INTRODUCTION

Efficiency, loosely defined, describes the ease with which a system performs its requisite tasks. By its more strict definition, it describes the ratio of the work produced by a system to the work expended on the system in performing that task. Several papers in this collection argue that the term ‘efficiency’ should be reserved only for the latter case. The former definition is more appropriately referred to as the effectiveness of the system. Thus, the efficiency of a respiratory system will refer only to the ratio of the mechanical work produced by the respiratory pump to the oxidative cost of producing that work. The effectiveness of this performance will also be a function of the many factors which influence the amount of gas exchanged between the environment and the blood for any given level of ventilation. This too can be regarded in terms of an input/output relationship and thus the effectiveness of a respiratory system can be defined as the ratio of the millilitres of O2 exchanged during ventilation to the millilitres of O2 required to power ventilation. The respiratory systems found in vertebrates reflect multiple experiments in design which attempt to optimize gas exchange across the respiratory surfaces given both the respiratory and non-respiratory constraints placed on the systems. Adaptive changes can be found at all levels which contribute both to the effectiveness and efficiency of each system. In the following article, select examples will be used to illustrate some of the trends which optimize the diffusive and convective processes involved in gas exchange (anatomical adaptations and their mechanical consequences) as well as the manner in which they are linked (physiological and behavioural adaptations).