Conclusion

Training the fishes increases their body length and weight and changes the gill morphology considerably. The gill volume of the trained fish is significantly increased and the space filling ration is highly significantly decreased. We conclude that the gills of the swimmer group are less compact than the gills of the control group. It is thus expected that the flow of oxygen-rich water is facilitated in the gills of the swimmers.
Endurance-training-related adaptations in the pathway of oxygen in mammals and fish are very similar. O2 uptake, heart, erythrocytes and muscles are all adapting in a comparable way. We confirm that the zebrafish is a good model to study human physiology. The gas exchange organ in our study showed high plasticity. We think that within the respiratory cascade in fish, the gills represent a bottle neck. Human lungs either have already sufficient volume to satisfy times of higher demand, or they would like to extend their volume but cannot because they are, in contrast to fish, locked in their thoracic cavity.