These are the types of questions that preoccupy the staff of the University of Pennsylvania’s Center for Resuscitation Science (CRS), a team of scientists, clinicians and engineers that’s revolutionizing the way we treat cardiac arrest and nudging forward the line between life and death. It all starts by learning what’s going on at the cellular level. According to Dr. Honglin Zhou, an assistant professor of emergency medicine at the University of Pennsylvania and an associate director of the CRS, scientists generally agree that, unlike the larger organisms they compose, there are clear ways to tell whether an individual human cell is dead. [Does Your Heart Really Stop When You Sneeze?]

Every cell has a tight outer membrane that serves to separate its own contents from its surroundings and filter out the molecules that are nonessential to its function or survival. As a cell nears the end of its life, this protective barrier will begin to weaken and, depending on the circumstances of a cell’s death, one of three things will happen: It will send an “eat me” signal to a specialized maintenance cell that will then devour and recycle the ailing cell’s contents; it will quarantine and consume itself in a kind of programmed altruistic suicide; or it will rupture abruptly and spill its contents into the surrounding tissue, causing severe inflammation and further tissue damage.

In all cases, when the integrity of the outer membrane is compromised, a cell’s fate is sealed. “When the permeability of the membrane has increased to the point that the cellular contents are leaking out, you have reached a point of no return,” Zhou said.