"Hanging upside down could cause blood to pool in the skull and pressure could build up inside the brain and eyes," Wright said. "This may result in congestion in the blood vessels causing stokes, or even rupturing of blood vessels causing bleeding. Seizures or death may also result."
The lower extremities have no such problem, of course; as long as we're up and about, our leg muscles do yeoman's work in squeezing the blood back to the heart.
"The lower extremities of man are endowed with constrictor mechanisms that protect the small arteries and capillaries from damage when we stand up," said Dr. Jay Cohn, professor of medicine at the University of Minnesota Medical School in Minneapolis.
"The brain does not have such constrictor mechanisms," he added. "Therefore, prolonged position with the head down may increase pressure in the small arteries and capillaries and lead to blood vessel rupture or blood leakage."
So as Blaine quite literally turns millions of years of evolution on its head, he will also be tempting injury and death. And any pre-existing health conditions could make the stunt even more risky.
"This could be worse if he has high blood pressure to start with," said Dr. Richard Smalling, director of interventional cardiovascular medicine at the Memorial Hermann Heart and Vascular Institute at the University of Texas Health Science Center in Houston.
"If he had any previously undiagnosed vascular abnormalities in his brain the risk would be magnified substantially. Hopefully these issues have already been investigated and he has been cleared medically."
Cardiovascular issues are only part of the picture. Wright said Blaine will also have to contend with a restricted ability to breathe, because his diaphragm will be upside down, with the weight of his intestines pushing upon it. And a lack of proper blood flow to his legs, if profound, may cause muscle tissue to die off as it is starved of oxygen.
And then there is the risk of plummeting to certain death.
"If you fall, there's nobody there to catch you," Ruden said. "You become street pizza."
Should Blaine survive the stunt, it will be the latest entry in an increasingly bizarre and death-defying resume.
"In the earlier part of his career he was buried alive, entombed in ice, he stood on top of a pole. All of this had the kind of flavor of death and resurrection."
"Some people think it's a death wish. I think it's the exact opposite -- a desire to live life at its fullest," Ruden said.
And Blaine is certainly not the first to try hanging upside down for extended periods of time. Indeed, an entire industry has been built around a practice called inversion therapy, in which people hang upside down for short periods of time, supposedly to remedy back pain and other ailments.
But while mostly harmless, this unconventional approach to health is not for everyone.
"[Inversion therapy] has its own side effects and adverse health risks, including headaches and bleeding into the retina," Wright said. "People with hypertension, glaucoma, heart disease or women who are pregnant should not try this. All others should check with their doctor before trying inversion therapy."
Still, Blaine is a man who has been buried alive and has spent days encased in a block of ice. In short, it is likely that he will have a few tricks up his sleeve -- or down it, as the case may be -- to avoid harming himself.