1012 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



fig. 9.4. A portion of the wall of a capillary (heart musclel 

 to show details of the interendothelial junction. The junction 

 provides a continuous channel connecting the inside of the 

 capillary with the outside basement membrane. The width of 

 the channel is about 100 A. The interior of the capillary is 

 almost filled with an erythrocyte. [From Fawcett (93).] 



cylindrical or perfectly uniform. Alternative models 

 utilizing different pore geometries or a limited distri- 

 bution of pore sizes could be devised to simulate ob- 

 served capillary permeability. The significant fact is 

 that both the hydrodynamic and diffusional char- 

 acteristics of the capillaries can be explained in terms 

 of a simple physical model which closely approxi- 

 mates the morphology of the capillary wall. The mean 

 pore radius calculated as above may be regarded as 

 analogous to the Einstein-Stokes molecular radius 

 (equation 7.5) which by itself tells nothing of the 

 actual shape of the molecule but is nevertheless 

 valuable for predicting kinetic behavior. 



The restricted pore areas shown in figure 9.2 repre- 

 sent only a minute fraction of the total capillary 

 surface, but they nevertheless provide for extremely 

 rapid transcapillary diffusion of small lipid-insoluble 

 molecules. The pore area per unit path length avail- 



able for diffusion of water through the capillary walls 

 of 100 g muscle is about 0.6 X io 5 cm (fig. 9.2). The 

 concentration of water available for diffusion in either 

 direction is about 55 molar (0.99 g/ml) and the diffu- 

 sion coefficient of water is 3.4 X io 5 cm 2 per sec. 

 Substitution of these values in Fick's diffusion equa- 

 tion leads to a calculated diffusion rate of 2 g per sec. 

 Since the total volume of plasma in the capillaries of 

 100 g of muscle is only about 1 ml, this suggests that 

 plasma water exchanges 2 times per sec or 1 20 times 

 per min with the interstitial water immediately sur- 

 rounding the capillaries. Similar calculations for 

 NaCl, urea, and glucose yield exchange rate of 60, 55, 

 and 30 times the plasma content of these substances 

 per minute. An alternative method of expressing the 

 results is in terms of the ratio of exchange rate to 

 plasma flow. The latter is generally in the range 2 to 4 

 ml per min per 1 00 g tissue. Taking 3 ml per min as 

 an average, we would estimate that the diffusion of 

 water, NaCl, urea, and glucose back and forth through 

 the capillary wall occurs at rates which are, respec- 

 tively, 40, 20, 18, and 10 times the rate at which 

 these substances are brought to the tissues by the in- 

 coming blood. In contrast, the extravascular circula- 

 tion of fluid caused by net filtration and absorption is 

 only about 2 per cent of the plasma flow as indicated 

 in figure 5.2. For this reason the rates of exchange of 

 small molecules between blood and tissues are but 

 little affected by simultaneous net fluid movement. 

 For example, //-aminohippurate and related sub- 

 stances diffuse rapidly out of the peritubular capil- 

 laries in the direction opposite to net fluid flow (peri- 

 tubular capillary reabsorption). The rates of clearance 

 of Na M of I 131 from skin are not appreciably affected 

 by concurrent edema formation (164). 



The permeability of biological membranes is usually 

 expressed in terms of flux rate per unit concentration 

 difference divided by the area, A m , of the entire mem- 

 brane surface (specific permeability coefficient ) 

 n . „ . D t A s 



P, -- 



-*-A„ 



(9.4) 



Ac ~ m A Ax 



m 



Values of P s for muscle capillaries are listed in table 

 9.1. Information of the type summarized in table 9.1 

 is not yet available from capillaries in regions other 

 than muscle, with the possible exception of renal 

 glomerular capillaries (section 10). Nevertheless, there 

 are many indications that permeability properties of 

 capillaries may differ greatly in different organs. 

 Studies of the rates at which labeled proteins or 

 dextran fractions appear in lymph from different 

 regions suggest that porosity of capillaries in visceral 

 organs may be considerably greater than in capillaries 



