In 2017, a nine-year-old girl from Devon was involved in a car crash that left her with a catastrophic brain injury. Keira had been a keen horse rider who loved animals, and would go out of her way to rescue insects in distress. After her family were told she was brain dead they immediately asked if she would be able to donate her organs, knowing it was what she would have wanted.

As Keira lay in intensive care, a young boy from Cheshire was on his eighth month in hospital with a dangerously enlarged heart. Before he became ill, nine-year-old Max loved to play football, climb trees and wrestle with his friends. But now, with acute cardiomyopathy brought on by a viral infection, Max was painfully thin and being kept alive with a mechanical heart pump. Not only did he not have the strength to leave his bed but, on his darkest days, he didn’t want to live. He and his family knew his heart could give out at any minute and his only hope was a transplant.

In The Story of a Heart, Dr Rachel Clarke writes about the feat of modern medicine that allowed Keira to give life to Max by donating her heart. As the author of Dear Life, about the reality of end-of-life care, and Breathtaking, an account of the Covid crisis (later adapted for TV by Line of Duty’s Jed Mercurio), Clarke has made her name telling tough medical stories in a way that is accessible and humane. As well as a tender account of two families linked by tragic circumstances, and the transfer of a human organ from one body to another, The Story of a Heart provides a detailed map of the surgical innovations, people and logistics that allowed that transplant to happen. It also examines our shifting understanding of this “toiling, tireless, muscular miracle” that is baked into our language, representing the gamut of human feelings: “Hearts sing, soar, race, burn, break, bleed, swell, hammer and melt. They can be won or lost, cut or trampled, and hewn from oak or stone or gold.”

Centuries ago, the heart was seen as the repository of human emotion and morality. Aristotle thought it the source of human intellect, pleasure and pain, while the 16th-century French surgeon Ambroise Paré called it “the chief mansion of the soul”. Though the science has moved on, the heart is still where we feel the physical sensations of fear and excitement, as adrenaline and increased blood pressure cause it to beat more rapidly.

As she traces the complex medical journeys of Keira and Max, Clarke takes regular detours into the lives of those looking after them – the book features a lengthy cast of paramedics, ICU staff, anaesthetists and surgeons – as well as the scientific discoveries that inform their treatment. Thus, after introducing us to Nick, a newly qualified doctor who happened upon the crash site and carefully removed Keira from the car to perform CPR, the narrative peels off to tell us about Dr James Styner, an orthopaedic surgeon and qualified pilot who, 41 years earlier, was involved in a light aircraft crash with his wife and four children. While receiving treatment at a small rural hospital, Styner watched a doctor pick up his semi-conscious son from a hospital trolley, causing his head to loll backwards. In that moment, it was clear to him that the medics had no idea how to safely respond to a serious trauma, and so he devised a uniform protocol for managing advanced trauma cases that is still used today.

Elsewhere, Clarke documents the achievements of Bjørn Ibsen, who masterminded artificial ventilation, which in turn led to the creation of the first intensive care units; the first operation on the human heart by Norwegian surgeon Axel Cappelen; and John Gibbon’s invention of the heart-lung machine, an external pump that moves blood around the body during heart surgery. All their innovations would contribute to Keira and Max’s treatment decades later.

There are moments, within this intricate tapestry, where Clarke’s evocative, empathetic writing makes you catch your breath. Taking in the spectacle of drivers in gridlocked traffic miraculously making way for an ambulance, she reflects on the unspoken knowledge “that nothing more substantial than the whisper of fate keeps the people they love from the horror of such a blue-lit dash, the vehicle emblazoned with the cruellest combination of words: children’s intensive care ambulance”.

And while there is much to be gleaned here about the minutiae of medical inventions and procedures, Clarke never loses sight of the human impact, marvelling at the “momentous and peculiarly banal” computer system that matches transplant donors with suitable recipients. “[These are] matters of life and death, in Excel spreadsheet form. The coordinator knows that the list of names she is about to generate will thud into the world like a mortal decree, a life-and-death diktat, plucking one child from the abyss while condemning others to remain on the precipice. It is extraordinary, God-like, terrifying.”

We know from the outset what happens to Keira and Max, though that doesn’t prevent the tension from rising as everything is put in place for the safe transfer of the heart. By the time Max is prepped for surgery and drifting into anaesthetised unconsciousness, his parents aren’t the only ones sobbing: I was in pieces. It’s a measure of Clarke’s careful, sensitive reporting that both Keira and Max’s parents not only agreed to the publication of this book but consented to be interviewed at length for it.

In one of the most remarkable passages, we find ourselves privy to the meeting of the two families – a rarity in transplant cases – three months after Max’s surgery. Joe, Keira’s father, is handed a stethoscope. Putting the buds in his ears, he lays the drum on Max’s chest and listens to his daughter’s heart: busy, pulsing, alive.