1022 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



.2 



1.0 



BLOOD FLOW ml/min/IOOg 



fig. 12.2. Steady-state oxygen consumption as a function of blood flow and tissue temperature 

 in the hind limb muscles of an anesthetized cat. Oxygen consumption was lowered when oxygen 

 supply (blood flow) was reduced below a critical value at each temperature. Critical oxygen pressures 

 in venous blood were 40, 28, and 16 mm Hg at oxygen consumptions of 0.3, 0.15, and 0.06 ml/min 

 respectively. A capillary -tissue pressure gradient of less than 5 mm Hg would suffice to supply these 

 rates of oxygen utilization by simple radial diffusion in tissue containing 100 perfused capillaries 

 per mm: (fig. 12. 1). The results indicate that the gradient of oxygen pressure from capillary blood 

 to tissue is greater than that predicted from the simplified model proposed by Krogh (183). Oxygen 

 saturation, S, measured by oximeter and gas analysis. Oxygen pressure in venous blood, Pv 0i , 

 estimated from measured oxygen dissociation curves at each temperature. Tissue temperature ad- 

 justed by passing femoral arterial blood through a heat exchanger. Blood flow adjusted by variable 

 arterial resistance. (From unpublished experiments by Rapela et al.) 



Urea and K 42 distribute in intracellular water and 

 for these substances the chief barrier to diffusion is 

 probably located at cell membranes in the tissues. 

 Table 12. 1 shows that blood-tissue permeabilities to 

 these substances are far less than respective capillary- 

 permeabilities. 



When blood-tissue permeability is large with respect 

 to blood flow, equation 12.1 approaches the limit C 

 = Q, and blood-tissue distribution is said to be flow 

 limited. This is the case for lipid-soluble molecules in 

 general (e.g., antipyrine, fig. 12.3) and provides the 

 theoretical basis for estimating regional blood flow 

 from blood or tissue clearances of these substances 

 (173). Johnson et al. (168) have shown that distribu- 

 tion of labeled water is blood flow limited in cardiac 

 and skeletal muscle and Sapirstein (311) has used the 



blood clearances of Rb s6 or K 42 as a measure of rela- 

 tive regional blood flow. The clearances of labeled 

 Xa or I from blood or interstitial space have also been 

 used for this purpose (72, 165, 172, 290, 362) but in 

 view of the interstitial component of blood-tissue 

 permeability this may not be justified. Several investi- 

 gators have measured fractional extractions (C/Q) of 

 test materials during single passage through vascular 

 beds of extremities (40, in), head (40), liver (40), 

 heart (44) and lungs (39). Equation 12.1 suggests 

 that the values so obtained reflect the exponential 

 ratios of blood-tissue permeability to blood flow under 

 the conditions of vascular tone prevailing at the time 

 of measurement. 



For large lipid molecules the chief barrier to tissue 

 distribution is the capillary wall and in this case PA m 



