EXCHANGE OF SUBSTANCES THROUGH CAPILLARY WALLS 



993 



B. Effects of Temperature on Filtration Coefficients 



As noted above, the Uarcy equation indicates that 

 flow through porous materials changes with tem- 

 perature in inverse proportion to viscosity, and accu- 

 rate measurements of flow through artificial, porous 

 membranes have confirmed this expectation (16, 81). 

 The filtration coefficients for capillaries, k c , and for 

 tissues, kt, include the viscosity of the filtered fluid 

 and should change with temperature. This was found 

 to be clearly the case by Pappenheimer (277) in the 

 perfused limb in which the capillary membranes 

 retain their normal impermeability to plasma pro- 

 teins over a temperature range of 8 to 44 C. The 

 ratio of the filtration coefficient measured at 36 ± 2 C 

 to that measured at 10 ± 2 C averaged 1.68 (se = 

 ±0.08). This value was within 10 per cent of the 

 ratio of viscosities of water at the two temperatures 

 (77 10 77 36 = 185) and the difference between the 

 two ratios was not significant (P > .05). 



In the frog mesentery Brown & Landis (23) had 

 observed earlier a decrease of filtration coefficient 

 (k c ) from .0070 to .00 1 g when temperature was re- 

 duced from 24 ± 2 C to o ± 2 C. However, as men- 

 tioned by Pappenheimer (276), the decrease in filtra- 

 tion coefficient was larger than that to be expected 

 theoretically and the scatter of values for individual 

 capillaries was too great for quantitative conclusions. 



In the human forearm, Landis & Gibbon (209) 



found the filtration coefficient (kt) almost halved as 

 the temperature of the plethysmograph (and of the 

 superficial tissues of the forearm) was changed from 

 44 or 45 C to 14 or 15 C. Brown el al. (24) extended 

 these observations to 4 to 5 C. Figure 6.5 (heavy line) 

 shows the filtration coefficients observed in the fore- 

 arm at plethysmograph temperatures over the total 

 range of 44 to 4 C. Using the data of Barcroft & Ed- 

 holm (9), the figures just above the bottom line of the 

 chart show the probable temperatures in muscle and 

 subcutaneous tissues corresponding to each surface 

 temperature. Starting from the filtration coefficient 

 at a tissue temperature of 35 to 36 C (surface tempera- 

 ture 34 to 35 C) the fainter, dash lines indicate coeffi- 

 cients of filtration calculated on the basis of deep 

 tissue temperature and the corresponding change in 

 viscosity of capillary filtrate. Observed and calculated 

 filtration coefficients agree fairly well at tissue tem- 

 peratures ranging from 35 to 17 C. Above 35 C the 

 observed coefficient is much higher than the calcu- 

 lated one, indicating that changes other than viscosity- 

 are involved. To be considered are such factors as 

 increased capillary pressure and filtering area (8) 

 secondary to vasodilatation or, possibly, opening of 

 arteriovenous anastomoses with elevation of small 

 vein pressure. Below a tissue temperature of 1 7 C, the 

 observed filtration coefficient does not decrease as 

 it should if viscosity alone were involved, but in- 



fig. 6.5. Effects of tempera- 

 ture on filtration coefficients, kt, 

 observed in the human forearm 

 (solid line), compared with fil- 

 tration coefficients to be expected 

 by calculation from change of 

 viscosity of water by reason of 

 temperature changes in muscle 

 ( — — ) and subcutaneous tis- 

 sue ( — - — - — ) [Calculated 

 from results of Landis & Gibbon 

 (209), Brown et al. (24) and, 

 for deep temperatures, Barcroft & 

 Edholm (9).] 



34 to 35 24 to 25 14 to 15 



TEMPERATURE OF WATER AROUN0 FOREARM C 

 PRESSURE PLETHYSMOGRAPH - COMBINED RESULTS 



