KNOXVILLE, Tenn. — It was around 4 a.m. Wednesday when Julia and Luther Davis heard the knock on their door that every parent dreads. 

Their son, Nathan, had been in a deadly crash and was rushed to UT Medical Center's intensive care unit. 

He was in the ICU for days before he died on Sunday morning.

"You had hope and then all of a sudden it was taken away from you," Luther Davis said. 

But in their heartbreak has emerged a story of hope. 

Their son--with piercing blue eyes and a "broad beaming smile"--was an organ donor.

"He wanted to give the gift of life to other people," his mom said. "And lo and behold, his daddy needs a kidney. And he's going to get Nathan's kidney." 

His dad, Luther, has been on dialysis for more than 5 years. His son's kidney is a perfect match.

"[The doctor] asked me 'do you want one of his kidneys?' I said, that's what he would have wanted," his dad said.  

Luther underwent the transplant at UT Medical Tuesday morning. It was a success -- and the kidney was beginning to function an hour after the surgery.

"We just wanted to thank the wonderfully compassionate team at UT Medical Center who have been instrumental throughout this whole process," Nathan's sister Gabriela said. "We knew that Luther was okay, by the grace of God, and we are so overwhelmingly grateful, in the face of this tragedy, for all of God's mercies and miracles shining through the darkness.”

"I'm glad. I'm so glad that there will be so many others saved by his donation," Luther said.  

Now the Davis family will prepare to bury their son--and welcome him home. 

"To know that part of him is going to come back and stay with us a while. That's a beautiful thing," Julia Davis said.