The constructal theory (of global optimization in the presence of local constraints) simply explains the complexity of the forms that arise in nature. For example, the shape of a cactus makes it possible for it to survive in land without water, or the shape of the lungs and bronchi, which maximize the uptake of oxygen in a very small volume, or the structure of a tree.

Flow-based systems (both animate and inanimate) depend, for their very survival, on their ability to maximize flow access on the one hand, and on the other, to modify their shape or morphology.

This theory allows the design and understanding of natural systems, heat sinks, communications networks, etc.

Constructal theory was developed by Adrian Bejan in the late 1990s. Bejan was teaching at MIT until 1976. He is currently associated with the Department of Mechanical Engineering at Duke University. His research area covers subjects such as minimization of entropy generation, energy analysis, condensation, convection in porous media, transition to turbulence, etc.

The term constructal is a neologism coined by Bejan that comes from the verb construere (Latin) and means to build.

Unlike biomimetic theories that reproduce shapes that appear in nature for use in man-made designs, constructal theory goes a little further back. It focuses on the principles that govern the appearance of these forms (and not merely the forms) and applies them to the design of similar flow systems.

Constructal theory explains how certain basic phenomena, optimized individually and collectively, can be used to construct more complex natural systems, within the restrictions imposed by the physics of the problem in each case.

According to constructive theory, the principle under which the geometry of natural systems can be deduced is constituted by the optimized form and structure obtained from engineering analysis. It is a predictive theory for geometry and rhythm in nature, in contrast to the rather descriptive techniques of fractal geometry, which has become fashionable in recent years, where a repetitive sequence of operations is assumed that generates an image similar to that of a tree with its branches.