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HANDBOOK 11F PHYSIOLOGY 



CIRCULATION II 



exposed for cannulation. Another problem is the 

 prediction, on a priori grounds, of the hemodynamic 

 behavior, i.e., the blood flow, the driving pressure, 

 and the resistance to perfusion of anastomotic channels 

 which vary so in length, caliber and, possibly, in 

 tone. However, one anatomical aspect of the ex- 

 panded collateral arterial circulation does lend itself 

 to physiological exploitation: the precapillary anas- 

 tomoses make it possible to measure that part of 

 the collateral arterial inflow which reaches the gas- 

 exchanging surfaces of the lung and is available for 

 respiratory gas exchange, i.e., the "effective" col- 

 lateral blood flow (31, 139). 



I 'enous Admixture 



In the normal pulmonary circulation a small 

 quantity of venous blood traverses anatomical chan- 

 nels to bypass the gas-exchanging surfaces of the 

 lungs, thereby reducing the oxygen tension of periph- 

 eral arterial blood. This shunt has diverse anatomical 

 origins: bronchial veins, anterior cardiac veins, 

 Thebesian veins, portal veins, mediastinal veins and 

 pulmonary arteries; in normal man and dog the 

 volume of shunted blood is generally considered to 

 be of the order of 2 per cent of the cardiac output 



(13. 2 3> 349)- 



In the rabbit and guinea pig, arteriovenous chan- 

 nels have been seen on the surface of the transil- 

 luminated lung (219). In the dog, the evidence for 

 such shunts is less direct and there is considerable 

 dispute concerning their size (349). Two types of 

 observations favor a large size: /) glass spheres, up 

 to 500 fi in diameter, reach the left heart following 

 injection into the pulmonary artery (322) ; 2) radi- 

 opaque material, forcefully injected through the 

 side vent of a wedged pulmonary arterial catheter, 

 traces a cine-angiographic course suggestive of short- 

 circuits (331). Opposed is the experimental evidence 

 that these channels are closer to 25 n than to 500 n 

 in diameter (38, 167, 349). A reasonable interpreta- 

 tion of the disparate results in the dog is that the 

 experimental conditions determine the degree of 

 patency of these channels and that ordinarily these 

 channels are virtually closed (95). 



The situation is somewhat more tenuous for the 

 human lung: on the one hand, large glass spheres 

 (up to 500 n in diameter) also traverse the isolated 

 human lung (322); on the other, is the inability of 

 painstaking histological examination to disclose the 

 channels (421) and the failure of physiological meas- 

 urements to obtain the high values for venous ad- 



mixture which would be consistent with the presence 

 of large, patent channels (23). If arteriovenous chan- 

 nels do exist in the normal human lung, they seem to 

 allow very little blood flow under ordinary conditions. 

 The small anatomical shunt in the normal animal 

 or man stands in marked contrast to the large size 

 which it may achieve in certain clinical states, such 

 as congenital right-to-left intracardiac shunts and 

 pulmonary hemangiomatosis (155, 226). Appreciable 

 shunting has also been demonstrated in those patients 

 with cirrhosis of the liver who develop portal-pul- 

 monary venous communications (63). 



Pulmonary \'asomotor Nerves 



There is no doubt about either the existence of 

 pulmonary vasomotor nerves or their ability to 

 change pulmonary vascular calibers when appro- 

 priately stimulated; only their physiological meaning 

 can be questioned (96, 382). 



The pulmonary vasomotor nerves have been most 

 intensively studied in the dog: both vasodilator and 

 vasoconstrictor fibers have been identified in the 

 upper sympathetic chain and in the vagus nerves 

 (95). Because of the complicated intermingling of 

 these fibers — not only with each other but also with 

 bronchial and cardiac fibers — electrical stimulation 

 often fails to separate vagal from sympathetic effects 

 on the one hand, and vasomotor from bronchomotor 

 and cardiac effects on the other (95). 



For comprehensive reviews of pulmonary and 

 pulmonary vascular innervation the reader is referred 

 elsewhere (96, 267). A few aspects are particularly 

 relevant to considerations of pulmonary hemody- 

 namics: a) the large pulmonary arteries and veins 

 are more richly innervated than their smaller counter- 

 parts (81, 147, 392); b) the muscular arteries and 

 arterioles are more richly innervated than the cor- 

 responding veins and venules (392); c) nerve endings 

 reach the medial and subendothelial layers of the 

 large arteries and veins (392); d) sensory nerves and 

 receptors have been identified in the airways and in 

 the large pulmonary arteries and veins (6, 80) ; e ) 

 the bronchial arteries are more richly innervated 

 than any other pulmonary vessels (292); and /) the 

 nerve supply to the bronchi exceeds that of the pul- 

 monary vessels (392). 



As a pharmacological device for estimating the 

 concentration of adrenergic nerve endings in the 

 different parts of the pulmonary vascular tree, Euler 

 & Lishajko (126) compared the concentrations of 

 norepinephrine in the central and in the peripheral 



