Each box represents a Blockchain ledger. The Patient Directory lists all pseudonymised patients about whom data is stored across all providers’ Blockchain ledgers. Associated with each patient is a pointer to all centres (represented by number) where that individual has had a medical interaction. Individual Centres store their own ledgers, containing more detailed metadata (represented by code, e.g. Data A1, which does not imply storage of sensitive medical information) about interactions with patients, including associated cloud-based data storage locations and access permission information. When a Centre joins the system (e.g. Centre 3), basic pseudonymised information about its patients is relayed to the Patient Directory, allowing other Centres to access that information, subject to any associated smart contract-based permissions.