Constructal Theory

The constructal theory puts forth the idea that the generation of design (configuration, pattern, geometry) in nature is a physics phenomenon that unites all animate and inanimate systems, and that this phenomenon is covered by the Constructal Law stated by Adrian Bejan in 1996: "For a finite-size (flow) system to persist in time (to live), its configuration must evolve such that it provides easier access to the imposed currents that flow through it."

Interestingly so... the examples they use to define this theory includes BOTH water and blood.... 

Some domains of application

Application	What flows	Tree channels	Interstitial spaces
Packages of electronics	Heat	High-conductivity inserts (blades, needles)	Low conductivity substrate
Urban traffic	People	Low-resistance street car traffic	Street walking in urban structure
River basins	Water	Low-resistance rivulet and rivers	Darcy flow through porous media
Lungs	Air	Low-resistance airways, bronchial passages	diffusion in alveoli tissues
Circulatory system	Blood	Low-resistance blood vessels, capillaries, arteries, veins	diffusion in capillaries tissues

Interstitial Space= "small opening or space between objects"