Todd Heap remembers catching a pass in the end zone ... and then darkness.

It was four years ago against Denver that Heap thought he had scored a touchdown before Broncos linebacker Al Wilson forced an incompletion by planting his shoulder underneath the Ravens tight end's chin.Heap's helmet flew off. His body fell backward to the turf. And he blacked out.

But like so many players in a sport that glorifies personal sacrifice for victory, Heap returned in a matter of plays, making more catches and absorbing more hits.

"I was like, `They're not going to stop me from getting on the field,' " said Heap, who was checked out on the sideline and cleared to play. "So the awareness has to be there to protect us from ourselves."

Tomorrow's preseason opener against the Philadelphia Eagles is the first time the Ravens will play in a game since recent studies claimed multiple traumatic concussions contributed to early dementia, severe depression and the deaths of four former players.

Ravens center Mike Flynn, who is in his 10th season, said he doesn't need statistical data to warn him about the dangers of concussions.

"I got to believe your body is not made to be hit like that, so there's going to be some long-term effects," Flynn said. "And if they tell me there isn't, I'll say, `Well, let's take your head and smash it for 11 years.' "

So, why does Flynn continue to play?

"We live in the moment and we think we're invincible," said Flynn, who thinks he gets concussions more frequently at this stage of his career. "But believe me, it's a constant debate with older players: `I love playing and I feel good now, but what is this really doing to me?' "

According to the NFL's mild traumatic brain injury (MTBI) committee, there is no evidence multiple concussions have a cumulative effect.

But the Center for the Study of Retired Athletes at the University of North Carolina contends that players who have suffered at least three concussions have triple the risk for neurological disorders later in life.

The MTBI committee criticized these findings because they are based on surveys and not scientific data.

The league is conducting its own study on retired players, but the results of this research could take two to three years.

"We don't know for sure the long-term effects of concussions as it relates to football," said Dr. Andrew Tucker, the Ravens' physician and a member of the MTBI committee. "Some of the data out there is interesting and it's concerning. But we have to build on that."

After a league-wide symposium in June, the NFL has taken steps to address the severity of concussions, starting in training camp this year.

The league has implemented a whistle-blower system, which allows injured players to anonymously notify the NFL if they are feeling forced to play when they are not ready to do so.

The NFL also will require all players to undergo baseline neuropsychological testing starting this season, which helps teams compare the mental functioning of an athlete before and after a concussion.

Other measures include enforcing rules that require players to properly buckle their chin strap (which reduces the chance of a player losing his helmet and injuring his head more seriously) and distributing a brochure to help educate players and their families about concussions.

"The most extreme form of prevention would be not to play the game because it is certainly a game that has risks," Tucker said. "And those risks will never be made zero."