The walls of the venous sinusoids are incom- 

 plete, having longitudinal slits between the reticular 

 cells which make up the lining of the sinuses. A third 

 view is a compromise between the open and closed 

 systems in that some of the capillaries are thought to 

 open into the intercellular spaces while others open 

 directly into the sinuses. 



It was hoped that the introduction of a technique 

 which permitted observation of the spleen of a living 

 animal might settle the controversy. In 1936, Knisely 

 (69) studied living transilluminated spleens of mice, 

 rats, and cats and reported that each vessel traced in 

 the spleen was connected to the arterial system and 

 the venous system. No vessels were seen to open out 

 into or pour blood into intercellular pulp spaces. 

 The lining of the arterioles, arterial capillaries, 

 capillaries, venous sinuses, and venules was readily 

 apparent through the microscope as a narrow, clear, 

 sharply refractile line, visible also during periods 

 when no blood was flowing through the vessels. 

 Knisely's conclusion was that the vascular system of 

 the spleen consisted of a series of completely inter- 

 connected, preformed, lined channels. He describes 

 spontaneous vasoconstriction in the arterial branches 



