  A model is proposed to minimize the total volume of the main distribution
networks of fluids in relation to the organ form. The minimization analysis
shows that the overall exterior form of distribution networks is a modified
ellipsoid, a geometric form that is a good approximation to the external
anatomy of the kidney and lung. The variational procedure implementing this
minimization is similar to the traditional isoperimetric theorems of geometry.
  A revised version of this preprint that expands Section 4 will be published
in the Journal of Biological Systems, World Scientific Publishing.
