An abrasive waterjet assembly has a cutting head assembly with a venting system for controlling the flow of abrasive within a cutting head body. The venting system includes one or more vents for regulating the pressure within a cutting head body to minimize, limit, or substantially eliminate any abrasive from reaching a jewel orifice. The vents include venting ports positioned between an orifice mount that retains the jewel orifice and a mixing region in which abrasive is mixed with a fluid jet produced by the jewel orifice. An isolator retained in the cutting head body further inhibits the upstream flow of abrasive, if any.