For a large number of problems, there will be some animal of choice, or a few such animals, on which it can be most conveniently studied.
The most direct evidence of the wonderful plasticity and elasticity of red corpuscles is obtained when they are watched in a current, where they can be caught against a projecting edge and bent by the pressure of the current flowing past them.
There can be little doubt that fishes swimming rapidly do not make respiratory movements at all, but obtain the necessary ventilation of the gills simply by opening the mouth.
It is no doubt technically possible to study metabolism and respiration of fishes during swimming at a constant rate, and of certain insects and birds during flight, and to obtain information similar to that obtained on man during work on a bicycle ergometer or a treadmill.
In the giraffe with a total height of 5 m., the heart is at a height of about 2.5 m., and it would be extremely interesting to know just how the giraffe avoids the development of filtration oedema in its long legs.
In many organisms, including man, the mechanical respiration and the circulation of the blood are 'regulated' so as to correspond to the demand of the moment.
The respiratory mechanisms of birds are definitely adapted to the function of flight, as evidenced by the fact that birds which do not fly (Apteryx, Penguins) show these adaptations in a greatly reduced form.
