1362 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



table i . Changes in the Circulation and Tissue Fluid 

 Volume in the Hind Limbs of the Cat During 

 Stimulation of the Sympathetic Vasoconstrictor Nerves 



remembered that skin and resting muscle are rela- 

 tively avascular tissues and only contain 14 per cent 

 of the blood volume. Measurements of the volume 

 of blood in the hind parts, by a radioisotopic method, 

 showed that they contained about 20 ml of blood. 

 Of this, 6.25 ml, that is 33 per cent, was expelled by 

 sympathetic activity. If the sympathetic could expel 

 33 per cent, the same proportion, from the splanchnic 

 area which contains a relatively enormous amount 

 of blood, it is clear that this mechanism would be of 

 great importance. 



But let us return to table 1 and to Mellander's 

 results. It will be seen that at 2 per sec hind-part 

 blood volume is reduced by 80 per cent of the maxi- 

 mal (line 9) and that resistance is not reduced by 80 

 per cent until the impulse frequency has been in- 

 creased to 8 per sec (line 4). 



To account for the changes in resistance and hind- 

 part blood volume found during maximal electrical 

 stimulation Mellander has calculated that the inter- 

 nal circumferences of an "average arteriole" and an 

 "average venule" would have to decrease by 35 per 

 cent and 20 per cent, respectively. This would happen 

 if the smooth muscle coat in both arterioles and ven- 

 ules shortened by 20 per cent. In the case of the 

 arteriole, owing to the protrusion inwards of the 

 inner wall layers (99), this would reduce the internal 

 circumference not by 20 per cent but by about 35 

 per cent. 



Table 1, line 12, shows the effect of sympathetic 

 chain stimulation on transcapillary fluid movement. 

 During stimulation tissue fluid entered the capillaries 

 and drained away, the amount being related to the 

 impulse frequency. The greater the impulse frequency 

 the more must capillary pressure have fallen. The 

 precapillary vessels must have constricted both 

 absolutely and relatively more than the postcapillary 

 vessels. The discrepancy must have increased as the 

 frequency increased. The falls in capillary pressure 

 corresponding to the different impulse frequencies 

 were determined as follows. In a control experiment 

 venous pressure was decreased by a known amount 

 by lowering the venous cannula, and the rate at 

 which fluid drained from the tissues out of the hind 

 parts was recorded. From this the rate at which fluid 

 entered the capillaries per 1 mm drop in capillary 

 pressure was calculated. This was the absorption 

 coefficient. Knowing both this and the rate of entry of 

 fluid into the capillaries recorded during the stimula- 

 tion at the different impulse frequencies, the cor- 

 responding falls in capillary pressure could be cal- 

 culated. These are shown in lines 13 and 14 and in 

 the tracing in figure 8. 



Another interesting point is that at the end of 2-min 

 stimulation the absorption of tissue fluid ceases. 

 Nevertheless during maximal physiological sympa- 

 thetic stimulation for 2 min the volume of tissue fluid 

 draining out of the hind parts is almost as much as 

 that expressed from the capacitance vessels (lines 8 

 and 12). 



Folkow & Mellander (98) have developed a tech- 

 nique for investigating the effect of a procedure upon 

 the capillary surface area. Maximal stimulation of 

 the sympathetic vasoconstrictors, they find, closes 

 many precapillary sphincters and reduces the capil- 

 lary surface area to about one-third under conditions 

 where blood flow is decreased to about one-sixth. 



CHEMICAL TRANSMISSION AT SYMPATHETIC VASOCON- 

 STRICTOR NERVE ENDINGS IN SKELETAL MUSCLE. Folkow 



