944 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



as the perfusion pressure was varied from 40 to 100 

 mm Hg. These findings suggest that a vasodilator 

 effect induced by the pump prevented the increase 

 in vasomotor tone that occurs normally as perfusion 

 pressure is increased from 35 to 85 mm Ha; ( 76 and 

 unpublished data). Autoregulation may be absent 

 from kidney and skeletal muscle beds in the presence 

 of strong extrinsic stimuli (69, 73, 111), and from 

 intestine during the initial period of the perfusion 

 (66). 



AUTOREGULATION IN DIFFERENT VASCULAR BEDS. 



Autoregulation in brain lias been both denied (22, 

 95) and supported (8, 26, 39, 71, 76). Autoregulation 

 has been observed in the kidney by many investigators 

 (58, 59, 65, 72, 97, 99, 105, 1 10, 1 16). Failure to note 

 autoregulation when the kidney(s) is perfused via the 

 aorta could be due to failure to ligate the two lumbar 

 arteries that arise from the aorta between the right 

 and left renal artery origins (58). 



Fairly potent autoregulation was observed in 

 skeletal muscle (27-29, 46, 109) and modest auto- 

 regulation in the intestine (66, 68, 101, 102) and in 

 the liver (36, 92); but significant autoregulation was 

 not elicited in skin (Rapela and Green, unpublished 

 data). It would be anticipated that myocardium 

 would demonstrate prominent autoregulation; how- 

 ever, to date this has not been demonstrated with 

 certainty (17, 21, 24, 43, 84, 85, 98). 



Muscle Flow 

 cm3/min l0 



Skin Flow 

 cm3/min 



37.5 



41.0 



1 



Arterial 



Pressure 



Constant 



Occlusion 

 I min. 



fig. 11. Records of flow in a dog skeletal muscle vascular 

 bed and in a cutaneous vascular bed (saphenous bed) before 

 and following a i-min period of occlusion of the arterial inflow. 

 Arterial pressure upstream from the point of occlusion re- 

 mained constant during these studies. 



fig. 12. Bar graphs of the magnitude of the reactive hyper- 

 emia computed from data such as that in fig. 1 1. The ordinate 

 values are the maximum flow during the postocclusion period 

 expressed as per cent of the control flow. The width of each 

 bar represents approximately the proportion of the cardiac 

 output which normally flows through the indicated vascular 

 bed; total is the integrated effect for the body as a whole. 

 [Reproduced from Green & Kepchar (45).] 



reactive hyperemia. Occlusion of the arterial supply 

 to a skeletal muscle bed for 30 sec to 1 min is followed, 

 upon restoration of the original pressure, by a marked 

 overshoot of flow (fig. 1 1, upper curve). This response 

 has been thought to represent a local reaction of the 

 arterial wall to changes in internal pressure (3). 

 However, it is more likely that the overshoot repre- 

 sents a special manifestation of autoregulation. When 

 muscle contracted during a period of occlusion the 

 excess 0> delivery during the period of reactive 

 hyperemia underpaid the O2 debt accumulated 

 during the period of occlusion, if the muscle was at 

 rest during the occlusion the reactive hyperemia 

 overpaid the debt (9, 108). 



Momentary overshoot of flow after a period of 

 occlusion was noted on occasion in the dog's paw. 

 However, comparison of weight changes (see below) 

 with the integral of the flow during the period of 

 overshoot suggested that the overshoot represented 

 refilling of small vessels, which had emptied by- 

 elastic recoil into the vein during the period of arterial 

 occlusion (114 and Rapela and Green, unpublished 

 data). 



Reactive hyperemia following temporary occlusion 

 of the arterial supply is maximal in myocardium and 

 brain (49, 84; and Rapela et «/., unpublished data), 

 active in skeletal muscle, present in the mesenteric 

 artery bed and in kidney, but is almost absent in 

 spleen, skin (fig. 1 1, lower curve), hepatic artery, and 

 portal vein vascular beds (45), as shown in figure 12. 



MECHANISMS RESPONSIBLE FOR AUTOREGULATION. Feed- 

 back loop. /) General concept. The engineer's feedback 

 loop provides a convenient way to visualize control 

 mechanisms (fig. 13). The controlled variable is that 



