What killed George Floyd during an encounter with Minneapolis police last May is at the heart of the trial of former officer Derek Chauvin, which resumes Monday morning with opening statements and the start of witness testimony.

The Hennepin County Medical Examiner's Office ruled Floyd's cause of death cardiac arrest, Floyd's family attorneys believe he died of asphyxiation and Chauvin's attorney has argued that he succumbed to a drug overdose and pre-existing health issues.

"Clearly there is a cause of death issue here — in fact, it is highly contested," Hennepin County District Judge Peter Cahill previously said as he presided over the case.

Some veteran defense attorneys have said Chauvin should be acquitted if asphyxiation can't be proven, given prosecutors' focus on Chauvin kneeling on Floyd's neck for more than nine minutes.

But prosecutors don't have to prove that Chauvin's actions alone caused Floyd's death, according to Minnesota's guidelines for jury instructions in criminal cases. According to the state and the defense's proposed jury instructions, "  'To cause' means to be a substantial causal factor in causing the death. … The fact that other causes contribute to the death does not relieve the defendant of criminal liability."

Doctors and scientists who study the cardiovascular and respiratory systems say they are complex, intimately interlinked and crucial to the heart's function.

Chauvin is charged with second-degree murder, third-degree murder and second-degree manslaughter.

"Anytime the neck is interfered with, I think the first thing people see or think is that your breathing is being interfered with, and that can be true," said Dr. David Penning, an assistant professor of biology at Missouri Southern State University. "But you do have big important veins and arteries [in the neck], and those also are susceptible to those same forces or pressure … ," he said.

Penning and cardiologists Dr. Geoffrey Barnes, an assistant professor at the University of Michigan, and Dr. Jonathan Marmur, a professor and chief of cardiology at the State University of New York Downstate Health Sciences University, spoke generally about the relationship between the cardiovascular and respiratory systems. They did not interpret the findings of Floyd's autopsy or determine his cause of death. "Breathing difficulties must be taken with the utmost seriousness …," Marmur said. "Untreated respiratory arrest will result in cardiac arrest and death."

Floyd's May 25 arrest was widely broadcast via a bystander's Facebook video, which conveyed to many observers that he died of asphyxiation as he repeatedly declared, "I can't breathe," while handcuffed and pinned stomach-down in the street under Chauvin's knee. Officer J. Alexander Kueng knelt on his back while officer Thomas Lane knelt and held onto his legs out of view of the video. Officer Tou Thao kept angry bystanders at bay.

On June 1, Hennepin County Chief Medical Examiner Andrew Baker ruled that Floyd, 46, died of cardiopulmonary arrest, sparking widespread public anger. Two days later, the county spent $4,325 to install emergency fencing around Baker's office downtown. The barricade remains.

Baker found that Floyd's cause of death was "cardiopulmonary arrest complicating law enforcement subdual, restraint, and neck compression." He also listed hardening and thickening of the artery walls, heart disease and drug use as "other significant conditions." Fentanyl and methamphetamine were found in Floyd's system.

Attorneys for Floyd's family commissioned a private autopsy that found he died of asphyxiation. Family attorney Ben Crump said at a June news conference that Baker went to "great length" to obscure Floyd's cause of death, and that disclosing the toxicology results was "an attempt to assassinate his character."

While Baker ruled the manner of death a homicide, an act caused by another person, it did little to quell the backlash.

The cause of Floyd's death is already emerging as an intensely divisive topic in the community. "With [Hennepin County Attorney] Mike Freeman and Dr. Andrew Baker at the helm, it appears, once again, systemic racism, white privilege, and collusion will in the day [sic]," a Wisconsin resident wrote in a letter addressed to Freeman, Baker and several others and obtained through a data practices request. "… the ENTIRE WORLD had access to the video which CLEARLY shows a man's life being taken by a thug in blue and the medical examiner comes up with 'underlying health conditions.' " Another person sent Baker a letter emblazoned with "I can't breath" [sic] three times, "Coverup" and "Time to Retire" on the envelope.

Penning is among a group of scientists who have debunked the long-held belief that snakes fatally asphyxiate animals constricted in their coils. Other scientists published two breakthrough studies in 2012 and 2015 finding that snakes killed their prey by using pressure to stop the flow of blood through the heart, causing circulatory arrest before the animal ran out of oxygen. Penning has continued to advance such studies, which aren't divorced from human biology. A 2017 study on people dying in grain elevators cited the 2015 paper.

Penning described the cardiovascular system as a series of rivers and streams; blocking flow in one area changes where blood travels and pools, sometimes to catastrophic effect.

"It's a really confusing system, and any effect in one area can lead to things you might not have expected," Penning said. "Anytime you apply force or pressure to the neck you're going to potentially interfere with breathing and the cardiovascular system, and of those two, the cardiovascular system is more sensitive …"

A lack of oxygen can damage or kill the heart muscle, triggering an abnormal electrical signal that causes cardiac arrest, said Barnes, a specialist at the University of Michigan's Frankel Cardiovascular Center.

Pressure on the neck can also cause blood pressure to drop and reduce blood flow to the heart, possibly resulting in a cardiac arrest, Marmur said. Chauvin's attorney, Eric Nelson, has argued in court filings that Floyd was resisting arrest and posed a possible threat to officers. He pointed to Baker's findings that no bruises were found on Floyd's neck and back as proof that his client is innocent. Floyd likely died of a drug overdose, Nelson wrote, adding that he had pre-existing health conditions and was positive for COVID-19.

"If [Floyd] were found dead at home alone and no other apparent causes [sic], this could be acceptable to call an [overdose]," said the notes of a meeting between Baker and prosecutors that Nelson filed with the court. "… I am not saying this killed him."

"Put simply, Mr. Floyd could not breathe because he had ingested a lethal dose of fentanyl and, possibly, a speedball," Nelson wrote.

The possibility of cardiac arrest in Floyd's death was not lost on some. "When doing the autopsy on Mr. Floyd please do not neglect the fact that a knee on the carotid artery can cause a fatal asystole or other arrhythmia — also a fatal stroke," a registered nurse from Pennsylvania wrote in a letter addressed to Baker.

Kueng, Lane and Thao are to be tried in one trial Aug. 23. They are charged with aiding and abetting second-degree murder and manslaughter