A heart transplant is more than a miracle of modern science – it’s an act of incalculable grace and a precious gift of life.

The most responsible act I’ve ever witnessed was performed by two people whom I never met. They were the parents of a young woman who died suddenly one December night 12 years ago. I think of them, whoever they were, every year around this time, the holiday season, because on that same night I was allowed to accompany the team of surgeons who harvested that young woman’s heart, the most precious gift all.

I was a journalist working on a magazine story about heart transplants; for a week I’d been on call, waiting for a tragedy that would beget someone else’s rebirth. It was late Saturday, only hours from the time when that ghoulish vigil would be handed over to the next, rotating team of harvest surgeons. And then all at once I was sitting beside the harvest surgeons in an emergency vehicle, racing in the wee hours over the George Washington Bridge toward a New Jersey hospital. I followed the surgeons down a series of brashly lit corridors into the operating room. One of them took me by the hand and placed me at the head of the operating table where the young woman lay, dead from a brain aneurysm she’d suffered earlier that night. Her head was covered, her lungs rose and fell by the force of a respirator, her heart still beat of its own accord.

I refused to let myself think about the individual before me, wouldn’t let myself consider the course of the night she’d been pursuing before everything went dark, or imagine what her long-term dreams might have been. Instead, I thought about the word grace: the grace of the parents to grant a long enough abeyance in their own private grieving process to allow other people the chance for more life; the grace of the surgeons dispatching their task so deftly. There was, too, the grace of my being able to see what few us ever do: the intricate biological underpinnings of our film-like waking lives.

The purest gifts, it’s often said, are those bestowed anonymously. I don’t know how many different lives that young woman ended up saving, the full bounty of the harvest that her parents had granted. I can only speak for the fate of her heart and the new life that I would watch it bestow.

The morning sun was just coming up behind the refineries along the New Jersey Turnpike as the harvest surgeons and I sped with our gift, wrapped in an ice-filled cooler in the trunk, toward a hospital in upper Manhattan. All I remember of that drive is that we had to stop at a tollbooth.

“But…” I stammered to the driver, a moonlighting New York City fireman, “we’ve got a heart in the….”

“Are you kiddin?” he answered. “Five-hundred dollar fine!”

My night was supposed to have ended at the doors of the Manhattan hospital’s transplant room; I had no press clearance to go further. But as the cart with our gift pushed open the operating-room doors, the transplant surgeon, the same one who had arranged my credentials for the harvest, looked up and spotted me.

“Hello, Charles,” he called out. “Why don’t you go scrub for another surgery?”

Once back in the OR, my hands gloved and held aloft as instructed, I was again ushered to the head of the table. A middle-aged man lay there before me. His head was covered, his lungs rose and fell by the force of a respirator, his blood flowed out and back into him through a bypass machine. And yet his chest was hollow. Empty. A man alive without his heart. It was sitting over on a side table in a metal bowl, sickly pale and distended. I looked again: it was still beating, in slow, ballooning heaves, like a fish dying on a dock.

“It’s still fighting for life,” the surgeon said as he turned now and lifted our gift from the ice into the light. It looked, by contrast, that much more lovely: ruby-hued, vigorous and vibrant, the very paradigm of a healthy human heart. I don’t know for how long by then it had been without blood, only that it must have been under four hours. Any longer, and a heart begins to die.

I watched the transplant surgeon move, like the harvesters before him, with precise and patient speed. And then he was pulling back and asking that the man’s blood be channeled back to its old, if newly inhabited, home. I stood staring, thinking of the warm blood surging now against the heart’s icy muscle walls. I wondered if she – I couldn’t help myself – if she would remember what to do.

Nothing happened. Moments passed. Still nothing.

“C’mon,” the surgeon whispered, reaching in now to give the heart his own warm hand’s gentle knead.

I didn’t think that was allowed somehow. He did it again. And again. We waited. And then, in sudden fits and starts, the heart began to remember, and the word grace came back to me once more. I thought of the grace period of extended life that this man was now receiving. And of the grace of my being able to witness such an act of responsibility on the part of that young woman’s parents: a responsibility to more than their own daughter. A responsibility to life itself.

It took a while before the heart’s rhythm was stabilized with the help of a few coaxing electrical jolts and a regimen of drugs. When the chief surgeon finally pulled back, his face bore an expression of both exhaustion and exhilaration.

“I will never tire of that moment,” he said in a near whisper, “when the heart comes to life again in someone else.”

And then, without any forewarning and for reasons I’ll never fully understand, the surgeon reached over, took hold of my right hand, and placed it on the beating heart. I’m not sure what exactly I expected: a kind of gelatinous warble, perhaps, or a dull thump. But I can still feel it now, the force of that young woman’s heart, like a fist against my palm, the reverberations running up and down the length of my arm. It was the force of true grace, of life beyond our own.

Charles Siebert is a contributing writer for The New York Times Magazine and is the author of several books including “A Man After His Own Heart: A True Story” and most recently, “The Wauchula Woods Accord: Toward a New Understanding of Animals.”