The Body: A Guide for Occupants

by Bill Bryson

We eat a lot of carbohydrates, but we use them up quickly, so the total amount in your body at any given time is modest—usually less than a pound. The main thing to bear in mind is that carbohydrates, upon being digested, are just more sugar—often quite a lot more. That means that a 150-gram serving of white rice or a small bowl of cornflakes will have the same effect on your blood glucose levels as nine teaspoons of sugar.

THE THIRD MEMBER of the trio, fats, are also made up of carbon, hydrogen, and oxygen, but in different proportions. This has the effect of making fat easier to store. When fats are broken down in the body, they are teamed up with cholesterol and proteins in a new molecule called lipoproteins, which travel through the body via the bloodstream. Lipoproteins come in two principal types: high density and low density.

Cholesterol is not as fundamentally evil as we tend to think it. Indeed, it is vital to a healthy life. Most of the cholesterol in your body is locked up in your cells, where it is doing useful work. Just a small part—about 7 percent—floats about in the bloodstream. Of that 7 percent, one-third is “good” cholesterol and two-thirds is “bad.”

Depending on where the fat ends up, it is known as subcutaneous (beneath the skin) or visceral (around the belly). For complex chemical reasons, visceral fat is much worse for you than the subcutaneous kind.

That seems an alarming decline until you realize that the most popular vegetable in America by a very wide margin is the French fry. (It accounts for a quarter of our entire vegetable intake.) These

Wrapped around the outside of it, like a wall around a garden, is six feet of broader-gauge plumbing known variously as the large intestine, bowel, or colon.

These additional receptors, known as photosensitive retinal ganglion cells, have nothing to do with vision but exist simply to detect brightness—to know when it is daytime and when night. They pass this information on to two tiny bundles of neurons within the brain, roughly the size of a pinhead, embedded in the hypothalamus and known as suprachiasmatic nuclei. These

In fact, according to genuine studies, men of college age think about sex nineteen times a day, roughly once every waking hour, which is about the same frequency as they think about food. College women think about food more often than they think about sex, but they don’t think about either terribly often.

Women (and we are talking here about healthy, fit women) carry about 50 percent more fat on their frames than fit, healthy men.

This not only makes the woman more agreeably soft and shapely to suitors but also gives her reserves of fat she can call upon for milk production during times of hardship.

Women’s bones wear out sooner, particularly after menopause, so they suffer more breaks in later life. Women get Alzheimer’s twice as often (partly because they also live longer) and experience higher rates of autoimmune diseases. They metabolize alcohol differently, which means they get intoxicated more ...

Women even tend to carry bags differently than men do. It is thought that their wider hips necessitate a less perpendicular carrying angle for their forearm so that their swinging a...

That’s why women generally carry bags with their palms facing forward (allowing their arms to be slightly splayed) while men...

Far more significantly, women and men have heart attacks in quite different ways. A woman suffering a heart attack is more likely to experience abdominal pain and nausea than a man, which makes it more probable that it will be misdiagnosed. In ...

Men have their own differences. They get Parkinson’s disease more often and commit suicide more, even though they suffer less from clinical depression. They are more vulnerable to infection than females (and not just humans but across nearly all species). That may indicate some hormonal or chromosomal difference that hasn’t yet been determined, or it may simply be that males on the whole lead riskier, more infection-prone lives. Men are also more likely to die from their infections and from physical injury, though again whether that is b...

Women are anatomically different in one other very significant way: they are the sacred keepers of human mitochondria—the vital little powerhouses of our cells. Sperm pass on none of their mitochondria during conception, so all mitochondrial information is transferred from generation to generation through mothers alone.

Over time, the mitochondrial pool for humans has shrunk so much that, almost unbelievably but rather wonderfully, we are all now descended from a single mitochondrial ancestor—a woman who lived in Africa about 200,000 years ago. You might have heard her referred to as Mitochondrial Eve. She is, in a sense, mother of us all. —

The vulva is the complete genital package—vaginal opening, labia, clitoris, and so on. The fleshy mound above the vulva is called the mons pubis. At the top of the vulva itself is the clitoris (probably from a Greek word for “hillock,” but there are other candidates), which is packed with some eight thousand nerve endings—more per unit of area than any other part of the female anatomy—and exists, as far as can be told, only to give pleasure. Most people, including females, are unaware that the visible part of the clitoris, called the glans, is literally only the tip of it.

The rest of the clitoris plunges into the interior and extends down both sides of the vagina for about five inches. Until the early twentieth century, “clitoris” seems generally to have been pronounced “kly-to-rus.”

The prostate, it might be said, produces seminal fluid throughout a man’s adulthood and anxiety in his later years. We shall discuss this latter attribute in a later chapter.

But on the other hand, they are blundering idiots. Shoot them into a womb and they seem curiously ill-prepared for the one task evolution has given them. They are terrible swimmers and appear to have almost no sense of direction.

Unaided, it could take a sperm ten minutes to swim across a space the width of one of the words on this page.

That’s why a male orgasm is such a vigorous endeavor. What seems to the man purely a burst of pleasure really is a kind of rocket launch. Once the sperm are expelled, it isn’t known whether they move about randomly until one strikes lucky or whether they are drawn to the waiting egg by some chemical signal.*1 In either case, overwhelmingly they fail. The chances of a successful fertilization from a single randomly timed act of sex have been calculated to be only about 3 per...

He suggested that the average young man’s sperm in the United States is about 90 percent faulty. Other studies in Denmark, Lithuania, Finland, Germany, and elsewhere have reported sharp falls in sperm counts.

That number falls to 1 million by the time of birth and continues to fall, though at a slower rate, through life. As she enters her childbearing years, a woman will have about 180,000 eggs primed and ready to go. Why she loses so many eggs along the way and yet enters her childbearing years with vastly more than she will ever need are two of life’s many imponderables. The bottom line is that as a woman ages, the number and quality of her eggs diminishes, and that can be a problem for those who postpone childbirth to the later stages of their productive years, which is exactly what is happening throughout the developed world.

In six nations—Italy, Ireland, Japan, Luxembourg, Singapore, and Switzerland—the average age of women at their first birth is now over thirty, and in six others—Denmark, Germany, Greece, Hong Kong, the Netherlands, and Sweden—it is just under.

(The United States is an outlier here. The average age of women at first birth there is 26.4, the...

Buried within these national averages are even greater variations within social or economic groupings. In Britain, for instance, the average age for women at first birth is 28...

As Carl Djerassi, the father of the contraceptive pill, noted in an essay in The New York Review of Books, by the age of thirty-five a woman’s stock of eggs is 95 percent exhausted and those that remain are more liable to produce faults or surprises, like multiple births. Once women pass thirty, they are much more likely to have twins. The one certainty of procreation is that the older both parties get, the more ...

One intriguing paradox of reproduction is that women are having babies later but preparing for it earlier. The age of first menstruation for women has fallen from fifteen in the late nineteenth century to just twelve and a half today, at least in the West. That is almost certainly because of improved nutrition. But what cannot be explained is that the rate has accelerated even further in more recent years. Just since 1980, the age of puberty has fallen in America by eighteen months. About 15 percent of girls now begin puberty by age seven. That could be a reason for alarm. According to the Baylor University Medical Center Proceedings, evidence suggests that the prolonged exposure to estrogen substantially increases the risk of breast and uterine cancer in later life.

The egg is one hundred times larger than the sperm it pairs with. Fortunately, the sperm doesn’t have to force its way in, but is welcomed like a long-lost if curiously diminutive friend. The sperm passes through an outer barrier called the zona pellucida and, all being well, fuses with the egg, which immediately activates a kind of electrical force field around itself to stop other sperm from getting through. The DNA from sperm and egg are combined into a new entity called a zygote. A new life has begun.

Success from this point is by no means assured. Perhaps as many as half of all conceptions are lost without being noticed. Without this, the rate of birth defects would be 12 percent instead of 2 percent. About 1 percent of implanted eggs end up stuck in the Fallopian tube, or somewhere else other than the womb, in what is known as an ectopic pregnancy (from a Greek word meaning “wrong place”). This can be very dangerous even now. Once it was a death sentence.

About one in 100 natural births result in fraternal twins, one in 250 in identical twins, one in 6,000 in triplets, and one in 500,000 in quadruplets, but fertility treatments greatly increase the likelihood of multiple births.

After three weeks, the budding embryo has a beating heart. After 102 days, it has eyes that can blink. In 280 days, you have a new child. Along the way, at about eight weeks, the developing infant stops being called an embryo (from Greek and Latin words meaning “swollen”) and starts being a fetus (from the Latin for “fruitful”). Altogether it takes just forty-one cycles of cell divisions

to get from conception to a fully formed little human.

“Despite its lavish spending, the United States has one of the highest rates of both infant and maternal death among industrialized nations,” according to The New York Times. The average cost of childbirth in the United States is about $30,000 for a conventional birth and $50,000 for a Cesarean, about three times the cost for either in the Netherlands. Yet American women are 70 percent more likely to die in childbirth than women in Europe and about three times more likely to suffer a pregnancy-related fatality than women in Britain, Germany, Japan, or the Czech Republic. Their infants are no less at risk. One of every 233 newborn babies dies in the United States, compared with just one in 450 in France and one in 909 in Japan. Even countries like Cuba (one in 345) and Lithuania (one in 385) do much better.

The consequences for the baby can be profound. Various studies have found that people born by C-section have substantially increased risks for type 1 diabetes, asthma, celiac disease, and even obesity and an eightfold greater risk of developing allergies.

On top of all that, about four women in every ten are given antibiotics during delivery, which means that doctors are declaring war on babies’ microbes just as they are acquiring them. We’ve no idea what consequences this has for their long-term health, but it’s unlikely to be good. There are concerns already that certain beneficial bacteria are becoming endangered. B. infantis, an important microbe in mother’s milk, is found in up to 90 percent of children in developing countries but as little as 30 percent in the developed world.

The oligosaccharides are produced purely for the benefit of the baby’s gut microbes—as bribes, in effect. As well as nurturing symbiotic bacteria, breast milk is full of antibodies. There is some evidence that a nursing mother absorbs a little of her suckling baby’s saliva through her breast ducts and that this is analyzed by her immune system, which adjusts the amount and types of antibodies she supplies to the baby, according to its needs. Isn’t life marvelous?

the summer of 2018, the administration of President Donald Trump provoked dismay among many health authorities by opposing an international resolution to encourage breast-feeding and reportedly threatened Ecuador, the sponsor of the initiative, with trade sanctions if it didn’t change its position. Cynics pointed out that the infant formula industry, which is worth $70 billion a year, might have had a hand in determining the U.S. position. A Department of Health and Human Services spokesperson denied that that was the case and said that America was merely “fighting to protect women’s abilities to make the best choices for the nutrition of their babies” and to make sure that they were not denied access to formula—something the resolution wouldn’t have done anyway.

The experience of pain begins just beneath the skin in specialized nerve endings known as nociceptors. (“Noci-” is from a Latin word meaning “hurt.”)

All that can be said is that signals from all types of pain are conveyed on to the spinal cord and brain by two different types of fibers—fast-conducting A delta fibers (they’re coated in myelin, so slicker, as it were) and slower-acting C fibers.

Nerve signals are not particularly swift. Light travels at 300 million meters per second, while nerve signals move at a decidedly more stately 120 meters a second—about 2.5 million times slower. Still, 120 meters a second is nearly 270 miles an hour, quite fast enough over the space of a human frame to be effectively instantaneous in most circumstances. Even so, as an aid to responding quickly, we have reflexes, which means that the central nervous system can intercept a signal and act on it before passing it on to the brain. That’s why if you touch something very undesirable, your hand recoils before your brain knows what’s going on. The spinal cord, in short, is not just a length of impassive cabling carrying messages between the body and the brain but an active and literally decisive part of your sensory apparatus.

This in turn led to the idea of proprioception—another Sherrington coinage—which is the body’s ability to know its own orientation in space. (Even with your eyes closed, you know whether you are lying down or whether your arms are outstretched and so

The nerves radiating out from this central hub—the ones that reach out to the other parts of your body—are the peripheral nervous system. The nervous system is additionally divided by function into the somatic nervous system, which is the part that controls voluntary actions (like scratching your head), and the autonomic nervous system, which controls all those things like heartbeats that you don’t have to think about because they are automatic.

The autonomic nervous system is further divided into sympathetic and parasympathetic systems. The sympathetic is the part that responds when the body needs sudden actions—what is generally referred to as the fight-or-flight response. The parasympathetic is sometimes referred to as the “rest and digest” or “feed and breed” system and looks after a miscellany of other, generally less urgent matters like digestion and waste disposal, the production of saliva and tears, and sexual arousal (which may be intense but not urgent in the fight-or-flight sense).

An oddity of human nerves is that those in the peripheral nervous system can heal and regrow when damaged, whereas the more vital one...

Pain, like the nervous system itself, is classified in a multiplicity of ways, and these vary in type and number from authority to authority. The most common category is nociceptive pain, which simply means stimulated pain. It’s the pain you get when you stub a toe or break your shoulder in a fall. It is sometimes referred to as “good” pain, in the sense that it is the kind of pain that tells you to rest the affected part and give it a chance to heal. A second type is inflammatory pain, for when tissue becomes swollen and red. A third category is dysfunctional pain, which is pain without external stimulus and that causes no nerve damage or inflammation. It is pain without evident purpose. A fourth kind of pain is neuropathic pain, in which nerves are damaged or grow sensitive, sometimes as a result of trauma, sometimes for no apparent reason.

There is a paradox at the heart of pain that makes its treatment particularly intractable. “When most parts of the body are damaged, they stop working—they switch off,” Tracey says. “But when nerves are damaged, they do exactly the opposite—they switch on. Sometimes they just won’t switch off, and that is when you get chronic pain.” In the worst cases, as Tracey puts it, it is as if the volume knob on their pain has been turned all the way up. Figuring out how to turn that volume down has proved to be one of the greatest frustrations in medical science.

Generally, we don’t feel pain in most of our internal organs. Any pain that arises from them is known as referred pain because it is “referred” to another part of the body. So the pain of coronary heart disease, for instance, may be felt in the arms or neck, sometimes in the