JOHNSON CITY, Tenn. -

Donated organs are a gift that can mean life or death for people waiting on a transplant list -- but we've learned fewer people are signing up to be donors.

An organ donation has been called a gift of life, and that gift is something John Caso has been waiting on for several years after going to the doctor thinking he had bronchitis. "The next day I was airlifted down to Duke University and put on the heart transplant list," he told us.

Caso tells us that was back in 2004, and life has been different ever since. "Since then life has been up and down. I was taken back off the transplant list back in 2006, and then in 2011 placed back on after going downhill a little with my health," he explained.

We learned Caso is in need of a heart; while he waits, he's gone on a low-sodium diet just like the doctor ordered. "That's the reason that I've been able to go on so long, because I have really followed what they've said to do," he said.

Caso isn't the only one. According to Donate Life, there are 120,000 Americans waiting on the transplant list; 2,000 are kids, and 2,500 are Tennesseeans. "It is not uncommon that someone would wait a couple of years until they could get a transplant, depending on what it is and how sick they are," adds Joy McCray with Donate Life.

McCray tells us people like John Caso waiting for a heart transplant could wait longer, because donated hearts are very rare. "They have to be the right size. They have to be a perfect match. It has to be able to fit and function for that person as well. We're very scared because we don't have many people signing up to be organ donors," she said.

Caso has advice to anyone who may be waiting for an organ donation, and even for people who are not -- he says listen to your doctor and never take life for granted. "I missed my kids growing up because I was always traveling. I missed their birthdays and things along that line, and if I had to do it all over again I would have slowed down," he said.

We learned there are eight organs, plus different tissues, that can be donated to help other people. If you would like more information about organ donation or if you want to sign up to be an organ donor and you’re in Tennessee, click here. If you’re in Virginia, click here.