  Fish gills are one of the most primitive gas/solute exchange organs, having
the highest ventilation volume, present in nature. Such performance is
attributed to a functional unit of gill - secondary lamella - that can extract
oxygen from an ambience even at a very low partial pressure. For centuries,
gills have stood as one of the simplest but an elegant gas/solute exchange
organs. Although the role of various morphometric factors of fish gills on
gas/solute exchange capabilities have been reported, there has been limited
understanding on what makes fish gills as an excellent gas/solute exchange
system. Therefore, in the current study, we have theoretically studied the
variation of few structural and parametric ratios, which were known to have
role in gas/solute exchange, with respect to the weight of fishes. Thereafter,
modelling and simulation of convection-diffusion transport through a two
dimensional model of secondary lamella were carried out to study different
factors affecting the performance of gills. The results obtained from both the
studies (theoretical and computational) were in good agreement with each other.
Thus, our study suggested that fish gills have optimized parametric ratios, at
multiple length scales, throughout an evolution to arrive at an organ with
enhanced mass transport capabilities. Further, our study also highlighted the
role of length of primary and secondary lamella, surface area of secondary
lamellae and inter-lamellar distance on gas/solute exchange capabilities of
fish gills. Thus, these defined morphological parameters and parametric ratios
could be exploited in future to design and develop efficient gas/solute
exchange microdevices.
