Under the new procedures, the sickest patients are still prioritized for receipt of livers donated in their local area and region, but they will also gain access to donated livers across the regional border, within a range of 170 miles.

Any modification to the system has life or death implications. Because the number of people in the United States who need livers vastly outnumbers the availability of donated organs, increasing accessibility to livers in one region inevitably means reducing it somewhere else.

Computer simulations conducted by U.N.O.S. suggest that New York City stands to gain the most from the changes, with an annual increase of 50 livers and a 21 percent decline in deaths for those on the waiting list. Places with a higher ratio of donor livers to recipients, among them the regions that include Ann Arbor, Mich., and Philadelphia, are likely to lose.

There are 58 so-called donor service areas in the United States, and their odd shapes and sizes reflect their origins, emerging organically from the nation’s first transplant centers in the 1960s and 1970s. For distribution purposes, the donor areas are configured into 11 larger regions.

In the early days of transplantation, there was little need for a complex distribution system because livers did not remain viable long enough to be transported very far. But medical advances have made it logistically possible to consider candidate organs from much longer distances.

When a person dies and donates a liver, potential recipients in the region are prioritized and the sickest gets first dibs. In the case of a tie, the system favors those in the local donor service area, and if none of the sickest patients can be matched with the liver, the organ is shared with other regions of the country.