Sisters Allison Kerr and Karen Owens have been inseparable all their lives.
They see each other every day and have shared every family milestone together.
What they also shared was a congenital heart condition which meant they both needed transplants.
Allison, 54, received her new heart last year but Karen, 57, is still waiting for hers.
The result has been a "rollercoaster" of emotions as the Lanarkshire sisters support each other through lifesaving treatment, Covid shielding and the thought of family reunions when lockdown is lifted.
Allison, of Coatbridge, told BBC Radio Scotland's Drivetime show: "Having someone else's heart inside me is very strange, but it's life-changing.
"I sometimes say to my husband and Karen that I can't believe I'm alive and not with the heart I was born with.
"The night before transplant, my heart rate was 27 and two days later when I woke up it was 110 beats per minute.
"It's very overwhelming and I get quite emotional thinking or speaking about it. The journey has been tough, but so worthwhile."
Karen, from Airdrie, was also diagnosed with the heart condition familial dilated cardiomyopathy in 2008.
It is the same condition which claimed the life of the sisters' father in 2005.
Karen had a defibrillator fitted four years ago and was told last November that she will need a transplant to survive.
She hopes her chances of receiving a new heart will increase under the opt-out system for organ donation which came into effect in Scotland in March.
"I saw how Allison had deteriorated in the year before she received her transplant. I think I knew it was inevitable it was going to happen to me after the journey she had," Karen said.
"Although I knew my heart was failing, it wasn't as advanced as Allison's. I wasn't on the transplant list.
"The day she went for her operation, it was myself and her husband who were with her.
"You are up and down - thinking this will be great then thinking, oh wow, there's another side to it and some people don't survive it.
"But to be there at the other end when she came out, and to watch her progress since then, has been absolutely phenomenal."
Now she can only wait and hope for a suitable donor to be found so she can have her own transplant.
"During the day, I don't often think about it," said Karen.
"But when you go to your bed and you are lying in the stillness and your heart misses some beats - which it does due to the condition we have - then I want a new heart.
"But then I think, don't be selfish - someone else has got to die for me to have this new heart, this gift."
Sister Allison says her life has been transformed by her transplant, which was carried out at NHS Golden Jubilee hospital in Clydebank.
"I went from being told I was getting a defibrillator on my 40th birthday to being told I needed a transplant on my 50th.
"But with my new heart, I now have my 60th birthday to look forward to.
"Without it, I simply wouldn't be here and I have so many people to thank for that, not least my donor."
It is estimated that about 500 patients are on the organ waiting list in Scotland at any one time.
Since a new law was introduced in March, everyone in Scotland is considered an automatic organ donor unless they opt out of the system.
It is now presumed that people have consented to donation unless they have stated otherwise.
Experts say this will mean that many more people can be given life-saving and life-changing transplants.
Dr Jane Cannon, a consultant transplant cardiologist at NHS Golden Jubilee, hopes Allison's story can be an inspiration to others.
She said: "To see people like Allison, who have been seriously ill, receive the gift of life and go home to their families, is always the end goal - and is the most rewarding part of the job.
"This is all thanks to organ donors whose humanity and selflessness makes this possible."
'Passion for the hills'
For now, the Covid shielding which has meant Allison has not been able to see her two stepsons and three grandchildren properly in 16 months goes on.
But she and her sister hope to one day resume their passion for walking in Scotland's hills together.
Allison said: "I hope my sister can have the same gift of life that I have been given.
"Life should never be taken for granted."