The Body: A Guide for Occupants

by Bill Bryson

The most remarkable part of all is your DNA. You have a metre of it packed into every cell, and so many cells that if you formed all the DNA in your body into a single fine strand it would stretch ten billion miles, to beyond Pluto.8 Think of it: there is enough of you to leave the solar system. You are in the most literal sense cosmic.

An interesting thing about touch is that the brain doesn’t just tell you how something feels, but how it ought to feel. That’s why the caress of a lover feels wonderful, but the same touch by a stranger would feel creepy or horrible. It’s also why it is so hard to tickle yourself.

Your sweat is 99.5 per cent water. The rest is about half salt and half other chemicals. Although salt is only a tiny part of your overall sweat, you can lose as much as twelve grams (three teaspoonsful) of it in a day in hot weather, which can be a dangerously high amount, so it is important to replenish salt as well as water.

If you put all the Earth’s microbes in one heap and all the other animal life in another, the microbe heap would be twenty-five times greater than the animal one.7 Make no mistake. This is a planet of microbes. We are here at their pleasure. They don’t need us at all. We’d be dead in a day without them.

As well as contact injuries, the brain is vulnerable to its own internal storms. Strokes and seizures are peculiarly human frailties. Most other mammals never suffer strokes, and for those that do it is a rare event. But for humans, it is the second most common cause of death globally, according to the World Health Organization. Why this should be is something of a mystery. As Daniel Lieberman observes in The Story of the Human Body, we have an excellent blood supply to the brain to minimize the risk of stroke and yet we get strokes.

All forms of heart failure can be cruelly sneaky. For about a quarter of victims the first (and, more unfortunately, last) time they know they have a heart problem is when they suffer a fatal heart attack.13 No less appallingly, more than half of all first heart attacks (fatal or otherwise) occur in people who are fit and healthy and have no known obvious risks. They don’t smoke or drink to excess, are not seriously overweight, and do not have chronically high blood pressure or even bad cholesterol readings, but they get a heart attack anyway. Living a virtuous life doesn’t guarantee that you will escape heart problems, it just improves your chances.

‘In Henry VIII’s reign puberty started at sixteen or seventeen. Now it is more commonly eleven. That’s almost certainly because of improved nutrition.’

The kidneys are invariably called the workhorses of the body. Each day they process about 180 litres of water – that is the amount a bath holds up to the overflow level – and 1.5kg of salt.15 They are startlingly small for the amount of work they do, weighing just 5 ounces (140g) each. They are not in the small of the back, as everyone thinks, but higher up, about at the bottom of the ribcage. The right kidney is always lower because it is pressed down upon by the asymmetrical liver.

‘Cartilage is remarkable, too. It is many times smoother than glass: it has a friction coefficient five times less than ice. Imagine playing ice hockey on a surface so smooth that the skaters went sixteen times as fast. That’s cartilage. But unlike ice, it isn’t brittle. It doesn’t crack under pressure as ice would. And you grow it yourself. It’s a living thing. None of this has been equalled in engineering or science. Most of the best technology that exists on Earth is right here inside us. And everybody takes it almost completely for granted.’

‘You shouldn’t ever try to kill yourself by cutting your wrists, by the way,’ he says. ‘All of those things going in are wrapped in a protective band called a fascial sheath, which makes it really hard to get to the arteries. Most people who cut their wrists fail to kill themselves, which is no doubt a good thing.’ He is briefly thoughtful. ‘It’s also really hard to kill yourself by jumping from a height,’ he adds. ‘The legs become a kind of crumple zone. You can make a real mess of yourself, but you are very likely to survive. Killing yourself is actually difficult. We are designed not to die.’

‘Bone is stronger than reinforced concrete,’ says Ben, ‘yet light enough to allow us to sprint.’ All your bones together will weigh no more than about 20 pounds (nine kilograms), yet most can withstand up to a ton of compression. ‘Bone is also the only tissue in the body that doesn’t scar,’ Ben adds. ‘If you break your leg, after it heals you cannot tell where the break was. There’s no practical benefit to that. Bone just seems to want to be perfect.’ Even more remarkably, bone will grow back and fill a void. ‘You can take up to thirty centimetres of bone out of a leg, and with an external frame and a kind of stretcher you can have it grow back,’ Ben says. ‘Nothing else in the body will do that.’ Bone, in short, is amazingly dynamic.

Today the average American walks only about a third of a mile (or half a kilometre) a day – and that’s walking of all types, including around the house and workplace.

According to the Economist, some American companies have begun offering rewards to employees who log a million steps a year on an activity tracker such as a Fitbit. That seems a pretty ambitious number but actually works out to just 2,740 steps a day, or a little over a mile. Even that, however, seems to be beyond many. ‘Some workers have reportedly strapped their Fitbits to their dogs to boost their activity scores,’ the Economist noted.

Even just standing burns an extra 107 calories an hour. Walking around burns 180.

Body size has a great deal to do with how we are affected by gravity. It will not have escaped your notice that a small bug that falls off a tabletop will land unharmed and continue on its way unperturbed. That is because its small size (strictly, its surface area-to-volume ratio) means that it is scarcely affected by gravity. What is less well known is that the same thing applies, albeit on a different scale, to small humans. A child half your height who falls and strikes her head will experience only 1/32 of the force of impact that a grown person would feel, which is part of the reason that children so often seem to be mercifully indestructible.

The existence of Unit 731 was a well-guarded secret, by Japanese and American officials alike, and would have remained forever unknown to the wider world if a student from Keio University in Tokyo hadn’t in 1984 come across a box of incriminating documents in a second-hand bookshop and brought them to the attention of others.27 By this time it was far too late to bring to justice Shiro Ishii. He had died in 1959, peacefully in his sleep, at the age of sixty-seven after nearly a decade and a half of untroubled post-war life.

Incidentally, the reason your nose runs in chilly weather is the same reason your bathroom windows run with water in chilly weather. In the case of your nose, warm air from your lungs meets cold air coming into the nostrils and condenses, resulting in a drip.

infectious droplets are more infectious to us when we pick them up by touch rather than when we breathe (or kiss) them in.

A cooked potato, for instance, is about twenty times more digestible than a raw one.

Even today vitamins are an ill-defined entity. The term describes 13 chemical oddments that we need to function smoothly but are unable to manufacture for ourselves.

We consume about 2.5 litres of a water a day, though we are not generally aware of it since about half is contained within our foods. The conviction that we should all drink eight glasses of water a day is the most enduring of dietary misunderstandings. The idea has been traced to a 1945 paper from America’s Food and Nutrition Board, which noted that that was the amount that the average person consumed in a day.18 ‘What happened,’ Dr Stanley Goldfarb of the University of Pennsylvania told the BBC Radio 4 programme More or Less in 2017, ‘was that people sort of confused the idea that this was the required intake. And the other confusion that occurred was then people said that it is not so much that you should take in eight ounces eight times a day, but that you should consume that in addition to whatever fluid you consume in association with your diet and your meals. And there was never any evidence for that.’

Drinking too much water can actually be dangerous.20 Normally your body manages fluid balance very well, but occasionally people take in so much water that the kidneys cannot get rid of it fast enough and they end up dangerously diluting the sodium levels in their blood, setting off a condition known as hyponatraemia. In 2007, a young woman in California named Jennifer Strange died after drinking six litres of water in three hours in a clearly ill-judged water-drinking competition held by a local radio station. Similarly in 2014, a high school football player in Georgia, complaining of cramps after practice, downed 7.5 litres of water and 7.5 of Gatorade, and soon afterwards fell into a coma and died.

An investigation by the American Food and Drug Administration in 2016 found that 84 per cent of chicken breasts, nearly 70 per cent of ground beef, and getting on for half of pork chops contained intestinal E. coli, which is not good news for anything but the coli.

According to a study by the United States Department of Agriculture, about a quarter of all chicken pieces sold in stores are contaminated with salmonella.7 There is no treatment for salmonella poisoning.

Incidentally, most nausea-inducing microbes need time to proliferate inside you before they make you sick. A few, like Staphylococcus aureus, can make you ill in as little as an hour, but most take at least twenty-four hours. As Dr Deborah Fisher of Duke University told the New York Times, ‘People tend to blame the last thing they ate, but it’s probably the thing before the last thing they ate.’8 Actually, many infestations take far longer than that to manifest. Listeriosis, which kills about three hundred people a year in America, can take up to seventy days to show symptoms, which makes tracking down a source of infection a nightmare.

The largest source of foodborne illness is not meat or eggs or mayonnaise, as is commonly thought, but green leafy vegetables. They account for one in five of all food illnesses.

A very few people suffer from a condition called REM sleep behaviour disorder in which the limbs don’t become paralysed, and they do indeed sometimes hurt themselves or their partners with their thrashing.

As well as normal overnight sleep, we also commonly indulge in snatches of wakeful-hours sleep in a state known as hypnagogia, a netherworld between waking and unconsciousness, often without being aware of it. Alarmingly, when a dozen airline pilots on long-haul flights were studied by sleep scientists almost all were found to have been asleep, or all but asleep, at various times during the flight without realizing it.

it is a curious fact, when you think about it, that no matter how profoundly unconscious we get, or how restless, we almost never fall out of bed, even unfamiliar beds in hotels and the like. We may be dead to the world, but some sentry within us keeps track of where the bed’s edge is and won’t let us roll over it (except in unusually drunk or fevered circumstances). Some part of us, it seems, pays heed to the outside world, even for the heaviest sleepers.

The best way to reduce snoring is to lose weight, sleep on your side and not drink alcohol before retiring.

‘On a Presidential visit to a farm, Mrs Coolidge asked her guide how many times the rooster copulated daily.1 “Dozens of times” was the reply. “Please tell that to the President,” Mrs Coolidge requested. When the President passed the pens and was told about the rooster, he asked: “Same hen every time?” “Oh no, Mr President, a different one each time.” The President nodded slowly, then said: “Tell that to Mrs Coolidge.”’

The chances of a successful fertilization from a single randomly timed act of sex have been calculated to be only about 3 per cent.

It is a curious fact that every woman is born with her lifetime’s supply of eggs already inside her. They are formed when she is still in the womb and sit in the ovaries for years and years before being called into play. The idea of women being born with a full load of eggs – the formal name is ova – was first suggested by the great and busy German anatomist Heinrich von Waldeyer-Hartz, but even he would have been astonished at just how quickly and abundantly eggs are formed within the growing child. A 20-week-old foetus will weigh no more than three or four ounces (about 100 grams) but will already have six million eggs inside her.

Just since 1980, the age of puberty has fallen in America by eighteen months. About 15 per cent of girls now begin puberty by age seven.

Perhaps as many as half of all conceptions are lost without being noticed. Without this, the rate of birth defects would be 12 per cent instead of 2 per cent.

An oddity of human nerves is that those in the peripheral nervous system can heal and regrow when damaged, whereas the more vital ones in the brain and spinal cord cannot.

Medical science offered very little in the way of safe, lasting relief back then. We are not much further along now. As Andrew Rice, a pain researcher at Imperial College London, told Nature in 2016, ‘The drugs we have relieve 50 per cent of pain in somewhere between one in four and one in seven of the patients we treat.14 That’s for the best drugs.’ In other words, some 75 per cent to 85 per cent of people get no benefit at all from even the best pain drugs, and those that do get benefit don’t usually get much. Pain relief, as Irene Tracey puts it, has been ‘a pharmacological graveyard’. Pharmaceutical companies have poured billions and billions into drug development but have not come up with a drug that controls pain effectively and doesn’t cause addictions.

Between 1999 and 2014, by one estimate, a quarter of a million Americans died from opioid overdoses.

The United States has 4 per cent of the world’s population but consumes 80 per cent of its opioids. About two million Americans are thought to be opioid addicts. Another ten million or so are users.

Opioid use has become such big business that we have now reached the surreal situation that pharmaceutical companies are producing drugs to alleviate the side effects of opioid overuse. Having helped to create millions of addicts, the industry is now profiting from medications designed to make their addiction a little more comfortable. So far the crisis doesn’t seem to be going away. Every year opioids (both legal and illegal) claim 45,000 or so American lives, far more than are killed in car crashes.

In London, life expectancy drops reliably by one year for every two stops travelled eastwards from Westminster on the District Line of the Underground.

Life in America is also much riskier, especially for young people. A US teenager is twice as likely to be killed in a car accident as a young person in a comparable country abroad, and is 82 times more likely to be killed by a gun.

Men face similarly unhappy prospects with prostate screening. The prostate is a small gland, about the size of a walnut and weighing just one ounce (less than 30g), which is chiefly involved in producing and distributing seminal fluid. It is tucked neatly – not to say inaccessibly – up against the bladder and wrapped around the urethra. Prostate cancer is the second leading cause of cancer death among men (after lung cancer), and grows more common as men get to their fifties and beyond. The problem is that the test for prostate cancer, called a PSA test, is not trustworthy. It measures levels in the blood of a chemical called prostate-specific antigen (PSA). A high PSA reading indicates a possibility of cancer – but only a possibility. The only way of confirming if cancer exists is with a biopsy, which involves sticking a long needle into the prostate via the rectum and withdrawing multiple tissue samples – not a procedure any man is likely to undertake eagerly. Since the needle can only be randomly inserted into the prostate, it is a matter of luck whether it strikes a tumour or not. If it does find a tumour, there is no telling with current technology if the cancer is aggressive or benign. On the basis of this uncertain information, a decision must be made on whether to surgically remove the prostate – a tricky operation with frequently dispiriting consequences – or treat it with radiation. Between 20 and 70 per cent of men suffer impotence or incontinence after treatmen...

A meta-analysis of six randomized control trials involving 382,000 men found that for every 1,000 men screened for prostate cancer about one life was saved – great news for that individual, but not so good for the large numbers of others who may spend the rest of their lives incontinent or impotent, the majority of them having undergone serious but possibly ineffectual treatments. All this isn’t to say that men should avoid PSA tests or women breast cancer screening. For all their flaws, they are the best tools available, and they do indubitably save lives, but those undergoing screenings should perhaps be made more aware of the shortcomings. As with any serious medical issue, if you have concerns you should consult a trusted physician.

There’s no question that people are living longer than ever. If you are a seventy-year-old man in America today, you have only a 2 per cent chance of dying in the next year. In 1940, that probability was reached at age fifty-six.3 In the developed world at large, 90 per cent of people reach their sixty-fifth birthday, the great majority of them in a healthy condition. But now it seems we have reached a point of diminishing returns. By one calculation, if we found a cure for all cancers tomorrow, it would add just 3.2 years to overall life expectancy.4 Eliminating every last form of heart disease would add only 5.5 years. That’s because people who die of these things tend to be old already, and if cancer or heart disease doesn’t get them, something else will. Of nothing is that more true than Alzheimer’s disease.5 Eradicating it altogether, according to the biologist Leonard Hayflick, would add just 19 days to life expectancy.

As Daniel Lieberman has noted, ‘For every year of added life that has been achieved since 1990, only 10 months is healthy.’

Most of us would almost certainly never have heard of either free radicals or antioxidants if a research chemist in California named Denham Harman had not, in 1945, read an article about ageing in his wife’s Ladies’ Home Journal and developed a theory that free radicals and antioxidants are at the heart of human ageing.12 Harman’s idea was never anything more than a hunch, and subsequent research proved it to be wrong, but nonetheless the idea has taken hold and will not go away. The sale of antioxidant supplements alone is now worth well over $2 billion a year. ‘It is a massive racket,’ David Gems of University College London told Nature in 2015.13 ‘The reason the notion of oxidation and ageing hangs around is because it is perpetuated by people making money out of it.’

It is a myth, incidentally, that menopause is triggered by women exhausting their supply of eggs.18 They still have eggs. Not many, to be sure, but more than enough to remain fertile. So it isn’t the literal running out of eggs that triggers the process (as even many doctors appear to believe). No one knows exactly what is the trigger.

As Daniel Lieberman told me, reaching 80 is largely a consequence of following a healthy lifestyle, but after that it is almost entirely a matter of genes.

It is a myth, and physiological impossibility, incidentally, that hair and nails continue to grow after death. Nothing grows after death.