Defensive tackle Haloti Ngata and left tackle Bryant McKinnie both made their 2012 training camp debut for the Ravens on Friday as both passed their conditioning tests and suited up in shells for practice.

Ngata was dealing with a hamstring injury sustained during a prior conditioning test attempt and McKinnie has been out with a back injury that resulted from a fall at his South Florida home.

"Bryant McKinnie passed with a diving effort at the end, I might add," said coach John Harbaugh. "It's a tough test. You have to be in great shape to pass that test, so congratulations to them."

Neither McKinnie or Ngata spoke following Friday's practice but will be available after Saturday's session that's open to the public at M&T Bank Stadium. Both players went through individual drills but didn't rotate in with the team. Harbaugh said both players have to be in shells for two practices before participating in team drills.

Jimmy Smith diagnosed: Cornerback Jimmy Smith injured his back during Thursday's practice, Harbaugh confirmed. Harbaugh said it's a muscular injury and added it's something Smith's dealt with dating back to his college playing days at Colorado. It's not believed to be serious though a timetable is uncertain. Defensive end Arthur Jones missed another practice and has a hip flexor injury. He was scheduled to have an MRI but the team has yet to receive the results.

Five in a row: Center Matt Birk missed his fifth consecutive practice and it's not just because of his age. Harbaugh said Birk's experienced some back spasms and that the team is holding him out. Birk may not practice until after Baltimore's first preseason game against Atlanta.

Logjam at WR: The top four receivers may be set on Baltimore's roster, leaving two open spots. Tandon Doss was though to have one of those locked up before a hamstring injury that's kept him out of practice. Wideouts Tommy Streeter, Deonte Thompson, Logan Payne and Dorian Graham are battling it out as Harbaugh and the coaches continue to evaluate them.

"They've all flashed plays, they've all made plays. The ability to stack a good practice on top of a good practice, or a good series on top of a good series, or a good play on top of a good play will be a huge determinant."

Absent from practice: OLB Courtney Upshaw (shoulder), FB Vonta Leach (back), CB Cary Williams (undisclosed), CB Asa Jackson (undisclosed), OL Jah Reid (calf), C Matt Birk (back), CB Jimmy Smith (back), DT Arthur Jones (hip flexor), LB Dannell Ellerbe (undisclosed), TE Dennis Pitta (hand), OLB Terrell Suggs (Achilles), RB Bernard Pierce (hamstring), WR David Reed (ACL), WR Tandon Doss (hamstring), LB Josh Bynes (undisclosed), WR Patrick Williams (undisclosed).

Marshal's message: Following practice, right guard Marshal Yanda told his teammates to, "Embrace the grind" of training camp. With injuries piling up and the summer heat beaming down, Harbaugh felt this statement was appropriate.

"It's a tough world out there," Harbaugh said. "People are struggling and we have a chance to play football. Embrace the grind."

Follow Ravens reporter Jason Butt on Twitter: @JasonButtCBS and @CBSSportsNFLBAL.