EXCHANGE OF SUBSTANCES THROUGH CAPILLARY WALLS 



1023 



14 CAPILLARY 

 CLEARANCE 



ml /min per lOOg 



PA 



fig. 12.3. Diffusion kinetics of anti- 

 pyrine, K", and urea in vasodilated 

 muscle. Permeability to antipyrine 

 (lipid-soluble) is so large that its 

 clearance is limited by rate of blood 

 flow. The clearances of K. 41 and urea 

 are limited, in part, by permeability 

 of cell membranes in extravascular 

 distribution volume. Less than 10% 

 of the diffusion barrier to urea is con- 

 tributed by the capillary wall (table 

 1 2.1). [Adapted from Renkin (299, 

 3°°)-] 



BLOOD FLOW, Q, ml /min per lOOg 



table 1 2. 1. Comparison of Blood-Tissue Permeability 

 with Capillary Permeability 



* From equation 12.1 f From table 9.1. 



= D s A,/ Ax. For these substances PA m is small com- 

 pared to normal rates of blood flow and equation 12.1 

 reduces to equation 8. 1 describing arterial disappear- 

 ance curves of large molecules. 



C. Nonuniform Distribution of Blood Flow in 

 Relation to Blood-Tissue Exchange 



The model discussed in the previous paragraphs was 

 designed to simulate effects of changes in flow velocity 

 through a constant number of open capillaries and the 

 results illustrated in figure 12.1 refer to widely dilated 

 blood vessels. At any given over-all blood flow, the 

 clearance of test molecules may be very much smaller 

 during vasoconstriction (299, 300). In supine, anes- 

 thetized dogs the fractional extraction of antipyrine 

 or D 2 from the circulation to extremities may be 

 only 0.6 to 0.8 (40) in contrast to values close to unity 



in perfused, vasodilated muscle (168) or the intact 

 human forearm (111). The fraction of total blood 

 flow passing through true (nutrient) capillaries is 

 subject to wide variation according to metabolic de- 

 mands of the tissue or to hemodynamic demands of 

 the organism as a whole. In some tissues, such as skin, 

 liver, or intestine, the nonnutrient fraction of total 

 blood flow may pass through arteriovenous anasto- 

 moses of potentially large caliber; in other tissues, 

 such as mesentery or muscle, effective physiological 

 shunts are formed by arteriovenous capillaries (388). 

 Nonuniform distribution of blood flow within single 

 organs may also occur between regions of different 

 function and metabolic rate, examples being medulla 

 and cortex of the kidney or gray and white matter of 

 the central nervous system. 



It is obvious that nonuniform alterations of blood 

 flow in the microcirculation will change the relations 

 between total blood flow and blood-tissue exchange 

 rates; conversely, it may be anticipated that quantita- 

 tive studies of effective tissue perfusion will depend 

 heavily upon information obtained from exchange 

 rates. At the present time, available information is 

 mostly qualitative and derives in large part from 

 observations on muscle. 



A striking example of nonuniform distribution of 

 blood flow in skeletal muscle can be observed following 

 electrical or reflex stimulation of sympathetic vaso- 

 constrictor nerves. Closure of precapillary sphincters, 



