“We’ve got a few minutes,” Baio continued, “and I know this is your first night in the hospital. So let’s go over a few things.”

“Great!” I replied. Our orientation leaders, a peppy group of second- and third-year residents, had instructed us to exude a demented degree of enthusiasm at all times, which wasn’t difficult now that my blood was more caffeine than hemoglobin.

“Just relax,” he said, “and take a look around.”

Together we scanned the fluorescent room, an enclosed space the size of a tennis court containing critically ill patients and nurses bustling between them. The perimeter, painted a regrettable shade of yellow, housed the patients in glass cubicles, while the center, where we were sitting, was mission control, filled with chairs, tables, and computers. “It’s just you and me tonight,” Baio said, whipping his stethoscope back and forth around his neck. “And 18 of the sickest patients in the hospital.”

Every night an intern and a second-year resident presided over the CCU. Tonight was our turn, as it would be every fourth night for the next month. All of the patients in the unit were on ventilators except one, a large Hispanic man who was riding a stationary bicycle and watching Judge Judy in his room. “These patients are receiving some of the most complex and sophisticated therapies in the world.” Baio reached for a bagel that was sitting on a platter nearby. “Patients get referred to the cardiac-care unit when hope is lost or after something devastating happens. Balloon pumps, ventricular assist devices, transplanted hearts, you name it.”

Until a few days ago, I had never set foot in a cardiac-care unit. Nothing about the setup looked terribly familiar. I continued to study the room, trying to decode the symphony of incessant beeps and alarms and wondering what each of them meant. It felt like I was sitting in the middle of a giant equation with infinite variables.

“These patients should all be dead,” Baio went on. “Almost every one of them is kept alive by an artificial method. And every day they’re going to try to die on us. But we’re going to keep them alive.” He paused for effect. “And that’s cool.”

It was cool. In college, I had studied molecular biophysics and briefly flirted with the idea of going to graduate school in that subject, using my degree to solve the structure of molecules that were too small to be seen under a microscope. But the field lost me when a professor, a young crystallographer, introduced the importance of imaginary numbers in biophysics. Try as I might, I just couldn’t wrap my head around that quixotic concept. I wanted to translate science into something more concrete, more tactile, to seek a profession where I could touch and see and feel. So I changed course and pursued medicine. And thus far, it had seemed like a wise decision. Nothing about this moment with Baio seemed imaginary. Quite the contrary, it felt excessively real.