The doctor who performed Trayvon Martin's autopsy says he has changed his opinion in recent weeks on two matters related to the teen's death.

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During testimony Friday in the George Zimmerman murder trial, Dr. Shiping Bao said he had changed his opinion about how long Martin was alive after being shot and the effect of marijuana detected in Martin's body at the time of his death.

The associate medical examiner said last November that he believed Martin was alive one to three minutes. But he testified Friday it was one to 10 minutes.

He also said that marijuana could have affected Martin physically or mentally, even though he said it didn't last year.

Earlier in the day, Zimmerman defense attorneys objected when Bao described Martin's condition after being shot.  Judge Debra Nelson sustained the objection, and Bao was directed away from that line of questioning by the state.

Bao said Martin didn't die immediately and his heart was still beating after the shot was fired.

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"He was still alive, he was still in pain," Bao said. The defense objected to Bao giving his opinion and Nelson sustained it after a bench conference.

Bao explained what an autopsy consists of to the jury. He then described Martin's autopsy in particular.

Bao said Martin was 5 feet 11 inches and 158 pounds when he died. Jurors were then shown a picture of Martin's body when it arrived at the medical examiner's office. Both Martin's mother, Sybrina Fulton, and brother, Jahvaris Fulton, left the courtroom. His father, Tracy Martin, stayed in the courtroom as the photos were shown.

Bao told jurors about the pictures and said Martin was healthy at the time of the shooting. He said Martin had a small abrasion on his fourth and fifth left finger.

Bao also said there was a zero chance for survival with Martin's gunshot wound.

"His heart was beating until there was no blood left," Bao said.

Bao said Martin was shot from intermediate range, defined as muzzle being between 0.4 inches to 4 feet away from the target. Bao said the muzzle had loose contact with clothing.

When asked by de la Rionda about the positioning of Zimmerman and Martin, Bao said, "I have no fact and zero opinion."

Bao then said a key statement that he didn't believe Martin "can move after shot." That would debunk the defense theory about Martin's hands being moved from stretched out, as Zimmerman says, to underneath his body, as he was found by paramedics.

Defense attorney Don West cross-examined Bao about who helped him with the autopsies, with Bao saying two technicians assisted him.

West focused on how Martin's body was transported from the crime scene. Martin's hands were not bagged to preserve evidence as they should be, Bao said. West asked what time the medical examiner's office got to the scene. Bao said according to the note, the office arrived about two hours after the shooting occurred.

"It's not my evidence.  It's not my opinion.  These are other people's," Bao said, as West asked about him reading the report. Bao said he can't read the report because it's not his note and he's under oath so if it was wrong it would be perjury.

"You cannot ask me a question I do not know," Bao said to West.

West then asked when the ME officer representative left. Bao said he didn't have that information. Bao then said he didn't remember the exact autopsy without his notes and he must rely on his report.

Bao said the clothing should have been put in a paper bag and not a plastic bag.

"There is no plastic bag other than the bag that carries the body," Bao said, adding if someone from his office put wet clothes in plastic bag, "they'd be fired the next day."

The defense requested to see Bao's notes he personally took from the stand. Bao, getting upset, said they could not see the notes. Nelson ruled that they could see the notes and as the defense read through several pages of Bao's notes, one attorney laughed.