You can see the exact moment “the light goes out” of George Floyd as ex-cop Derek Chauvin pressed a knee into his neck, a respiratory expert testified Thursday.

“You can see his eyes,” said Dr. Martin Tobin, who specializes in pulmonary and critical care medicine, at Chauvin’s murder trial in Hennepin County District Court in Minneapolis as jurors were shown the disturbing clip.

“He’s conscious and then you see that he isn’t. That’s the moment the light goes out of his body.”

The footage shows Chauvin’s knee grinding the left side of Floyd’s face into the pavement — as Floyd’s eyes flicker ever so slightly before they go blank.

Just minutes earlier on May 25 of last year, Floyd, 46, had been arrested for passing a fake $20 bill at Cups Foods, a Minneapolis convenience store.

After resisting police officers’ attempts to get him in the back of a police cruiser, he was dragged to the ground, handcuffed behind his back, and positioned face-down against the pavement. Chauvin’s knees were pressed against Floyd’s neck and back for 9 minutes and 29 seconds.

Alarmed bystanders begged Chauvin and the three other cops to relent as Floyd said “I can’t breathe” 27 times and called out for his mom.

Tobin said Thursday that even a healthier person couldn’t have survived the oxygen deprivation caused by the restrictive positioning.

The doctor described how Floyd would have felt like he was gasping for air through a straw as he slowly lost consciousness.

“He’s constantly cranking up the right side of his body, you can see there, to try and get some air into the right side of his chest, he’s making repeated struggling movements,” said Tobin, as prosecutors played the footage.

After about five minutes prone on the pavement, Floyd kicked out his leg in an extension then, seconds later, took his last breath.

“You’re seeing here fatal injury to the brain from lack of oxygen,” said Tobin of the involuntary reaction.

Floyd was soon dead, but Chauvin’s knee remained on his neck for more than four minutes.

After studying the footage, Tobin concluded that Floyd died of oxygen deprivation caused by the way police had restrained him.

The defense has claimed that Floyd died of a drug overdose — given the concentration of fentanyl in his blood — and a heart ailment.

On cross-examination, defense lawyer Eric Nelson suggested it was unfair to expect cops to have the expertise of a medical doctor, asking: “You understand Minneapolis Police Officers aren’t even EMTs?”

“Correct,” Tobin replied.

Nelson also pointed out that the many hours Tobin had to review the video footage of the confrontation and examine medical records isn’t comparable to the split-second decisions a cop makes on the ground.

He also pressed Tobin on whether Floyd’s drug use, heart disease, cigarette addiction and recent COVID-19 infection could have played a role in his demise. 

In the afternoon, forensic toxicologist Daniel Isenschmid, of NMS laboratory, took the stand and said that Floyd had in his blood a large amount of fentanyl — 11 nanograms per milliliter — but a small amount of methamphetamine and marijuana.

He said that some of the fentanyl had been metabolized. Typically with fentanyl overdose deaths, a person dies before the body has a chance to start breaking down the drug, Isenschmid told jurors.

The lab analyzed 2,345 fentanyl DUI cases from 2020 and found that about 25 percent of the drivers had a higher concentration of fentanyl in their blood than Floyd did at the time of his death.

The final prosecution witness Thursday was Dr. Bill Smock, who also told jurors that Floyd died of oxygen deprivation — not a drug overdose or a heart condition. 

“Mr. Floyd died from positional asphyxia, which is a fancy way of saying he died because he had no oxygen left in his body,” said Smock, an ER doctor who specializes in forensic medicine. 

On Friday, prosecutors are expected to call Hennepin County Chief Medical Examiner Dr. Andrew Baker, who performed Floyd’s autopsy and ruled that his cause of death was a heart attack. 

Chauvin, who is charged with second-degree murder, third-degree murder and second-degree manslaughter, faces up to 40 years in prison.

Three other former Minneapolis police officers — Tou Thao, Thomas Lane and J. Alexander Kueng — are due to stand trial on charges of aiding and abetting murder and manslaughter in the case.