A Boston police officer wheels an injured boy down Boylston Street as medical workers carry an injured runner. (Charles Krupa / AP)

The Boston Marathon bombing injured more than 170 people, many sustaining critical wounds that required amputations. That makes it all the more remarkable that, as Atul Gawande points out, all have survived.

It's remarkable, and the product of years of preparation. In the years after the September 11 terrorist attacks, hospitals have invested significant resources in preparing for trauma situations like this one.. "Almost every hospital has a surgeon or nurse or medic with battlefield experience," Gawande writes.

Massachusetts General Hospital, one of the top hospitals in the country, once brought in Israeli doctors to overhaul its disaster response procedures. On Monday, Gawande reports from a number of hospitals, those preparations paid off. Here's what happened at Brigham and Women's Hospital, where he works:

They broke up into teams of six or so people, one trauma team for each patient. A senior nurse and physician stood at the door to the ambulance bay triaging the patients going to the teams. The operating-room director handled triage to, and communication with, the operating rooms. Another staff member saw the need for a traffic cop and began shooing extra clinicians into the waiting room where they could stand by to be called upon.

Another factor that could have been at play: The strength of Massachusetts' (and Boston's) hospital system. A non-profit called Leapfrog grades hospitals each year on safety, based on their ability to reduce things like infections and medication errors. Leapfrog gave 83 percent of Massachusetts' hospitals an "A," making the state's hospitals the safest in the country. Nationally, only 30 percent of hospitals got this top grade.

Boston's hospitals are safe—and also numerous. The Advisory Board, a hospital consulting firm, came up with this map of hospitals that absorbed patients.

Six of the eight hospitals pictured above are within approximately two miles of the bomb site.

After the Aurora shooting, Gawande recalls the chief of medicine at his hospital giving a lecture titled "Are We Ready?"

"In Boston," he concludes, "It turns out we all were."