  Transport of fluid through a pipe is essential for the operation of
macroscale machines and microfluidic devices. Conventional fluids only flow in
response to external pressure. We demonstrate that an active isotropic fluid,
comprised of microtubules and molecular motors, autonomously flows through
meter-long three-dimensional channels. We establish control over the magnitude,
velocity profile and direction of the self-organized flows, and correlate these
to the structure of the extensile microtubule bundles. The inherently
three-dimensional transition from bulk-turbulent to confined-coherent flows
occurs concomitantly with a transition in the bundle orientational order near
the surface, and is controlled by a scale-invariant criterion related to the
channel profile. The non-equilibrium transition of confined isotropic active
fluids can be used to engineer self-organized soft machines.
