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Re: RTs (lengthy)



Rob Meyerson  wrote

>Confused...
>
>In watching a presentation on the Discovery Channel (I know, not the best 
>source of 
>info), the narrator said that the function of the RT's is simply to give the 
>animal a better 
>sense of smell (more places to put olfactory receptors).  Is this merely a 
>case of 
>double-duty, with the RT's performing multiple roles?
>

Nasal turbinates (a term that includes both respiratory and olfactory 
turbinates) perform multiple functions. The functionality is primarily 
determined by the structure of the mucous membrane. The type of
function a turbinate performs cannot be determined from the
bony/cartilaginous structure which is essentially for support. The number
of turbinates vary from species to species and also within the same
species (humans can have 2 to 4 turbinates, though most have three). In
humans, the surface structure of the inferior surface of the inferior
turbinate is totally different from the rest. This area is covered by a flat
epithelium overlying a rich capillary plexus that facilitates transmembrane
exchange of heat and water. Part of the capillary networks coalesce
forming the so called blood lakes - another adaptation for the same
purpose. A very similar surface structure occurs in the Little's area of the
nasal septum (not part of the turbinates) which must also perform the
same function. Thus, nasal turbinates are not essential for rapid
exchange of heat and water during respiration.

The rest of the surfaces of the nasal turbinates are covered with
columnar epithelium containing secretary glands. Both surfaces of the
superior turbinate contain nerve endings from the olfactory nerve and this
area provides for an increased surface of olfaction. This turbinate also
protects the openings of the frontal sinuses from direct exposure to
inhaled air. The middle turbinates similarly protect the entry points of the
maxillary sinuses.

Animals with a single turbinate can use parts of its surface for different 
purposes. There is no reason why a single turbinate, if covered on the
two surfaces with two different kinds of epithelium (mucous membrane),
should not perform two different functions. Epithelial structure may even
be different in parts of the same surface - in which case the same
surface would perform more than one function.

Of course, the structure of the mucous membrane in a fossil specimen is 
impossible to determine. Thus the function of the nasal turbinates in
extinct animals - whether they were respiratory, olfactory, or both - can
only be guessed. 

Dr Gautam Majumdar  MBBS, MD, MRCP, FRCPath
gautam@majumdar.demon.co.uk