"Sepsis is the beginning of a domino effect," said Dr. Hassan Makhzoumi, a pulmonary specialist at St. Joseph's Medical Center in Towson.
As bacteria release toxins, the blood vessels respond by widening.
The heart, which depends on tight vessels to maintain adequate blood pressure, works harder to supply blood to organs and tissues.
But the effort is often futile, especially when the heart belongs to an elderly patient like the pope who has battled Parkinson's disease, suffered respiratory infections, had breathing and feeding tubes installed and has been too weak to speak.
"The heart is working, and in spite of that, it's not meeting the demands," said Makhzoumi, a critical care specialist who is head of pulmonary medicine at St. Joseph's. "The cardiac output should triple in a state of sepsis to provide the necessary blood."
Early yesterday, the Vatican said that the pope was suffering from heart failure - a weak heart - and that his kidney's were shutting down. All of this is typical for a patient succumbing to septic shock.
"Kidney failure frequently happens when you have very low blood pressure and your kidneys don't get adequate blood flow," said Dr. Erika Feller, a heart-failure specialist at the University of Maryland Medical Center.
Unstable blood pressure is what ultimately dooms patients suffering from septic shock, she said. The kidneys, liver and the brain all rely on blood to function - but shut down when the supply is insufficient.
Septic shock has been increasing for decades, in part because the population has been aging.
It now ranks as the most common cause of death in intensive-care units and the 13th-most-frequent killer overall in the United States, according to the University of Texas Southwestern Medical School.
Though the exact number of deaths caused by septic shock is not known, estimates range from 50,000 to 100,000.
Many doctors have observed that the pope's problems of the past month were likely consequences of advanced Parkinson's disease, a nerve disorder that he has battled for more than a decade.
Though it initially causes an unsteady gait and hand tremors, Parkinson's eventually makes victims prone to infections as they lose mobility and have trouble swallowing and keeping secretions out of their lungs.
Infections, said Feller, are also common among patients who have been hospitalized and had tubes such as urinary catheters inserted.
"He probably got the urinary infection that way," she said. "That may be contributing to his kidney failure."
Breathing tubes such as the one the Pope had inserted into his neck also provide routes for infection.
"And in the hospital, people in general are exposed to infectious sources," Feller said.
Doctors began treating the pope's urinary tract infection with antibiotics on Thursday, but the drugs apparently failed to prevent bacteria from breaking out into the bloodstream.
Poor nutrition may have reduced his ability to overcome the infection, even though he was armed with antibiotics. It's a common problem, said Makhzoumi, in elderly patients who, like the pope, have had trouble swallowing and lack appetite.
But cascading symptoms of septic shock can continue even when bacteria have been wiped out.
"Unfortunately, the switch has been thrown on, and we do not have the means to turn off this switch of sepsis," he said.