The main objective of this book is to present some of the fundamental structural aspects in the design of respiratory organs while debating and speculating on when, how and why these states were founded. The author's main thesis is that the modern gas exchangers are products of protracted processes that have entailed adaptation to specific environments and lifestyles. Only those feasible designs that have proven adequately compentent in meeting demands for molecular oxygen have been preserved. The author's approach has been to look into the limiting functional properties as regards the respiratory capacities of gas exchangers while finding out the specific structural adaptations that have evolved to meet the metabolic needs or to look into form and discern how it limits function.