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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



innervated by the sympathetic vasoconstrictor system, 

 can stop nutrient blood flow through large areas of 

 capillary bed, leaving blood to flow through arterio- 

 venous thoroughfare channels or other regions of 

 low metabolic rate. Under these conditions, total 

 blood flow is reduced but oxygen saturation of venous 

 blood approaches that of arterial blood (26, 274, 294) 

 and respiratory gas exchange may be reduced to one- 

 half or less of its normal value. In diving mammals, 

 drastic vasoconstriction of this type greatly reduces 

 tissue gas exchange for periods of one-half hour or 

 more (322, 323). This is in contrast to uniform reduc- 

 tion of total blood flow caused by decrease in arterial 

 pressure or infusion of vasoconstrictor drugs (274); 

 under these conditions, oxygen extraction is increased 

 and oxygen utilization remains relatively constant 

 over a wide range of blood flow (fig. 12.2). 



Recent investigations of the rates at which labeled 

 ions are removed from interstitial space in muscle also 

 suggest that vasomotor nerves control the distribution 

 of blood between nutrient and nonnutrient circula- 

 tions (165). There is general correspondence between 

 clearance of Na 24 or I 131 and over-all blood flow when 

 flow is altered by pressure, reactive hyperemia, or 



exercise (72, 165, 172, 290, 362). However, activation 

 of the vasomotor system to muscle or skin generally 

 results in large changes of flow without corresponding 

 changes of clearance. Hyman el al. (165) have shown 

 that I 131 clearance may actually decrease during large 

 increases of flow caused by activation of the sympa- 

 thetic vasodilator system. They suggest that vasodi- 

 lator nerves act primarily to increase flow through 

 arteriovenous thoroughfare channels. 



Studies of this type are only beginning and they 

 point to new directions for research on the peripheral 

 circulation. The principal function of the circulation 

 is to provide for exchange of materials between blood 

 and tissues and it seems logical to study this function 

 directly in terms of exchange rates. Such studies will 

 only be meaningful, however, if the limitations im- 

 posed by over-all permeability are considered in rela- 

 tion to tissue perfusion. In the present article we have 

 provided a quantitative background for assessing the 

 role played by capillary permeability in the distribu- 

 tion process, indicating only briefly the contributions 

 of interstitial diffusion, cellular permeability, or 

 chemical reaction velocity. 



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