Vascular lining helps maintain body temperature: leatherback sea turtle
 

The vascular lining in the trachea of adult leatherback sea turtles helps them maintain body temperature while foraging in cold water via counter-current exchange.

   
  "[T]he trachea is lined throughout by a continuous vascular plexus. This  contains a high proportion of longitudinally arranged, large-diameter  blood vessels lying mainly in the deeper two-thirds of the mucosa, with  prominent cross-connections between them. The arrangement is consistent  with their functioning as a counter-current arrangement, retaining heat  and maintaining body temperature…We believe that the vascular lining of the long adult leatherback trachea functions in analogous fashion to nasal turbinates." (Davenport et al. 2009:3445-6)
  Learn more about this functional adaptation.