Ashcroft begins with "Life at the Top", with the challenges humans face at high altitudes — in aircraft and balloons, mountain climbing, and in permanent settlements. Abrupt depressurisation (in aircraft accidents) is extremely dangerous, but even with more gradual ascent mountain sickness is a danger. This is not well-understood — no way is known of predicting who will suffer from it — but involves changes in breathing regulation, which is driven by oxygen concentrations at sea level but by carbon dioxide at low pressures. Indigenous people may have genetic and developmental adaptations to living at altitude, but others can acclimatise: some can climb Everest without oxygen. Birds cope with low pressures and rapid changes in pressure with a more complex breathing system involving air sacs as well as lungs.
"Life under Pressure" is about ventures underwater, from free diving and early use of diving bells in salvage and war to modern scuba-diving. The major problem here is the bends (when too rapid decompression causes nitrogen bubbles to form in the blood) but other concerns include nitrogen and oxygen toxicity and high pressure nervous syndrome. Much of our understanding of these has come from scientists who used themselves as their own experimental subjects. Special gas mixtures and careful decompression now allow long-term life underwater, with North Sea oil rig workers spending whole months on the ocean floor. Sea lions and other diving mammals have special adaptations to cope with pressure changes — and of course some animals live at much greater depths, which humans can only reach inside rigid submersibles.
As "Life in the Hot Zone" explains, humans die from heatstroke if their core temperature rises above 42 degrees. To keep cool, humans can restrict blood flow and sweat — if water is available, which is why that is critical at high temperatures. Other topics touched on include the role of elevated temperatures induced by fever, the development of the thermometer, salamanders and bombardier beetles, malignant hyperthermia (where people can shiver themselves to death; also found in pigs as porcine stress syndrome), and the physics of heat transfer (and how firewalking works).
Human responses to "Life in the Cold" include shivering, goose pimples, restriction of the blood flow, and (in babies) the burning of brown fat. The dangers include hypothermia, trench foot, and frostbite — outside the tropics, cold is often as much of a danger as drowning after marine accidents. People can survive surprisingly low temperatures, however, with cases where those long given up for dead have revived. And animals such as penguins, polar bears, and fish have anatomical adaptations to the cold.
In "Life in the Fast Lane" Ashcroft considers energetic extremes, sporting records and limits. She provides non-technical explanations of how muscle works, how energy production links such metabolites as ATP, glycogen, lactic acid, and creatine phosphate, and how the oxygen supply works. She also touches on fitness and training, differences between sprinters and distance runners, gender differences, drug use, and the abilities of animals (and how allometry allows ants and fleas to perform amazing relative feats).
"The Final Frontier" looks at life in space. Life has to be protected from huge temperature variations, airlessness, and radiation. As well as practical problems it poses for everyday living, weightlessness affects the balance and induces muscle and bone loss. And then of course there are the accelerations ("g-forces") necessary to climb into orbit (also found in bungee jumping and high-performance military aircraft) and the problems astronauts face on re-entry and return to a normal environment.
A final chapter "The Outer Limits" turns to non-human life, especially bacteria, in even more extreme environments: in superheated water around deep sea thermal vents, acidic and basic conditions, frozen solid, and possibly on other planets. (Those interested in microbes in extreme conditions should check out Postgate's The Outer Reaches of Life.)
No attempt is made to link these chapters together, and the topics within chapters are often disconnected — even those not boxed as digressions. Readers will pick up from Life at the Extremes a variety of isolated facts rather than any kind of systematic understanding. Some of these are, however, quite intriguing — for example a brief account of the Ama traditional female divers of Japan. Some minor problems could have been fixed by tighter editing: Greek hoplite armour did not weigh anything remotely like "110-130 kilos"; we don't really need to know that Boyle's laboratory in Oxford was close to Ashcroft's own (the blurb already tells us that she is "Professor of Physiology at Oxford"); and the occasional flippancy seems inappropriate — "Unfortunately for science (and himself), Lavoiser [sic] met his death early, at the hands of Madame Guillotine". None of this, however, should stop a broad range of people enjoying Life at the Extremes.