A doctor caring for an 8-year-old boy whose arm was reattached after it was bitten off by a shark says he is still at risk of dying from brain swelling.

"Because of the shark injuries and loss of blood associated with that, his brain did go through a period of time with a very low amount of blood flow," Dr. Rex Northup said.

Northup said the next few days are crucial because brain swelling can lead to death if the pressure in the skull exceeds blood pressure, which would stop bloodflow to the brain and cause death.

Jesse Arbogast, of Ocean Springs, Miss., was swimming at Gulf Islands National Seashore in the Florida Panhandle Friday evening when he was attacked by a 7-foot bull shark, which tore off his arm.

Doctors were able to reattach the arm, but Jessie went into kidney failure on Sunday and has been undergoing dialysis since. He remains unconscious and in critical but stable condition.

Northup, chief of pediatric care at Sacred Heart hospital, said Jessie arrived Friday night with no blood pressure, no pulse and damage to "literally his entire body" because of severe blood loss.

"It's going to be a very tough situation to pull him through," Northup said at a press conference in Pensacola, Fla.

"Our hope is we don't see [him] deteriorating or worsening. If we can get another several days behind us where things don't deteriorate, we'll be happy with that."

Northup described himself as "cautiously optimistic" and said Jesse's reattached arm and severely gashed leg are healing well. He said circulation in the arm and leg was good, although Jessie likely won't be able to use the arm for up to 18 months.

The boy has had six operations since Friday's attack.

"He has done a little bit of a spontaneous eye opening and blinking of his eyes, but at this point is not coherent," Northup said.

Jessie's uncle, Vance Flosensier, of Mobile, Ala., wrestled the shark to shore after the Friday attack.

"He's a big guy. He got hold of it and tossed it ashore," District Ranger Supervisor John Bandurski said.

Ranger Jared Klein then shot the shark four times with a 9 mm pistol, and pried its jaw open with a police baton. Volunteer firefighter Tony Thomas used a clamp to pull the boy's severed arm out of the shark's gullet.

The boy had no pulse and no blood pressure when he was airlifted to Baptist Hospital about 30 minutes after the attack, said Dr. Jack Tyson, a surgeon.

"He had essentially lost all of his blood," Tyson said. He was transferred to Sacred Heart because Baptist Hospital, where surgeons reattached his arm, is not equipped to treat kidney failure.

Ian Rogers, the plastic surgeon who helped reattach the arm, had said he was hopeful the boy could regain near normal use of his arm in 18 to 20 months with extensive therapy.

The Associated Press contributed to this report.