XRP acts as a bridge currency to other currencies. It does not discriminate between any fiat/cryptocurrency, which makes it easy for any currency to be exchanged for another. Each currency on the ecosystem has its own gateway e.g. CADBluzelle, BTCbitstamp, and USDsnapswap.
If David wanted bitcoins as payment for the services rendered to Lawrence, Lawrence does not necessarily have to be in possession of any bitcoins. He can send the payment to his gateway in Canadian dollars (CAD), and David can receive bitcoins from his gateway. One gateway is not needed to initiate a complete transaction; multiple gateways can be used, forming a chain of trust rippling across the users.