88 4 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



fig. 1 6. Vasa vasorum in the 

 wall of the aorta of the horse. 

 China ink, thick cuts. Left: 

 longitudinal section; right: cross 

 section. [Straubesand (87).] 



1.5mm 



6.0mm < 



Interna 



> Media 



\ fig. 17. Schematic longitudinal section through the aortic 

 wall at the origin of an intercostal artery. / = intercostal 

 artery; V = vas vasis externum; a = outer branch; ; = inner 

 branch. [Schonenberger & Muller (82).] 



stance to the basal side of the endothelial cells, where 

 thev release their contents to the wall tissue. This 

 transport is called cytopempsis. Another possibility 

 of active transport through the endothelium may be 

 by a similar mechanism which was described by 

 Ussing (92) for the frog skin. He demonstrated that 

 sodium is actively transported across the skin cells 

 l>\ a carrier system located in the cell membrane. This 

 would mean that ions or other materials enter the 

 endothelial cell passively through the surface mem- 

 brane, along a concentration gradient, and are then 

 actively transported out of the cell and into the wall 

 tissue against a concentration gradient. Sawyer & 

 Yalmont (77) have published evidence for such a 

 mechanism in the canine thoracic aorta and vena 

 cava, where the net flux of sodium or chloride ions 



fig. 18. Cross section through the thoracic aorta of the dog, 

 showing a vas vasis internum. There is no branching of capil- 

 laries in the intima and the innermost part of the media. The 

 dark masses in the outer third of the media are accumulations 

 of injected material that has broken out of the capillaries. 

 [Woerner (101 I 



in the aorta is from the inside to the outside. In the 

 vena cava it is in the opposite direction (for a possible 

 explanation of this contrasting behavior see below). 

 The variety of theories about the transport of material 

 across the endothelial lining shows that much work 

 remains to be done. Most of these theories are deduced 

 from experimental results on capillary endothelial 

 cells but it may be that transport differs in the capil- 

 lary endothelium and the endothelium of larger 

 vessels. There is also the possibility that different 

 tissues use different transport mechanisms. (See also 

 Chapter 29.) 



The further transport through the intima and the 

 media may be passive in the intercellular space, 



