Cities persist by routing flows through heterogeneous media. Water crosses watersheds, air and roots move through soil pores, parcels pass through hubs and streets, and instructions circulate through digital systems that coordinate workers. These flows are not secondary to urban form; they constitute the operational city. A surface, a soil profile, a micro-hub or an algorithm matters because it changes where movement can occur, at what speed, with what resistance and under whose control.

The material ground of metabolic routing is permeability. Leopold shows that urbanisation alters hydrological timing by replacing absorptive and distributed conditions with surfaces and systems that accelerate runoff. Paradelo and colleagues make the same problem visible inside the soil itself: compaction, sealing, organic matter and preferential channels reorganise the capacity of ground to admit and transmit water. Permeability is therefore designed, whether intentionally or not. Logistics introduces another form of routing in which urban space is converted into a network of intermediary positions. Castillo’s micro-hubs demonstrate that location and movement cannot be planned independently. A strategically placed node changes the efficiency of an entire distribution system. The city becomes topological: what matters is not only distance but the arrangement of transfer points, transport modes and permissions through which goods can change state and direction. Algorithmic management reveals that routing also acts upon people. A delivery itinerary, a warehouse instruction or a healthcare scheduling system can reduce uncertainty for an organisation while narrowing discretion for workers. Digital coordination redistributes agency along the chain. Efficiency therefore cannot be assessed only through travel distance or throughput; it must include the authority embedded in the route and the capacity of situated actors to modify it. Socioplastics can use metabolic routing to understand how a large corpus becomes traversable. Bibliographic clusters, readers, DOI anchors, indexes and cross-links function as intermediary nodes that shorten conceptual distances and make transitions among fields possible. The quality of the system depends not only on accumulation but on permeability: whether users can move between scales, disciplines and media without losing provenance or becoming trapped in one classificatory route. Metabolic routing adds a material and organisational criterion to Socioplastics. Flows require channels, but channels always shape what they carry. Excessive sealing creates runoff; excessive algorithmic prescription reduces autonomy; poorly located hubs increase friction; weak indexing leaves knowledge stranded. A productive field maintains structured permeability—enough resistance to preserve distinctions and enough connectivity to permit movement. The concept adds an urban metabolism of knowledge in which routing, maintenance and agency become inseparable.