Medically and legally, Jahi McMath has passed away.

Yet for her family, and for an array of organizations and physicians who question the medical establishment’s practice of pronouncing patients in her condition deceased, McMath has not gone anywhere. Her lungs continue to breathe with the help of a ventilator, and her heart is still beating.

Children’s Hospital and Research Center at Oakland released the 13-year-old girl to the county coroner Sunday evening, according to a statement from the hospital. Complying with a court order, the coroner then let McMath’s family reclaim her body, still connected to the ventilator.

The family’s attorney has declined to say where she is now, except that she is under medical supervision. It isn’t clear what will happen to her next — whether she’ll go to another facility for continued care, or how long she can remain in her current state.

What is clear is that sometime after her tonsillectomy to treat sleep apnea went awry last month, McMath entered a condition that is controversial among medical ethicists and often confusing for the general public.

She is brain-dead, according to several doctors who have examined her. Here we try to answer a few questions about what the diagnosis means.

What is brain death?

By law, a person who is brain-dead is deceased. It’s a condition made possible by modern medicine, in which a patient’s brain has ceased functioning but his or her organs are maintained by machinery and sometimes drugs.

Brain death differs from a persistent vegetative state, which was the condition of Terri Schiavo before her husband removed her from life support in 2005 in a well-known case. Unlike Schiavo, and unlike a comatose patient such as former Israeli prime minister Ariel Sharon, a brain-dead patient has absolutely no activity in his or her brain.

Also, while comatose patients sometimes wake up, brain death is permanent. Thomas Nakagawa, who specializes in pediatric intensive care at Wake Forest Baptist Medical Center, compared brain death to gangrene in a limb. When blood flow is lost, the tissue dies and begins to decay.

“There comes a point in time when there’s nothing left to do. The same thing occurs in the brain,” Nakagawa said.

The brain is particularly fragile because it is enclosed by the skull. If it is damaged and begins to swell (whether because of trauma or, as in McMath’s case, because of a loss of blood flow and oxygen), then the pressure from the walls of the skull can injure the organ further.

How long will McMath’s heart continue to beat?

Generally, a brain-dead person’s heart will stop in a few days or weeks, but there have been patients whose hearts have continued to beat for much longer following the diagnosis of brain death.

A 1998 study by Alan Shewmon, a neurologist at the University of California, Los Angeles, documented dozens of cases in which the hearts of brain-dead patients continued to beat for more than a week. A large majority stopped within a year, but some continued for as long as 14 years. In one case, a pregnant woman who became brain-dead carried her child to term.

Don’t brain-dead people sometimes wake up?

“There is no hope for recovery from brain death. It’s death,” said David Greer, a neurologist at Yale School of Medicine.

Physicians do occasionally make mistakes. An inexperienced doctor might confuse the effects of a powerful sedative for the destruction of the brain and wrongly pronounce a patient dead. When the drug wears off, the patient’s brain might begin to function again to some degree.

Greer believes that such errors are very rare but that because they often make headlines, they give families of patients a false sense of hope. He has devoted much of his time to developing and promoting a rigorous, standard procedure for diagnosing brain death.

How do you tell if someone is brain-dead?

A brain-dead person isn’t stiff or cold. The person still has a pulse and continues to breathe. “It just looks like the patient is asleep,” Nakagawa said.

He is the principal author of a widely accepted set of guidelines for determining whether children and infants are brain-dead, a procedure that Children’s in Oakland follows, according to court records. The procedure is designed to test some of the patient’s most primitive functions to be sure the brain has stopped working entirely.

Some elements of the procedure would be uncomfortable, if not excruciating, for someone who is fully awake. McMath’s doctors would have pressed firmly on her brow and on the joints between her jaw and her skull to make sure she did not respond to pain. They would have inserted a tube into her windpipe to make sure she can no longer cough, and they would have filled her ears with ice water to make sure her eyes do not move reflexively.

If your heart is beating and you’re still breathing, are you really dead?

The definition of brain death is based on the idea that a patient whose brain has permanently and entirely stopped working is no longer alive. Yet in some religious faiths, including certain interpretations of orthodox Judaism, life ends with the heartbeat.

McMath’s uncle told the Associated Press the that the family was hoping the girl could miraculously recover.

They have arranged for an air ambulance to fly her to a facility in New York that has agreed to keep her attached to a ventilator, but surgeons at Children’s refused to perform the necessary procedures to prepare her for the flight. They argue it is unethical to operate on the dead.

Some medical experts have likewise argued that a person is dead only when he or she no longer has a pulse.

“In my professional opinion Jahi is not a cadaver,” wrote Paul Byrne, a physician and former president of the Catholic Medical Association, in court papers. “Her heart beats thousands of times a day.”

Byrne visited McMath but was not able to examine her because he is not licensed to practice medicine in California.

What will happen to McMath now?

Her family will have to find a physician willing to prepare her for the flight to a new facility. Beyond that, it is difficult to know how long her heart will continue to beat.

Eventually, McMath’s pulse will cease, and her family, along with the medical staff who have tended to her for the past weeks, will have to come to terms with her death, one way or another.

“There are families that never come to the realization that their loved one has died, or that their child has died, even after the funeral,” said Nakagawa of Wake Forest.

But the death of a child following complications from a tonsillectomy is bound to be even more difficult to accept — especially if she looks as though she’s merely sleeping.