A 27-year-old Anderson man who was struck by a big rig Friday night in the middle of Interstate 5 was going to have his right arm amputated, the California Highway Patrol reported today.

Todd Mort also suffered head and chest injuries when he was hit about 6:40 p.m. in the southbound lanes of I-5 just south of Shasta Dam Boulevard in Shasta Lake, a CHP spokesman said.

Mort was a passenger in a vehicle going north on I-5 when he was either asked to get out or requested to get out because of an argument with the driver, the spokesman said.

Mort called his girlfriend for a ride while he was standing on the right shoulder of northbound I-5.

His girlfriend was traveling south on I-5 south of Shasta Dam Boulevard as Mort crossed the freeway to meet her, the spokesman said.

Mort began waving his arms to get his girlfriend's attention while he was standing in the middle of southbound I-5. With his back to traffic, Mort was hit by the left-side mirror of a tractor-trailer driven by Mandel Rodriguez, 49, of Daly City, the CHP said.

The mirror hit Mort's extended right arm and he was thrown to the pavement. The driver pulled to the right shoulder and stopped.

Mort was taken to Mercy Medical Center in Redding and diagnosed with head injuries, chest trauma and his right arm was to be amputated, the CHP spokesman said. He was listed in serious condition today, a Mercy nursing supervisor said.