Everyone knows the stirring heroic story by now of the kidnapping and rescue of Captain Richard Phillips. But there is a medical back story that has been submerged beneath the tremendous skill shown by the Navy SEALS. That is the endurance of Captain Phillips himself.

Consider that Phillips was held captive on the Maersk Alabama lifeboat for 5 days beginning last Wednesday, and during that time, he was reportedly subject to temperatures approaching 100 degrees. It is doubtful that he received much food or drink, and he was clearly under great stress. I doubt he slept more than a few hours per night.

These medical observations only deepen my admiration for this man and his acts of heroism. He was clearly dehydrated, undernourished, and exhausted at the time of his rescue, and during his previous attempt at escape. He is not only a man of great courage, but also one of stamina and strength.

From a purely physiological point of view, we all have an untapped reserve of strength and endurance that may help us to survive under challenging circumstances. As I described in my book "False Alarm; the Truth About the Epidemic of Fear," the mechanism is known as fight or flight, or acute stress response. In response to a bodily threat we experience an outpouring of hormones, adrenaline, noradrenaline, the steroid cortisol. Our eyes dilate to see better, our hearts beat faster and harder, we are ready to fight or to flee.

None of us will ever respond any better than Captain Phillips did. His body was as strong as his mental resolve.

Consider that this man is 53 years old, older than this doctor. Compare that with the Navy SEALS, many of whom are in their late teens or early twenties and in top physical condition. Phillips appears to be in good physical condition, but not compared to one of the SEALS. I don't know whether Phillips exercises regularly, but I suspect he does, and he looks fit. But nothing could have prepared him for this ordeal, except for his great courage, and the spare powerpack that the body and brain have known as fight or flight. A good thing for America, Phillips and the navy chose to fight.

Dr. Marc Siegel is an internist and associate professor of medicine at the NYU School of Medicine. He is a FOX News medical contributor and writes a health column for LA Times, where he examines TV and movies for medical accuracy. Dr. Siegel is the author of "False Alarm: The Truth About the Epidemic of Fear"and "Bird Flu: Everything You Need to Know About the Next Pandemic." Read more at www.doctorsiegel.com

Dr. Marc Siegel, a practicing internist, joined FOX News Channel (FNC) as a contributor in 2008..