The human body’s most compassionate gift is the interdependence of its parts. As organs in the torso fail, the brain likewise shuts down. With the exception of the minority of people who suffer sudden death, the vast majority of us experience a slumberous slippage from life. We may be able to sense people at the bedside on a spiritual level, but we are not fully awake in the moments, and often hours, before we die.
Every major organ in the body — heart, lungs, liver, kidneys — has the capacity to shut off the brain. It’s a biological veto system. When the heart stops pumping, blood pressure drops throughout the body. Like electricity on a city block, service goes out everywhere, including the brain. When the liver or kidneys fail, toxic electrolytes and metabolites build up in the body and cloud awareness. Failing lungs decrease oxygen and increase carbon dioxide in the blood, both of which slow cognitive function.
The mysterious exception is “terminal lucidity,” a term coined by the biologist Michael Nahm in 2009 to describe the brief state of clarity and energy that sometimes precedes death. Alexander Batthyány, another contemporary expert on dying, calls it “the light before the end of the tunnel.”
The Gentler Symptoms of Dying - NYTimes.com