It’s 6.45am at Camden ambulance station in north London, and the day shift is just beginning. Andy Donovan, who will drive the ambulance I will accompany for the next nine hours, is making me a cup of tea. His more senior paramedic partner, Dean Lowes, is running a few minutes late. When he does arrive, Lowes looks very sorry for himself: he’s got an ear infection, picked up on a friend’s stag weekend in Budapest. Lowes is the ambulance’s first case of the day. They nip off to the nearby Royal Free hospital in Hampstead to get some ear drops. Paramedic, heal thyself.

All this delays us for more than an hour, and we’re not ready to “go green” – telling the London ambulance service’s call centre near Waterloo station that they are available for a job – until after 8.30am. Lowes, who along with Donovan is featured in the BBC’s new three-part series on London’s overstretched ambulance service, is suitably embarrassed. “This never happens to me,” he says. “I’m never ill.” But full marks for at least getting here. Crewing an ambulance is challenging at the best of times.

Soon after going green, our first assignment comes in, flashing up on a monitor at the front of the ambulance. It’s just about as unpleasant as it could be. One word: “HANGING”, and the location. It is a “Red One” – the top-priority call sign, meaning life-threatening. Lowes and Donovan’s speed of reaction is electrifying. One moment, Lowes had been playing a Kings of Leon track on his mobile and saying how much he liked the band; the next, the ambulance is tearing south towards King’s Cross.

The call comes through at 8.49, and we get to the scene five minutes later. My heart sinks when I realise it is student accommodation. Two policemen are arriving simultaneously, and we all head up two floors in the lift to a stuffy, antiseptic white corridor. I go up with the policemen, who are bemoaning the fact their car was the closest to the scene. “You had a feeling it was going to be a funny day,” one says to the other. “You said you had a feeling in your bones.” “Yeah,” says the other with a grim laugh, “I should shut the fuck up.” In situations like this, black humour is sometimes the only way out.

Lowes, as senior paramedic, is first into the little study-bedroom. He has to decide if the student, who appears to have hanged himself, is dead, or, in the official language they use, to declare “life extinct”. It takes him just moments to satisfy himself that he is. The student is pronounced dead at 8.57. I can’t bring myself to look at the body – the young man is fully clothed – for too long. What strikes me most is how peaceful he looks, and how red his hands are – the blood drains down to the hands and feet, a sign he has been dead for several hours.

Within minutes there are half a dozen police on the scene, taking a statement from the traumatised fellow student who discovered the body, talking to the staff in the hall of residence, looking through the young man’s possessions to establish his identity. It has ceased to be a medical emergency and become a police inquiry – and a personal tragedy for the family who do not yet know what has happened. It appears the young man, who was 23, was anxious about a dissertation he had failed to deliver. What a terrible, pointless waste.

This is a shocking beginning. A suicide by hanging is rare. It is the first Lowes has witnessed. “He looked like a wax dummy,” he says as we wait downstairs while he does the paperwork to certify the death. “It’s when you see his passport and the picture of how he looked when he was alive that it hits you. That humanises it.” Having been a body, he becomes a person. “I try not to look at a dead person’s effects too much,” says Lowes, “because you start to build a little story about them.” “You can’t go into it too deeply,” adds Donovan. “There’s a lot of stuff you lock in the box.”

A paramedic team leader turns up. He doesn’t say so, but Lowes and Donovan know he is there for their welfare – to make sure that having to deal with the young man’s death has not affected them too severely. “If you want to take a bit of a break, that’s fine,” the team leader tells them. They don’t particularly, although they do have a fag standing next to their ambulance. The morning is hot, and people stroll past the student block, laughing in the late-summer sunshine, not realising that inside a promising young life has been extinguished.

By 11am, they are ready to roll again. They go green, pressing the button that declares the ambulance available, and in a second – literally – their next assignment flashes up. It’s another Red One – a cardiac arrest in West Hampstead, a couple of miles to the north. The siren screams, I lurch around in the back of the ambulance feeling sick, and Donovan swears at the vehicles that block his way, costing him vital seconds that could mean the difference between life and death.

The job is undeniably exciting, or at least seems so to me. Horrible, of course – no one wants to discover dead bodies – but also fascinating because of its unpredictable nature. You have no idea where you will go next or what you will have to deal with. “That’s the beauty of it,” Donovan had told me earlier. “You never know what you’re going to from job to job.”

It’s like roulette, I suggest, and he tells me that is exactly what they call the last job of the day. If you go green with, say, half an hour left of your shift, the call centre will play “red roulette”. Instead of giving you a less urgent call (categorised from C1 to C4, depending on the degree of seriousness), they will give you something life-threatening. It seems mad, but the logic is that whatever you do is likely to take hours – every callout seems to generate a mountain of paperwork – so you may as well go to something that is worth your while. A practical, if heartless, way of looking at it. Paramedics often work 12-hour shifts, and I can’t imagine what it’s like to get a final Red One at the very end.

We get to West Hampstead in about six minutes. Another ambulance is already on the scene, as well as team leader April Barter, who has come by car. I bumped into her earlier at the ambulance station in Camden, and she was complaining about having nothing to do that morning. Now she has something to do. A man in his 60s has had a cardiac arrest – a heart attack in which his heart has stopped completely – and the struggle is on to save him.

The man is lucky. I hadn’t realised where we were when we arrived, but then it dawns on me – it’s a bridge club, and dozens of middle-aged and elderly card players are watching the paramedics’ attempts to revive their fellow participant. Even before the first crew arrived, some medically trained members of the club had starting giving him CPR (cardiopulmonary resuscitation), that pounding of the chest that aims to kick the heart back into action. Without their prompt action, he would be dead. The paramedics continue the CPR – by now his chest looks as if it has caved in, but apparently this is quite normal – and administer defibrillation, an electric shock designed to correct his heart rhythm.

After half an hour of attention, his heart is functioning again and he can be taken to hospital. He is still unconscious, but has a reasonable chance of surviving. His bridge partner, who tells me they had just played a very successful rubber, offers to go to hospital with him, while his wife is given the news at home. There is an impressive calm at the club as the man is carried out. Who knew bridge could be an extreme sport – or bridge players so unflappable?

A cardiac arrest involving two crews generates an especially large volume of paperwork, and we are stuck outside the Royal Free for more than an hour while all the forms are filled in. Soon after we deliver the man to the hospital’s heart centre, Barter tells me he has regained consciousness. “The fact he’s awake, his eyes are open, he’s moving around tells us his brain has more oxygen. Although potentially it has been starved of oxygen for a short period of time, that’s a really positive sign and it’s a potentially good outcome for him. That’s a massive lift for us. Good times.” She says she is “buzzing”. “If I can make a difference to one person in a day, then I go home happy,” Donovan told me earlier. It looks as if he and his colleagues have made that difference today.

It is that difference, rather than the material rewards of the job, that attracts Lowes and Donovan. “You don’t do this job if you want money,” says Lowes, who is 37 and comes from the north-east of England. “It has other benefits. You go home and you sleep at night. You don’t take any work home with you. You might take some kind of emotional stuff away at the end of the day but, as far as the working day is concerned, once you’re finished, you’re finished.”

Lowes, who is a fully qualified paramedic, tells me he earned £36,000 last year. Thirty-year-old Donovan, a friendly, buoyant east Londoner who is one rung below his partner in terms of clinical qualifications, says he earns £20,000 a year basic, which rises to about £28,000 with the inner-London allowance, rest-break compensation (they will typically work through their breaks) and overtime, lots of overtime. They are contracted to work 37-and-a-half hours a week, but can do up to 56. Without the overtime, they would struggle financially. The staff need the relentless pressure on the service to earn enough to live.

The upside, apart from the drama of the job and the satisfaction of saving lives and helping people at moments of crisis, is the flexibility. “There are a lot of other things out there that I wouldn’t want to do, sitting behind a desk being one of them,” says Donovan. “At least in this job you’ve got a little bit of freedom. Once you’re out on the road, you’re your own boss.”

While the paperwork for the cardiac arrest case is being done, I talk to Gary Nicholls, one of the paramedics in the first crew to arrive. He has clocked up almost 24 years on the job – Lowes and Donovan have each done seven. “You never know what the next call is going to be,” he says. “That’s what keeps us interested. But it doesn’t matter what comes down on the screen, we can always deal with it. The workload can be relentless, but your colleagues are there to get you through the shift.”

The London ambulance service was put into “special measures” last year because of a number of failings, including staff shortages, poor response times, lack of leadership and concern that the service was ill-prepared to deal with a major terrorist attack. The cynic in me thinks the BBC series – and my ride in the ambulance today – are part of the PR fightback, and maybe they are, but there is no doubting the commitment of the dozen or so paramedics and backup staff I meet. This is a service under pressure, but by no means one that has lost heart. Nicholls really does believe they can deal with anything, including his first job that day – chasing a naked man who it was feared was high on drugs across Hampstead Heath.

It is already well past 1pm. The complexity of the jobs, the paperwork and the fact that you need a bit of a breather mean crews will only do four or five callouts in a nine-hour shift, and six or eight in a 12-hour one. Just before 1.30pm, Donovan and Lowes go green again. This time it’s a Red Two – slightly less urgent but still potentially life-threatening, a woman in Kilburn with chest pains and breathing problems.

She is sitting on the stairs of her house when we arrive six minutes later. She looks remarkably well, and within about two minutes of arriving Lowes has diagnosed an anxiety attack. She had a heart bypass operation five years earlier, and clearly fears a heart attack. She has already been to hospital for a checkup that day, and now wants to go back, despite getting the all-clear earlier. It is unlikely there is a serious problem, but Lowes and Donovan can’t take any chances, so take her to the Royal Free. It’s not their most productive couple of hours, but they talk to her respectfully, calm her down, deliver her to A&E and fill in a fresh set of forms.

It’s now 3 o’clock, and we’re on our way back to Kilburn. This time they’ve received a less urgent C2 call, after an earlier Red One to another fatality was aborted. The monitor in the ambulance advises “man in his 70s with severe behavioural change”. When we get to the flat, we find an elderly man close to collapse – probably through dehydration – and his wife at the end of her tether. She thinks he has undiagnosed dementia, and there are suggestions he can be violent towards his family, though today he can barely raise himself from the sofa.

It is an example of the social work side of paramedics’ work. They check him over physically, but he is in reasonable shape apart from the dehydration. What he may need very soon is a place in a care home. That is the shadow that falls across the conversation Lowes has in the corridor with the man’s wife – the sad but all-too-common conclusion of a 50-year marriage – while Donovan talks to the man’s son about sport.

As their appearance in the BBC series shows, they are very good at being de facto social workers, counselling the anxious, the elderly, the confused, the demented. “When I first started this job, going into people’s homes took a bit of getting used to,” says Donovan, “but because you’re wearing a uniform, in the eyes of the public you’re a goodie. You’re welcomed into most situations, whether it’s for social reasons or for emergencies.”

Lowes calls the man’s GP, who promises to come round. For the moment, there is nothing more that can be done. More paperwork and another cigarette in the afternoon sun. The shift is drawing to a close, and the crew do not fancy any red roulette. There is a general callout for an ambulance, any ambulance, to go to Victoria station, where a girl has fainted. They decide it’s too far and head back to base. That’s enough excitement for the day.

As we drive back to Camden, their monitor is reporting that University College hospital has been temporarily closed, St Mary’s in Paddington is accordingly under severe pressure and the Royal Free is “breaching” – A&E is missing its waiting targets and patients are being left in ambulances longer than they should be. It’s going to be quite a night shift, and Lowes and Donovan are happy to be out of it. But tomorrow they will be back, and who knows where the spin of the roulette wheel will take them?

Ambulance starts on BBC1 at 9pm on 27 September.

In the UK, the Samaritans can be contacted on 116 123. In the US, the National Suicide Prevention Hotline is 1-800-273-8255. In Australia, the crisis support service Lifeline is on 13 11 14.