Scaling of the brain and the eye cooling system in birds: a morphometric analysis of the rete ophthalmicum.

A morphometric analysis has been made of the rete ophthalmicum which is a functional arteriovenous heat exchange system in the heads of birds. The morphological data were obtained from histological sections of the rete from 40 species of birds covering a weight range of 20,000-fold. The maximal number of retial arteries, the length, the diameter of the arteries, the total arterial cross-sectional area, and the maximal exchange area have been expressed in terms of allometric equations. Other parameters important for the countercurrent heat exchange effect have been estimated or calculated. By use of simple physical laws for ideal heat exchangers, it is shown that the temperature decrease of the arterial blood passing the rete is rather constant over a wide range in body weight, but larger than previously reported body-to-brain temperature differences. The results are consistent with the heat exchange function of the rete and provide support for the idea that the rete, besides serving as a cooling system for the brain, also may be important in reducing heat loss from the eye.