title: Identification of vascular sphincters at the junction between alveolar capillaries and pulmonary venules of the mouse lung.
abstract: BACKGROUND
  A peculiar feature of lung circulation in the lung is the pronounced variations in blood volume observed in alveolar capillaries that occur because of the changes in the conformation of the alveolar wall that are associated with the respiratory movements. This phenomenon has led to the postulate that mechanisms of postcapillary control of blood flow are to be present in the lung vessels. In the present study we searched for microanatomical evidence of vascular sphincters in the deep lung tissue of mice, namely in alveolar capillaries and pulmonary veins.
  
  
  METHODS
  We have used scanning electron microscopy (SEM) to examine two types of samples of normal lung tissue of CD-1 mice: 1) vascular corrosion casts made by vascular perfusion with Mercox resin, and 2) routinely made gold/platinum-coated replicas of sectioned lung tissue.
  
  
  RESULTS
  Careful scrutiny of the vessels of the deep lung tissue led to the identification of sphincters in alveolar capillaries. These sphincters were located at the junction between capillary and pulmonary veins. They corresponded to areas of the vascular wall showing circular swellings where a radial organization was observed, since they were made up of alternating grooves and bulges. Transmission electron microscopy showed that smooth muscle cells participated in the formation of the sphincters.
  
  
  CONCLUSIONS
  Our data reveal a new location for vascular sphincters in pulmonary vessels and, because these novel sphincters are located at the capillary-vein junction, they offer a structural setting for the existence of postcapillary control of blood flow in the pulmonary circulation of mice.
authors.affiliation: Department of Anatomy, Abel Salazar Institute for the Biomedical Sciences, University of Porto, Portugal.
authors.forename: M N
authors.initials: MN
authors.lastname: Peáo
authors.forename: A P
authors.initials: AP
authors.lastname: Aguas
authors.forename: C M
authors.initials: CM
authors.lastname: de Sá
authors.forename: N R
authors.initials: NR
authors.lastname: Grande
mesh_terms: D000818:Animals; D002196:Capillaries / Q000033:anatomy & histology; D016319:Corrosion Casting; D004730:Endothelium, Vascular / Q000033:anatomy & histology*; D005260:Female; D051379:Mice; D008854:Microscopy, Electron; D008855:Microscopy, Electron, Scanning; D011650:Pulmonary Alveoli / Q000098:blood supply*; D014699:Venules / Q000033:anatomy & histology