Constructal Law in Biosystems: The Physics of Design and Evolution in Flow Architectures

Introduced by Bejan in 1996, the constructal law asserts that a finite-size flow system must evolve into configurations that provide easier access to the currents that flow through it in order to persist in time. This paper synthesizes the application of this principle to biosystems across multiple levels of biological organization, unifying evidence from physiology, animal locomotion, plant biology, microbiology, cellular biology, neurobiology, and social biology within a common framework. Tree-shaped vascular and bronchial networks; Kleiber's 3/4 power law of metabolism; the 1/4 power laws of breathing and heartbeat; the unified mass scaling of running, swimming, and flying; the architecture of root-trunk-canopy systems; the morphology of bacterial colonies and stony corals; the optimal microvasculature of the brain; the stepwise growth of cell clusters; the geometry of social-insect nests; and the design of athletic performance can be placed within a single physical framework. Several of these are derived as theorems of that law, while others are better described as interpretations or analogical extensions of it. Rather than competing with natural selection or the laws of thermodynamics, the constructal law completes them by placing design and evolution within physics as a first principle. The framework offers a deterministic, falsifiable platform for predicting biological architecture and for engineering biomedical flow systems such as vascularized tissue scaffolds and chemotherapeutic protocols.