Fresh from Class AA Harrisburg, Nate Karns lasted 4 1/3 innings and kept the Orioles from inflicting any serious damage while his offense awakened to the tune of nine runs. (Jonathan Newton/The Washington Post)

As Nate Karns walked from the bullpen to the home dugout Tuesday night, the Nationals Park crowd along the first base line stood and cheered the Texan by way of Harrisburg and a major leaguer by way of survival. The Washington Nationals, in need of a stopgap start, had pulled him from Class AA. Karns had arrived because he had been read a pitcher’s death sentence — shoulder surgery — and come out the other side chucking mid-90s fastballs.

Karns, 25, collided with another rookie fresh from the Eastern League, Kevin Gausman, the prized right arm of the Baltimore Orioles. As the Nationals pummeled Gausman in a 9-3 victory, Karns made a debut representative of his path to the majors: He plodded forward, and as he allowed three runs over 41 / 3 blurry innings, he survived.

“I don’t remember much,” Karns said.

The Nationals summoned Karns out of desperation, but he will stay for at least one more start. With Ross Detwiler still affected by the oblique strain that landed him on the 15-day disabled list earlier this month, the Nationals scheduled Karns to start Sunday in Atlanta.

“I thought the kid pitched great,” Manager Davey Johnson said. “He should be proud of what he did.”

The Post Sports Live crew discusses what would happen to the Nationals’ lineup if Bryce Harper keeps suffering minor injuries. (Post Sports Live)

More dominant starts await Karns, based on the promise of the 95-mph heat coming out of his right hand. Tuesday night, he left the heavy lifting to his lineup — which provided the Nationals’ highest run total since April 15 – and bullpen.

First baseman Adam LaRoche launched a mammoth, three-run homer in the first and added a solo blast in the eighth, his 10th of the year. After the Orioles tied it in the fourth, scuffling reserves Tyler Moore and Roger Bernadina — hitting a combined .273 coming in — launched back-to-back homers, giving the Nationals control for good.

Karns pitched before a crowd of 35,644 – about 30,000 more than had ever watched him before – that included his parents, David and Tambra, who had flown in from Dallas.

“My mom was probably an emotional wreck,” Karns said. “She made a lot of sacrifices for me to get here. I’m glad I could reward her with my debut.”

Monday afternoon, Karns’s manager at Harrisburg, Matt LeCroy, gave him an odd chore. First, LeCroy told him his start would be postponed. Then he instructed Karns to chart pitches thrown to Orioles’ hitters as they played the Nationals.

“I was kind of thrown off by that,” Karns said. “I was like, okay. I’ll chart the big league game, and we’ll see how this is going to help me against Akron.”

Finally, LeCroy let him know: He was headed to the big leagues. When all 6 feet, 5 inches of Karns climbed the mound after a 1-hour, 21-minute rain delay, it provided an organizational success. The Nationals had signed him out of Texas Tech in 2009 with a bonus above his 12th-round slot. He missed all of 2010 and most of 2011 after a labral tear in his right shoulder.

Karns rehabbed through grueling, sweaty days in Viera, Fla. Last fall, after he punched up a 2.17 ERA at Class A, the Nationals named him their minor league pitcher of the year. In the spring, added to the 40-man roster, he went to Viera again, and, “he showed he could get major league hitters out,” General Manager Mike Rizzo said.

“He’s come a very long way,” Nationals director of player development Doug Harris said. “He persevered, worked his tail off every day through some times it would have been easy for him to take a different path. He’s seeing the fruits of his labor.”

So there was Karns, strutting in from the bullpen. He slapped the first high five he saw waiting in the dugout — Stephen Strasburg’s meaty right hand. The song of his choice, “La Grange” by ZZ Top, blared while he threw his warmup pitches. Teammates reminded to slow the game down, pitch at his pace.

“It looked like he had walked through the shower before he even took the mound,” LaRoche said. “I’m sure his world was spinning pretty fast.”

In a 1-2-3 first inning, Karns attacked. He threw Manny Machado a 97-mph fastball. He induced all grounders, typical of his style. Over 2161 / 3 innings in the minors, Karns had allowed eight homers. He “pitched downhill,” Harris said, throwing fastballs at a downward angle.

“He was doing exactly what he needed to do,” catcher Kurt Suzuki said. “He was having fun.”

The Orioles scored one in the second, which Karns escaped with two strikeouts to leave runners on the corners. In the fourth, Chris Davis blistered a 95-mph fastball into the home bullpen. Two batters later, J.J. Hardy tomahawked a 92-mph heater over the left-center field wall. Suddenly, Karns faced a tie.

After LaRoche walked in the fourth, Moore launched Gausman’s 0-2 slider over the left field fence, his second homer. Bernadina, hitting .121 entering the game, followed with a laser over the right field wall — his first. When the Nationals finished with Gausman, they led, 7-3.

Karns needed three more outs for a win. Worn from the adrenaline of his debut, he walked two Orioles in the fifth. Johnson removed him, and Zach Duke induced a double play from Nick Markakis, the start of 42 / 3 scoreless innings of relief.

Karns watched from the dugout, content he had helped the Nationals win, proud he had made it to the majors and ready for more.

“It’s been a long road,” Karns said. “I’m just glad the Nats stuck with me.”