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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



eem 





fig. 8. Structure of small pre- and postcapillary vessels. Upper half. Comparison of a 30 fi pre- 

 capillary vessel (left) and postcapillary vessel (center) with systemic arterial branches (right) of the same 

 size. The pulmonary pre- and postcapillary vessels are structurally similar; they are strikingly differ- 

 ent from the systemic arterial branches. Lower half. Comparison of a 57 11 pulmonary artery (left) and 

 a corresponding pulmonary vein (center with systemic arteriole (right) of the same size. Muscle cannot 

 be identified in the vein, ec, endothelial cells; ef, elastic libers; ™, smooth muscle; iem, internal clastic 

 membrane; eem, external elastic membrane; ac, alveolar capillaries. (Elastic tissue stain. 585 X) 

 [Courtesy of E. R. Weibel (132).] 



muscle in the capillary walls. Without such con- 

 tractile elements, it is unlikely that the capillaries 

 can contract actively in the conventional manner of 

 muscle-containing vessels (400); on the contrary, 

 capillary lumens are more apt to be passively nar- 

 rowed by swelling of endothelial cells, by perivascular 

 transudates (114), by raised alveolar pressures (351), 

 and by the pushes and pulls of adjacent structures 



(397)- 



According to Weibel, the "typical" alveolus in 

 man is more like the cell of a honeycomb than a 

 sphere (422). It measures approximately 200 to 



250 fi in diameter. Each alveolus is lined by a con- 

 tinuous epithelium (40 to 65 m,u thick) which changes 

 its submicroscopic appearance upon appropriate 

 stimulation (276, 375). In the human lung approx- 

 imately 300 million alveoli are juxtaposed to ap- 

 proximately the same number of capillary segments. 

 After the age of 8 years, an increase in the size of the 

 lung seems to involve an increase in the dimensions 

 of existing alveolar-capillary units rather than in 

 their number (422). 



The thickness of the alveolar-capillary interface is 

 of the order of 285 to 640 rn.fi (375): not all of the 



