On asymmetric and incomplete bronchial trees*

Multiple forms of life depend on branched flow networks for physiological transport processes. Human airways are one among them and is a brilliant example of how evolution has developed optimal designs for biological systems. Existing body of research on anatomy of bronchial trees and air-flow physics within them has discovered the existence as well as benefits of asymmetry in branch divisions. The present study builds morphometric parameter-based mathematical models of the bronchial trees to study the effects of asymmetry and incompleteness due to anatomical limitations of the body on the area available for gas exchange, resistance to fluid flow, volume occupied the bronchial tree and particle deposition within the bronchial tree. We observe that maximum surface area for gas exchange, minimum resistance and minimum volume are obtained at either symmetry or very small asymmetry. But with increasing asymmetry, deposition of inhaled foreign particles in the non-terminal airways is enhanced. This aids in self-defence of the host against virus laden aerosols as well. Thus, we can appreciate how natural asymmetry present in human lungs provides an optimal design taking care of maximising gas exchange and protection while minimising resistance and volume.