At 6:54 a.m., Brian Coffee, a vintage-car collector in Dayton, Ohio, e-mailed the transplant team at the University of Pennsylvania veterinary hospital.

"We're sending you our thoughts and prayers. Give Princess and Charles a big kiss for us."

Princess, 8, whose coat is tortoiseshell, was in her cage "making muffins," as cat people like to say. Front paws padded up and down, as if kneading dough, a sign of calm.

Princess is aptly named, for she was about to receive a rarest jewel: a lifesaving kidney transplant.

Penn Vet is one of three veterinary hospitals in the world doing kidney transplants in cats. Surgeon Lillian Aronson has done all 150 since Penn began in 1998. Princess would be 151.

Charles, the donor cat, wasn't quite as calm as Princess in his adjacent cage in the transplant office. He is a country cat, from a shelter in York County that supplies donor cats to Penn. Being a cat, he couldn't consent to having his organ taken, and at that moment, he wasn't too pleased.

Were he able to reflect, however, his attitude might have changed. He avoids the possibility of euthanasia, a reality for millions of cats and dogs, and, like all donor cats at Penn, goes home with the animal whose life he is saving.

The American Society for the Prevention of Cruelty to Animals supports such "ethical" transplants, as long as the shelter cat gets a good home. People for the Ethical Treatment of Animals opposes them. "They have their organs removed against their will," an official said.

Charles, an orange tabby, was wearing a cone collar because he kept trying to pull the IV out of his paw with his teeth. Nurse Lynn Beale, transplant-team coordinator, who cares lovingly for the donor cats, scooped him out of his cage and cooed: "It's OK, baby. It's OK."

After calming him in her lap, she carried him to the operating room.

Penn's Matthew J. Ryan Veterinary Hospital treats 31,000 animals a year, including 13,000 in its emergency room. Patients are mostly dogs and cats, but also rabbits, ferrets, lizards, birds, even zoo animals - a jaguar, lion, kangaroo, and aardvark.

Penn Vet has more than 60 veterinarians, many of whom do valuable research in cancer, neurological disorders, and gene therapy. "Our clinicians treat your pet with breakthrough science, and their work is often translated by human doctors into lifesaving human treatments," reads a hospital brochure.

Human transplant began with animal experiments, but these surgeries benefit pets and owners, not science. "I can't really say that anything we have learned from these patients is something they didn't already know in humans," Aronson said.

Isn't it an extravagance?

"You could say that about any procedure that's not a spay-neuter," Aronson said. "Why do we do minimally invasive surgery? Why should we do emergency surgery? Nothing's better than to take an animal that has been suffering and improve its quality of life. These are our patients. We're trying to save their lives. And you see the emotional ties these families have. You feel like you're saving the life of a family member."

In Operating Room 3, Charles lay on one table, Princess on another, both covered in sterile blue paper, exposing only the surgical area, their bellies.

Skytron lights were overhead. Trays with scalpels, clamps, and scissors flanked operating tables. Ten people on the transplant team were in gown, mask, hat, and booties.

Charles went first. Aronson cut a six-inch incision and inspected his kidney, making sure it would be suitable for transplant.

All donor cats go through testing, even, yes, a CT or "cat" scan. They must be disease-free and have "architectural perfection" in their kidneys.

Charles was perfect. He was covered for warmth and monitored; his kidney would be taken in hours.

Princess was next.

Aronson worked by looking through a microscope, and the image was shown on a large screen for others to see.

Surgical resident Chloe Wormser assisted.

Imaging before surgery showed an extra blood vessel possibly in the way. Now, Aronson was exploring it. "Could be a problem," she told her team.

"This is a first," she said. The room was quiet except for the amplified heartbeats as she considered options. "We're going to isolate it and [move] it out of our surgery field," she concluded. "We're not going to touch it."

Princess was 10 trim pounds, but in Aronson's view was a "fat" cat - meaning she had a deep abdomen with organs hard to reach.

It took Aronson until 1 p.m. to have Princess ready for the big moment.

In 2004, Aronson did a transplant on a cat from York. Its owner was involved with a shelter there, and York became the source of donor cats.

Beale,  who owns two dogs, a cat, and a horse that pokes his head through her kitchen window on warm days,  drives to York a few times a year to pick up donor cats. Beale renames each batch, always thematically.

She's done vegetables: Rutabaga, Zucchini, Bok Choy; beans: Pinto, Garbanzo, Butterbean; pastas: Ziti, Tortellini, Rigatoni.

Last year, driving home, she heard Jim Croce's "Bad, Bad Leroy Brown" on the radio and began singing at the top of her lungs. The cat on the front seat began howling in protest. On the spot, Beale named him Leroy Brown, and his mates James Brown, Charlie Brown, Zac Brown, and Sawyer Brown.

The current batch is named after the housekeeping staff at Penn Vet.

Leroy Brown, by the way, donated his left kidney to a cat owned by Robin Quivers, sidekick to radio personality Howard Stern. The rich and famous have brought cats in for transplant from as far as Brazil and the Middle East, but Aronson and Beale say most people who pony up $18,000 to $25,000, the average cost, are just ordinary Americans who love their pets.

   Beale said pet owners have raided their 401(k)s and taken out second mortgages. A young couple, newlyweds, said Beale, "were using money that they had gotten at their wedding for a down payment on a house."

She turned them down, but they insisted. "The cost," said Beale, "usually does not deter people."

The average age of a cat receiving a transplant at Penn Vet is 8. Ninety-two percent have left the hospital; 70 percent are alive after one year; the median survival is 1,000 days.

Invariably, Aronson said, the recipient family is so grateful, it wants to adopt the donor cat.

About 1:30 p.m., Aronson removed Charles' kidney.

At its simplest, Aronson made three cuts - to the artery leading into the kidney, the vein leading out, and the ureter that connected to the bladder.

Aronson cupped the kidney in two hands and carried it to Princess. It looked the size of a robin's egg, gray without blood.

The pressure was on.

Once Aronson clamped the artery and vein in Princess - where the surgeon planned to attach the new kidney - much of the blood supply to her hind legs would be restricted.

So suturing was a race against the clock.

Stitching must be so tight that there are no leaks. The best way to go fast was to go slow.

As Aronson worked on Princess, Susan Volk, a fellow surgeon, tended to Charles. By 2:45, he was all sewn up. His breathing tube was removed, and he began waking up.

"Hi, gorgeous!" Beale said, and carried him off the table. He is expected to live a full and healthy life with one kidney.

There was a visitor that day in surgery.

Stephen Guy, a human transplant surgeon at Hahnemann University Hospital, had come to watch.

"So what do you think?" Aronson asked as she worked.

"It's what we do, only smaller," he said.

By 2:50 p.m., the artery and vein from the new kidney were sewn tight.

"Venous clamp being removed," Aronson said.

The kidney expanded and turned a healthy red as it filled with blood.

Almost immediately the kidney began working, as urine flowed through the ureter, still unattached.

"We like pee," said Aronson.

Connecting the ureter to the bladder was delicate work. A cat ureter is smaller in diameter than the needle in a flu shot. Aronson used suturing thread as thin as human hair.

 At home that morning, Aronson's children, 10, 9, 6, and 4, had, as they do every morning before a transplant, serenaded her: "Don't worry. You'll do great! Cha Cha Cha." At moments like this, she appreciated the support. 

About 4:30 p.m., after more than six hours, Princess' surgery was done. Masks came off. Princess was breathing on her own.

Beale was first to speak to her. "Oh, there she is. She's my girl."

Princess was wheeled into the ICU.

The surgeon from Hahnemann thanked Aronson. He had taken many photos. He was going to put together a slide show for his team, and invite Aronson.

"We'll have a spirited discussion," he said.

Aronson called the family. "We just finished, and everything went really well," she told Coffee.

"Oh, wonderful," he said.

"It was a little challenging, but she did really well. Kidney was making nice urine at the end."

"That's great," said Coffee. "Get some rest. We really appreciate what you've done."

"Go eat something," Aronson said, "because I'm sure you haven't eaten anything."

"I bet you haven't either," he replied.

Aronson hung up.

"Now it's up to her," she said. "We've done our part."

Coffee and his wife and two children live in a 4,500-square-foot home in Dayton with three rooms dedicated to their cats and a full-time employee who drives them to the vet and looks after them.

The Coffees got their first six cats at a local shelter, but then people started taking them cats, and strays would somehow show up. Princess appeared four years ago. Their vet found evidence of five broken ribs, suggesting abuse.

Two strays were pregnant, their kittens too cute to give up. Soon, they had 27 cats.

Coffee says he owns antique cars in garages and museums around the country. One of their cats is named Bentley because he appeared when Coffee was in his garage, by the Bentley.

In 2004, one of their sweetest cats, Ginger, died needing a kidney transplant.

"She has a husband in our group named Jack," Balazs said. "And they loved each other and they were a couple, and he sat by her and adored her from the moment we rescued her."

Ginger was too sick for a transplant. The Coffees resolved to save Princess if they could.

Their own veterinarian in Dayton suggested they just donate the money to area shelters, rather than spend it on one cat. Kathryn Balazs said she and her husband were generous with many charities for humans and animals and did not see this as extravagant.

"Every one of these cats is part of our family," she said.

The 27 names:

Jack, Romeo, Burt, Tupper (looks like a British gentleman), Lilly, Daisy, Emma, Tony, Lucy, Jimmy, Ava, Rosie, Charlie, Misha, Hannah, Frankie, Molly, Bella, Lola, Owen, Winnie, Lexi, Bentley, Kayla,  Sophie , Cheddar (he's orange), and Princess.

And now Charles, 28.

On Friday, Nov. 7, eight days after surgery, the Coffees came to take the cats home. The couple and their daughter, Madison, 7, walked into Beale's office and fell to their knees.

Mother and daughter went to Charles. Dad went to Princess.

Coffee, 6-feet-6, 240 pounds, poked his head in her cage and scratched her chin.

"That's my girl," he purred. "Long road, huh?"

Princess, quietly convalescing, opened her eyes wide. Her ears perked, and she got up and gave him a gentle head butt.

Charles, shy, wasn't given a chance to be timid with Madison, who soon climbed inside his cage.

"Mommy, do you think Charles will be sleeping with me tonight?"

"Oh, I bet he will be."

Finally, it was time to go.

The cats were placed in their travel carriers for the flight. Beale instructed Balazs, a dermatologist, in all of Princess' medicines. Then hugs all around.

"Thank you so much, of course," said Coffee.

"Thank you for saving Princess," said Balazs.

"Anytime," said Aronson.

"That's how we roll," said Beale.

Aronson went off to another surgery. Beale walked them out.

"Get home safe," Beale said, closing their rental-car door. She stood in the autumn sunshine, watching, as the Coffees and their cats drove away.