"Pressure built up perhaps too quickly for the brain to accommodate," said Dr. Jonathan Fellus, who specializes in brain injury at the Kessler Institute for Rehabilitation in New Jersey.
Fellus said that when left untreated, fluid can be deadly for brain injury patients.
Giffords was brought by a specially equipped private plane from Tucson to Houston's Hobby Airport last Friday and immediately was transported to the Texas Medical Center's intensive care unit by medical helicopter.
Before departing for Houston Friday morning, Giffords left the Tucson, Ariz., hospital where she had been treated since suffering a gunshot to her head in an attack over two weeks ago. She was the last shooting victim to leave the hospital.
Along her roughly 10-mile route to the Air Force base, crowds of supporters stood on the curb, cheering and waving flags and signs as her motorcade passed by. Giffords' ambulance was escorted by a squadron of military veterans on motorcycles.
"We could hear applause in the ambulance with Gabby, and she responded very well to that, smiling and even tearing a little bit," said Dr. Randall Friese, one of the doctors who treated Giffords in Tucson. "It was very emotional and very special."
At a press conference in Houston late Friday afternoon, Giffords' medical team said that the trip went flawlessly and that while she has a long road ahead, she has great potential for rehabilitation.
Giffords' husband, Kelly, who lives in Houston, said last Thursday that she soon would be "back at work" and predicted she would be walking around in a couple of weeks.
"I'm extremely hopeful that Gabby will make a full recovery," he said. "She is a fighter like nobody else I know."
In other signs of progress, Giffords was taken outside Thursday for the first time since the attack so she could get some sunlight. She stood up Wednesday for the first time with assistance and looked out of a hospital window.
Giffords already has been able to scroll through an iPad, one of many steps described by her doctors as "fantastic achievements forward."
The Associated Press contributed to this report.