RESISTANCE AND CAPACITANCE PHENOMENA IN VASCULAR BEDS 



945 



measurement which the controlling mechanism is 

 attempting to keep constant (such as arterial pres- 

 sure, or tissue Os tension). The level of the controlled 

 variable may be disturbed by various loads, i.e., 

 bleeding in the case of arterial pressure, or variations 

 of tissue metabolism in the case of tissue Oo tension. 

 The detector senses continuously the magnitude of the 

 variable and feeds the information to a summator or 

 discriminator where it is compared with the desired 

 value (set point) and modified by signals from other 

 loops. The resulting signal is then fed to an effector 

 which controls the activity of whatever process 

 is necessary to maintain the controlled variable 

 constant. 



Input from 

 other loops 



Set 

 point 



Ui 



I 



control 



signal (s) 



Feedback signal 



Load 



Detector 



fig. 13. Schematized diagram for a feedback loop (see text 

 or discussion). 



2) Present evidence suggests that, in skeletal 

 muscle, a possible controlled variable in the feedback 

 loop for autoregulation is the tissue oxygen tension 

 (69). The detector is unknown; but the feedback loop 

 may involve adenine formed from adenosine triphos- 

 phate (ATP) in the presence of insufficient oxygen. 

 The summation point may be the receptor site on the 

 arteriolar smooth muscle, and the effector may be 

 the arteriole which controls the rate of blood flow by 

 means of which the controlled variable is regulated 



(5)- 



3) There are other possible controlled variables in 

 the feedback loop for autoregulation. The increased 

 flow that occurs in skeletal muscle during and follow- 

 ing a tetanic contraction may represent another 

 manifestation of this pattern of autoregulation 

 although axon reflexes in the sympathetic nerve 

 supply have been postulated as playing a role (62). 

 The vasodilation that occurs in skeletal muscle during 

 activation of the patellar reflex is thought to be due 

 to the same mechanism as that of the postcontraction 

 hyperemia ( 107). 



The controlled variable responsible for auto- 

 regulation (and reactive hyperemia) may vary with 

 different vascular beds. Tissue oxygen tension 

 appears to be the controlled variable in the myo- 



® 



e/vjw/wt: 



95" 

 Arterial 

 Pressure 

 mm Hg 



Coronary 



Flow 



em'/min 



-0 



Control 



90 sec 5% O2 



Mean Flow 

 cm3/min 30 



67 



Control 



Wean Flow 

 cm3/min 21 



I mg NaCN 



into Coronory Artery 



fig. 1 4. Left anterior coronary artery inflows in the dog. A : in response to a 90-sec period of 

 breathing 5% Oo. B: effects of an intra-arterial injection of 1 mg of sodium cyanide. Note the calibra- 

 tion for flow is nonlinear (the deflection is approximately proportional to the square of the flow). 

 [Modified after Green & Wegria (49).] 



