[020 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



Q ,ml /min per lOOg Tissue 



20 10 



fig. 1 2. 1. Steady-state radial diffu- 

 sion of oxygen (Qo : ) as a function of 

 capillary density (abscissa) and radial 

 oxygen pressure gradient from capil- 

 laries to tissue (ordinate). [Graph con- 

 structed from the Krogh-Erlang equa- 

 tion (183).] 



90 

 80 

 70 

 60 

 50 



40 

 30 



20 



20 



30 40 60 80 100 140 200 300 Intercopillary distarce, 2R, u. 



usual in frozen sections of muscles from hind limbs of an- 

 esthetized cats. Many of the higher estimates imply capillary 

 blood volumes in the range 5 to 15 per cent of tissue volume. 

 In most skeletal muscles the entire blood volume is less than 

 4 per cent of tissue volume (328, 335) and at least half of this 

 may be accounted for by large blood vessels (243). Even taking 

 low estimates for capillary density, however, (200/mm 2 at 

 rest, 600/mm 2 in activity) the oxygen pressure gradient pre- 

 dicted by the Krogh-Erlang model would be less than 5 mm Hg 

 at rest and less than 20 mm Hg in maximum work, leading to 

 tissue oxygen pressures of 10 to 30 mm Hg in the outermost 

 regions of each diffusion cylinder. 



The Krogh-Erlang model provides a theoretical 

 basis for analysis of the blood-tissue gas exchange, but 

 several lines of evidence suggest that factors other than 

 simple radial diffusion in a homogeneous medium 

 may be involved. In the case of skeletal muscle, Milli- 



P c , c opi 1 1 ory 2 pressure, mm Hg 



P R .tissue 2 pressure at R 



Q n , Op consumption ml /sec per ml tissue 



u 2 



a ,0 t solubility =2. 8 « 10"' ml /ml «mm Hg" 1 



D , tissue 2 diffusion coeff. = 1.5 x 10"' cm J /sec ( ) 



R , radius of diffusion cylinder 



r .capillary radius = 4 u. 



kan (253) showed that intracellular myoglobin rapidly 

 becomes desaturated during contraction of the soleus 

 muscle in the cat. Since the half saturation pressure 

 of myoglobin at physiological pH is only 3 mm Hg 

 this implies that intracellular oxygen tension falls to 

 extremely low values during contraction. Lactic acid 

 increases rapidly in venous blood from contracting 

 muscle (7, 181), also indicating that oxygen supply 

 cannot keep up with demand at high rates of metabo- 

 lism, despite normal oxygen pressures in venous blood. 

 Mechanical reduction of blood flow to resting muscle 

 may cause substantial reduction of steady-state oxygen 

 consumption even when the blood vessels are dilated 

 and when venous oxygen pressure is sufficiently high 

 to meet the diffusion requirements estimated from the 

 Krogh-Erlang model (275, 359) (fig. 12.2). Inter- 



