The heavy bag shudders as it swings slightly, suspended by a chain. Jason Quirk punches hard, lost in a physical workout that has two purposes.

He’s training for his pro boxing debut Saturday night at the Portland Expo. His gloved left hand slams into the heavy bag. He follows with a right, then a succession of rights, maybe against an imaginary opponent.

Getting lost in concentration during his workout also helps Quirk forget a more important part of his 25-year-old life. He is a rookie firefighter for the Portland Fire Department who was called to the Noyes Street house fire that took the lives of six people.

Nearly 10 years of fighting for the Portland Boxing Club as an amateur has prepared him for his pro debut. Very little in his four years of active duty with the Coast Guard or his six months with the PFD gave him the experience to deal with Maine’s deadliest fire in 40 years.

“I try not to think about it,” said Quirk, a middleweight. “I try to compartmentalize. When I’m in the gym, I’m strictly working out. That’s where my mind is.

“It is a little hard. No question it was a tragic event.”

He was off-duty Nov. 1, but was called in for the third alarm on Noyes Street. The fire was out, but the site needed to be cleaned up. Quirk was there when the victims were taken from the house. The photos of firefighters with their helmets removed in respect for the deceased were poignant.

“I don’t want to go into details,” said Quirk. “It was a pretty somber scene.”

He drew strength from his fellow firefighters. He was the new guy but he had passed another test.

Portland firefighters were not unlike Quirk’s fellow fighters at the Portland Boxing Club.

“It is like a locker room here,” said Tony DiPietro, a firefighter at Portland’s Bramhall firehouse. “We kid with each other, we make sure no one gets a big head. But we know we have a job to do and we know we’re all here for each other.”

Quirk is in a pool of firefighters, assigned to different firehouses in Portland based on who might be on vacation or sick. He works two 24-hour shifts a week.

About a dozen firefighters from the Bramhall station have bought tickets to Saturday’s fight card. The Russell Lamour-Ahsandi Gibbs fight for the IBA North American Middleweight Championship is the main event. But to Portland firefighters, Quirk is the draw.

“He’s one of us,” said DiPietro. “He’s a good kid, pretty modest. He hasn’t really been talking up this fight. We’re going to show our support.”

After playing football and hockey for Scarborough High School, Quirk turned to boxing for its personal challenge of stepping into the ring alone. He was 15 when he first walked through the gym door. He fought his first fight about a year later.

Maybe one in 100 return week after week and year after year, said Bob Russo, owner of the Portland Boxing Club and its head trainer. Quirk had to prove himself.

“He’s respectful, hard-working, organized, disciplined. … I can’t say enough about Jason,” Russo said. “His parents go overboard doing things for people and he’s a product of them.

“In the ring he’s a big puncher. He can bang,” Russo said. “He’s got a cut-your-teeth kind of opponent.” Meaning, Russo didn’t match Quirk with a veteran fighter.

Quirk said he’s won 27 of his 40 amateur fights. He was about to fight for the New England Golden Gloves championship and the invitation to the national tournament when the call came from the fire department. There was an opening and he was asked to fill it. Quirk didn’t think twice and dropped out of the Golden Gloves tournament.

He had enlisted in the Coast Guard for four years of active duty to serve his country. He joined the fire department to serve his community.

One way or another, he wanted to have an impact on lives.