Bill has always been close to his daughter Laura. Thirty-eight years ago, he held her minutes after she was born, and marvelled at her vulnerability; her back barely bigger than the palm of his hand. "Now I can't believe that things have come full circle," he says. "Who'd have thought that that tiny baby would have saved my life?"

Last October, Laura donated her kidney to Bill, now 63, after he was diagnosed with polycystic kidney disease (PKD), a genetic disorder leading to kidney failure. "My dad kept saying he didn't want my kidney, he didn't want me to go through any heartache and pain. I was adamant I wanted to do it; I'm a mother of two and I wanted my dad to watch them growing up. Deep down I knew he felt it was his only chance." Laura had two sisters but they both had young babies, while her brother also suffered from PKD. So Laura was the obvious donor.

Then the bombshell dropped. After a series of blood and tissue tests, the hospital told them that Bill wasn't actually Laura's biological father. It was a secret her mother, now divorced from Bill, had kept from both of them. "Neither of us had any idea. I was gutted," says Laura. "I challenged my mum but she just wouldn't talk about it. If anything, it made me more determined than ever. I wanted to be part of him; to have a real part of me existing in him."

Bill no longer tried to dissuade her, now that he knew how much it meant to her. "I felt such a mix of emotions knowing what she was about to do; gratitude, love, worry if anything should happen to her. I didn't want her to think my feelings towards her as a father had changed. I'll always be her Dad; the rest is history. It still seems a little unreal that part of her is ticking away inside of me happily and healthily."

Laura and Bill have no regrets about their decision, but there have been emotional consequences. Laura found her mother's deceit difficult to reconcile and hasn't told her other siblings about the discovery for fear it would break the family apart.

Family sacrifice of this kind isn't unusual and neither are the emotional complexities that follow. "Often when relations make the decision to donate, it is an instant response that can be uninformed. They don't always take on board the psychological effects," says Celia Eggeling, renal counsellor for the South West Thames Renal and Tranplantation Unit. Understandably, it is difficult to be rational here; a loved one falls ill and the desire to help is overwhelming. Beyond the health concerns, other considerations seem trivial.

In terms of related organ donation, these instincts are being encouraged. More than 5,000 people are waiting for a kidney transplant, yet fewer than 2,000 can be carried out each year. Transplants from living donors are highly successful and already account for one in four of all kidney transplants in the UK. When living donors and patients are close relatives, the chance of a successful transplant are improved.

Of course, there is relief when an operation is successful and a loved one has a new lease of life - but there can be more uncomfortable emotions, too. Apprehension, doubt and anxiety are all likely to creep in before and after a donation. What is crucial in any physical family sacrifice is the strength of the relationship between giver and recipient. After Emma Wellesley, 35, was diagnosed with a rare lung complaint at the age of 19, she knew she wouldn't be able to have children naturally - her lungs and heart would be too weak to sustain a pregnancy. So when Emma wanted children, her mother, Annie Casserley, 53, offered to act as a surrogate mother, carrying an embryo from her daughter and son-in-law.

"Mum was the only person in the world I would have trusted. I don't think it's changed anything between us, except now we see each other more. When it comes to telling [my daughter] Annie Trinity, I'm not bothered. She'll know she was surrounded by people who loved her," says Emma. Her mother had a healthy pregnancy and gave birth by caesarean last September. Despite the extraordinary conditions, she felt surprisingly detached and single-minded about her undertaking. "I couldn't bear to see Emma heartbroken. When I was pregnant, I was taking care of Emma's baby in my womb, looking after her for a while - in the same way I'd look after my other two grandchildren."

However the motivation behind some family sacrifices are less clear-cut to those involved. Last year, Greg Sanukonanu, 34, donated a kidney to his sister Celestina Lepcha, 29, with the help of St George's Hospital in south London. Greg took six months' unpaid leave from his job at a boarding school in India, and stayed in London, leaving behind his wife and five-year-old son. It was an enormous commitment but Greg was happy to make it: "I was ready to do something for her because, as a family, we are close." Initially Celestina felt uncomfortable with his offer. "I felt if anything had gone wrong, what would I say to his family?" Fortunately, all went well. "After the operation I was in tears, I was so thankful to him. I couldn't explain to him how grateful I felt."

Yet, deep down, Celestina also felt puzzled. She hadn't been close to Greg when they were growing up. "We didn't get along. He used to have a short-temper and we would fight." Celestina hopes they will be closer since the operation, even though Greg is back in India. "One thing is clear: although he doesn't show it, I know he loves me."

There are numerous reasons why relatives step forward to donate and the dynamics of every family's history will have a bearing. "There can be rivalry and competitiveness," explains Eggeling. "One parent or sibling can feel they should donate as a way of getting back into a family if they've been estranged. There is also a weeding-out process; a pecking order about who is the most suitable depending on if they are married, with children or single."

What one tends to assume is that the instinct to prolong or improve the life of a child, sibling, parent and partner is all-powerful. But there are situations where relatives feel unable to donate. This is most likely to happen when family pressure to go ahead has been too great to bear. Jean Mitchell, a counsellor at a hospital in Surrey, recalls a case where a brother had agreed to give his sister a kidney but refused a few days before the operation. Scared at the prospect of losing an organ, he was overwhelmed by family expectation. "He felt he had to give in to her and hadn't been given enough choice," recalls Jean.

In another case, she says, an only son with three younger siblings was the perfect kidney match for his father. "He felt unable to proceed and it was such a slap in the face to his parents. He had always felt that his wishes had been ignored; that he was always bowing to his father's wishes."

Two years later, the family are still searching for an outside donor. "We had to tell him how courageous he was being," says Jean. "Our role is to protect the potential donor, even if it means the rest of the family vent their anger and disappointment on us." She has also come across parents who refuse to donate to a child. "One mother said, 'My daughter's been ill since she was a child, now it's my son's turn to have some of me.' Her daughter is still on dialysis."

One dynamic is clear. Relatives, especially children, are more happy to give when they feel they have control and less likely to when they feel they are relatively powerless within their family. "Young children usually want to do what their parents tell them but as they get older they have different views. They need to know they can say, 'No', explains Diane Melvin, a consultant clinical psychologist at Great Ormond Street hospital for children.

When children aren't old enough to make an informed decision, different emotional and ethical issues have to be resolved. Last year Ethan Barnes, now six, donated bone marrow cells to his sister Willow, three, who suffers from Hurler's syndrome, a rare genetic condition where excess sugars build up in the heart, liver and kidneys.

Willow was diagnosed at 21 months, and the search for a donor began. "When they told us Ethan was a match it was scary," says their mother, Charlotte. "He wanted to help Willow but we did think, what are we doing to him?

"I wondered if I was using Ethan, exploiting him to help Willow." Since the operation last October any doubts have passed; Ethan recovered almost immediately and Willow is now healthy and active. "Yet if it had gone wrong, we didn't have a clue how to explain it to Ethan, how to stop him feeling guilty if she hadn't survived."

"It's tough on families," says Diane. "A transplant will have an effect on everyone - immediately and in the long term." Particularly if the donation is unsuccessful. "People may feel; my cells or my organ wasn't good enough," she says. But where there is a healthy amount of emotional independence and understanding, family relationships can be enriched.

Certainly, Laura doesn't regret her decision to donate to Bill. "My father is a changed man. It makes me feel so satisfied, watching him go from strength to strength. But our relationship hasn't changed at all; in light of everything, he'll always be my dad."

Jean Mitchell and Laura and Bill are pseudonyms.