"It also gives you a runner's high kind of feeling, which is why I've seen people who don't notice the pain or injury until they calm down," he said.
Dr. Alasdair K.T. Conn, the chief of emergency services at Massachusetts General Hospital, said he treated two individuals who walked away from a subway accident, only to later find that they both had broken necks.
"Patients get caught up in the moment. They have this sort of adrenaline rush and they just don't feel pain for a few hours," said Conn. "It's almost sort of disassociated from their body until they settle down.
"They kept saying they were fine or 'I have a bit of neck pain,'" said Conn. "They didn't feel the pain."
"It's amazing," he added.
Often, patients will seek medical attention for one injury without knowing they have another -- and sometimes much more serious -- injury elsewhere.
For example, Conn said he once treated a man who had fallen and broken his wrist and was in agony over the pain in his arm -- again, so much so that he did not feel the pain of a broken neck.
"That's the classic example of a distracting injury," said Conn. "Yes, you have more than one injury, but one is more painful than the other, and the adrenaline of the moment causes your pain threshold to go up."
"Distraction from pain is well established in medicine," said O'Brien. "I've had a young woman who had both a broken neck and broken femur, who didn't notice her neck pain because the leg fracture was so intensely painful."
Several doctors told ABCNews.com that the body's natural instinct is to fight -- even during the most excruciating pain -- or divorce themselves from their wounds.
"There are these moments that have windows where that serious injury can be overcome," said Dr. Stephen Hargarten, professor and chair of emergency medicine at the Medical College of Wisconsin. "The pain part of it is related to the body's extraordinary ability to manage pain in the way of producing endorphins, which are our own bodies' pain meds, and the adrenaline, which helps us fight to stay alive."
"The adrenaline rush is a real thing," added O'Brien. "The desire or drive to stay alive and stay conscious can indeed block pain perception."