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HANDBOOK OF PHYSIOLOGY -" CIRCULATION II 



decreased and vasodilators increased the size of the 

 arteriovenous connections. The vasoconstrictor effect 

 of adrenaline was evident in direct observations of 

 living vessels in rabbits' ears and it was found in the 

 same preparations that histamine and acetylcholine 

 dilated these shunts (61). This has been confirmed. 

 According to Curri et al. (40) serotonin introduced 

 intravenously in a dose of 8 mg resulted in a cessation 

 of rhythmic activity of the intercalated segments. 



Role in Bodily Economy 



Surely one function of the arteriovenous shunts as a 

 component of the microcirculation is concerned with 

 regulation of regional blood flow, as exemplified in 

 erectile tissue. In general, when the shunts are open 

 the capillary bed may be largely or entirely bypassed, 

 and total blood flow traversing the part may be 

 maximally increased. This phenomenon, according to 

 Grant (61, 62), helps to maintain the temperature of 

 the extremity when exposed to extreme cold. Also it 

 has been known since 1840, from the observation of 

 Julius Robert Mayer, surgeon to the threemaster 

 "Java," that venous blood tends to become "arteri- 

 alized" in the tropics, indicating a dilated state of the 

 arteriovenous connections (71). Thus, a thermoregu- 

 latory function has been suggested for these structures. 

 Many of the arteriovenous anastomoses are however 

 deeply situated, for example, in the periosteum or 

 even within parenchymatous organs, and must have 

 other than thermoregulatory functions. A third 

 physiological role which has been considered, but 

 which has not been truly demonstrated or tested 

 experimentally, is in the regulation of blood pressure. 

 It seems logical that if sufficient numbers of the direct 

 arteriovenous anastomoses are widely open, systemic 

 arterial blood pressure might fall. 



It was suggested by Schumacher (160), largely on 

 theoretical grounds, that the specialized epithelioid 

 cells might have a secretory function — more specifi- 

 cally that they could secrete acetylcholine. Luckner & 

 Staubesand (no) found in extracts of the coccygeal 

 body a substance with the biological properties of 

 acetylcholine in concentrations of 9000 jug per g. 

 Indeed Schumacher (160) conceived that the pulsa- 

 tion of the arteriovenous anastomoses was a mecha- 

 nism to maintain a level of the short-lived acetyl- 

 choline in the blood. This concept is of interest in that 

 the epithelioid cells are rather widely distributed in 

 small groups within the walls of arteries, for example 

 at the vascular pole of the glomerulus, where they 



had been described first by Ruyter (149) and later by 

 Goormaghtigh (59). 



Also rather theoretical is the idea that the connect- 

 ing segments may be pressoreceptors, i.e., that the 

 metabolism of the cells could be altered by variations 

 in pressure, and that this effect could somehow be 

 transmitted to the associated extensive neural plexuses. 



Schumacher (160) thought that cells of the non- 

 chromaffin paraganglia were analogous to the epi- 

 thelioid cells of the intercalated segments, but there 

 is no evidence that the carotid body is related to 

 arteriovenous anastomoses, although some of these 

 structures exist in its connective tissue capsule (1). 



Clearly, there is much to be learned in the domain 

 of function of the arteriovenous anastomoses. Further 

 study doubtless will be highly rewarding. 



ABNORMAL ARTERIOVENOUS COMMUNICATIONS 



Arteriovenous connections of unusual size or loca- 

 tion can occur as single or multiple lesions and, 

 especially when multiple, can be familial (Osler- 

 Weber-Rendu disease). The lesions vary from insig- 

 nificant blue or purple spots on the skin or mucous 

 membranes to complex cirsoid masses with the 

 arrangement of hemangiomas. These are important 

 chiefly because they can bleed, as for example into 

 the gastrointestinal tract. 



In the lung, the pulmonary arteries and veins can 

 come into free anastomosis with a right to left shunt. 

 When of sufficient size, there are the expected conse- 

 quences of desaturation of systemic arterial blood, 

 cyanosis, polycythemia, clubbing, and at times 

 thrombotic complications. Cardiac failure does not 

 occur unless immense numbers of the arteriovenous 

 fistulas are present (66). These can be of such small 

 size as to be undetectable by angiography. 



It was known to Virchow that acquired hemangio- 

 mas with a cavernous component also are the seat of 

 arteriovenous communications, as indicated by the 

 bright color of the effluent blood. Fistulation also can 

 occur within certain neoplasms, especially when they 

 become necrotic or hemorrhagic as in the case of 

 chorionepithelioma. A bruit may then become audi- 

 ble over the lesion. 



The "cutaneous arterial spider" has been recog- 

 nized to consist in part of arteriovenous connections. 

 The arterial component has in its walls specialized 

 "glomus cells" (epithelioid cells), like other arterio- 

 venous shunts. Such structures develop commonly in 



