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HANDBOOK. OF PHYSIOLOGY 



CIRCULATION II 



fig. 20. Net-shaped small channels with a diameter of 

 1-2 n in the adventitia of the peroneal artery. Capillaries of the 

 sheath and the arterial sheath partly removed. The small 

 channels were connected with the capillaries of the sheath. 

 a: Wall capillaries in the stratum longitudinale hbroelasticum; 

 b: network of small channels; c: arterial sheath and capillaries 

 of the sheath, in situ. [Lang (51).] 



to maintain a pressure which is necessary to exceed the 

 tissue pressure. If the capillaries are too far from the 

 origin, the pressure will be too low to supply the wall 

 efficiently. At greater distances from the intercostal 

 arteries, the vasa vasorum interna, with their short 

 delivery system, may provide sufficient blood supply. 

 Schonenberger & M tiller (82) have also deter- 

 mined the flow and resistance in the vasa vasorum of 

 the cow's aorta. Flow increases and resistance 

 decreases, with rising pressure, with a maximum at 



about 140 mm Hg (distended vessels). At higher 

 pressures, a decrease of flow and an increase of 

 resistance occurs (collapsed vessels). The maximal 

 flow seems to occur at the systolic blood pressure 

 level. This indicates that nutrition of the vascular 

 wall may be problematic in hypertension, if the 

 diastolic pressure exceeds the physiological systolic 

 pressure, since the flow in the vasa vasorum may never 

 reach the maximum. The inner region of the wall, 

 which is not nourished by the vasa vasorum, also 

 suffers ischemic changes, including a compensating 

 increase in vascularity. 



The lymphatics certainly play no role in the circu- 

 lation of the tissue fluid within the vessel wall itself, 

 as they do in other organs. The very small channels 

 which arise from capillaries (see above) and which 

 have a diameter of 1 to 3 fi may function like the 

 lymphatics with local drainage (51). The mechanical 

 or hydrostatic pressure gradients are irrelevant to 

 this diffusion transport and determine only the 

 direction of flow in the vasculature of the vessel wall — 

 whether the supply to a capillary comes from an 

 internal or an external arterial branch. 



The situation in the veins is quite different from 

 that in the arteries. There are no channels of supply 

 from the lumen of the vein as is the case in the artery 

 and, since the venous blood is depleted of oxygen and 

 nutrients, the supply by diffusion through the intima 

 is nonexistent or very limited. The pressure gradient 

 is from the external arterial plexus to the capillary 

 plexus, extending as far as the intima. Venous 

 drainage is into small venae vasorum rather than into 

 the lumen of the large vein. The interstitial space is 

 probably drained by "lymphatics", although the fluid 

 may pass directly across the intima and into the 

 lumen. The pressure gradient is favorable for this 

 movement of fluid and it might nourish the non- 

 vascular inner wall. This concept agrees very well 

 with the experiments of Sawyer & Valmont (77), who 

 have found a net transport of Na and CI from the 

 outside to the inside in the canine vena cava. 



REFERENCES 



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 Alexander, R. S. The participation of the venomotor 

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'954- 



Bader, H. Uber die Reversibilitat der plastischen Deh- 



nung des glatten Muskels. Z. Biol. 1 10: 347-355, 1958. 



4. Bader, H. Die Abhangigkeit des Verhaltnisses von 

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5. Bader, H., and E. Kapal. Uber die Bedeutung der 

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6. Bader, H., and E. Kapal. Experimentelle Untersuch- 

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 mischlauchen. 2. Mitteilung. Z. Biol. 109: 325-331, 1957. 



