Were it not for the real-life tragedy and horror involved, this documentary could almost be a bleak and often hilarious black comedy. Film-maker Luke Lorentzen follows the crew of one of Mexico City’s dozens of private ambulances, guys who zoom recklessly through the streets to the site of an emergency, either directly called via the private medical insurance firm with whom the unfortunate person is insured, or after they have checked the police scanner alerting them to a likely disaster.

One such crew is the Ochoa family, dad Fer at the wheel, teenage son Juan in overall charge and his kid brother Josué somewhat pointlessly along for the ride, confidently handling their dodgily maintained equipment. (Whether he should be in school is a moot point.)

Often the guys will challenge a rival ambulance to a race to a possible job, almost screaming with excitement, with Juan shouting at cyclists and pedestrians to get out of their way through a loudhailer. And then ruthless Juan has to tell the sobbing victims and their friends how desperately serious it all is, how they need to take the patient to a certain superior private hospital (which might not be the closest) and how they should just “sign everything” when they arrive. Sometimes they don’t, which means Juan has to beg for the “emergency ride” fee of 3,800 pesos (£160) and he has no legal means of enforcing that without a signature. Incredibly, when a mother brings in her desperately injured daughter who dies en route, the Ochoas find themselves wheedling some money out of her.

Yet somehow Lorentzen shows that it is not the Ochoa family who are the bad guys, but the whole rotten system; and the private-ambulance cowboys are themselves desperate for cash, having to spend a good deal on bribing the cops. This is what we could have to look forward to with a privatised NHS.

• Midnight Family is released in the UK on 21 February.