950 



HANDBOOK ciF 1'IIYSIOLOGY 



CIRCULATION II 



agents, whereas all other beds show a significant 

 response (fig. 18). Secondary dilation and the dilation 

 following adrenergic blockade which occur with 

 epinephrine are due probably to excitation of beta 

 receptors; this effect is of significant magnitude in 

 skeletal muscle bed and of lesser magnitude in cardiac, 

 mesenteric arterial, and splenic vascular beds (fig. 19). 

 Sympathetic constrictor fibers exert maximal effects 

 in kidney, skin, and mesenteric beds. Significant 

 constriction is noted in muscle, spleen, and hepatic 



fig. 18. Bar graph of responses in various vascular beds to 

 an intra-arterial injection of 1-10 fig of epinephrine. Light- 

 colored bars — initial responses; dark-colored bars — secondary 

 (dilator) responses. See legend to fig. 12 for additional infor- 

 mation. [Reproduced from (45).] 



arterial and portal venous beds; but no significant 

 reduction of flow occurs in cerebral or cardiac beds 

 (fig. 20). 



In the dog, unequivocal cholinergic dilator 

 responses in response to sympathetic nerve stimulation 

 are seen only in skeletal muscle (fig. 20) (45). How- 

 ever, Beck & Brody (4) believe that another active 

 dilator pathway can be demonstrated in the dog's 

 hind quarters perfused at a constant rate of flow. 

 Dilation, primarily induced by this pathway, is most 

 readily obtained by release of tracheal occlusion and 

 is not blocked readily by atropine. It is claimed that 

 in man there is a cutaneous vasodilator innervation 

 (61, 94). It is claimed also that when sweat fibers are 

 activated there is a release of bradykinin which 

 causes secondary dilation in blood vessels adjacent to 

 the sweat glands (34). However, Senay el al. (103) 

 often found marked dissociation between sweating 

 and vasodilation in the forearm. 



SEGMENTAL RESISTANCES IN VASCULAR BEDS 



Methods 



The behavior of the small vessels, i.e., the arterioles 

 and venules, may be inferred from comparison of the 

 pressures recorded from fine catheters fed distally into 

 small arteries and veins via the large vessels, with the 

 lateral pressures recorded from the larger vessels 

 (53, 57). Additional information is yielded when 

 flows are measured simultaneously (14, 15). Micro- 

 puncture studies have been used to obtain the 

 pressures in the portal venous tree, portal venules, and 

 central veins of the liver (83). 



0?N 





400 

 300 



200- 

 150 



100 

 80 



o PER CENT OF TOTAL FLOW 



Heart 4 7, Brain 3 8, Muscle 29.6, Mes. Art. 19 7, Spleen 14 

 Skin 9 4, Kidney 26 3, Hep Art. 5.2, (Portal V. 21.1) 



fig. 19. Bar graphs of responses to intra-arterial injections of isoproterenol (B) compared with 

 those to epinephrine during adrenergic blockade with phenoxybenzamine (Dibenzyline) (.4). The 

 ordinate values are the maximum flows in response to the agent, expressed as per cent of the control 

 flow. The last bar (unshaded) represents the computed integrated effect on the total body flow 

 I cardiac output). The sequence of the bars appears in the figure; the figures on the abscissa represent 

 the per cent of cardiac output which flowed through the bed in the control period. See legend to fig. 

 12 for further explanation. [Reproduced from (45).] 



PER CENT OF TOTAL FLOW 



