The anatomy of the nasal conchae with their mucosal vasculature, as well as that of the ophthalmic retia mirabile of grey (Halichoerus grypus) and harp (Phoca groenlandica) seals were investigated by dissection, injection and histological techniques.
The highly convoluted nasal conchae had a surface area of approximately 25 cm2.kgw-1, usually with an air space of less than 1.0 mm between the conchal lamellae. A longitudinally oriented, extensive conchal mucosal rete is supplied caudally by the sphenopalatine artery. A plexus of retial veins runs adjacent to, and in close contact with, the retial arteries. The venous plexus may be drained caudally, primarily by the sphenopalatine/descending palatine veins, and/or rostrally, by the dorsal nasal vein. Microscopical examination revealed that the arterial and venous retial vessels communicate mainly through capillaries, but some arterio-venous anastomoses were detected in the alar fold region. The nasal conchae provide the means for efficient conservation of heat and water by nasal temporal counter-current heat exchange. This is due to a thermal gradient, which will exist along the nasal mucosa, as long as the animal is breathing air colder than body temperature. We suggest that the blood usually runs in a counter-current fashion in the conchal mucosal retia, whereby the temperature gradient along the mucosa may be maintained. Moreover, the steepness of this gradient may be altered by changes in nasal mucosal blood flow so as to control accurately the extent of respiratory heat and water loss. Both species possess a complex ophthalmic rete mirabile. Arterial supply to the rete is by the external ophthalmic artery, while venous drainage may occur by way of several anastomosing veins. The vascular arrangement suggests that counter-current heat exchange may take place between arterial and venous blood. Such exchange would allow the temperature of the eye to be maintained at a lower level than that of the ophthalmic arterial blood and, hence, reduce heat loss from the large pinniped eyes.