The Body: A Guide for Occupants

“It may be slightly surprising to think it, but our skin is our largest organ, and possibly the most versatile. It keeps our insides in and bad things out. It cushions blows. It gives us our sense of touch, bringing us pleasure and warmth and pain and nearly everything else that makes us vital. It produces melanin to shield us from the sun’s rays. It repairs itself when we abuse it. It accounts for such beauty as we can muster. It looks after us.”

The skin is a shield and cushion that also helps regulate body temperature, assists the immune system in fighting pathogens, alerts its owner to changing conditions in the environment, and communicates with others through blushing, sweating, and aromas. The skin thus serves a high number of functions, coordinates them smoothly, and does so largely in the background.

“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.”

Without microbes, much of our food would go undigested; without them pulling vital nitrogen from the air and putting it into edible plants, most organisms, including humans, would quickly die off. Though individual microbes are too tiny to see without a microscope, all of them together outweigh all the visible plants and animals 25 times over. 

“While we have the overwhelming impression that the greenness of the trees and the blueness of the sky are streaming through our eyes as through an open window, yet the particles of light impacting on the retina are colorless, just as the waves of sound impacting on the eardrum are silent and scent molecules have no smell. They are all invisible, weightless, subatomic particles of matter traveling through space.”

A continuing source of wonder and mystery to science is how three pounds of wrinkly goo inside the skull converts impacts of subatomic particles on the human sensory apparatus into light, sound, touch, smell, and taste. The world in itself contains no colors, no heat or cold, no noises or hardness or softness, but we perceive things that way because of how our brains translate stimuli into sensations. 

“It’s curious that we always speak of our five senses because we have way more than that. We have a sense of balance, of acceleration and deceleration, of where we are in space (what is known as proprioception), of time passing, of appetite. Altogether (and depending on how you count them) we have as many as 33 systems within us that let us know where we are and how we are doing.”

The five classic senses referred to are vision, hearing, smell, taste, and touch, four of which originate in organs in the head, along with balance, acceleration, proprioception, and time sense. Touch, pressure, and pain occur all over; touch especially in the hands, which reserve disproportionately large regions of the brain for processing the various sensations that hands receive. Appetite manifests partly in the stomach. The body, then, contains a vastly more complex set of sense receptors than popularly believed. 

“The fact is that odors and flavors are created entirely inside our heads. Think of something delicious—a moist, gooey, warm chocolate brownie fresh from the oven, say. Take a bite and savor the velvety smoothness, the rich heady waft of chocolate that fills your head. Now consider the fact that none of those flavors or aromas actually exist. All that is really going in your mouth is texture and chemicals. It is your brain that reads these scentless, flavorless molecules and vivifies them for your pleasure. Your brownie is sheet music. It is your brain that makes it a symphony. As with so much else, you experience the world that your brain allows you to experience.”

The entire range of human sensory experience is the nervous system’s interpretation of energy, chemicals, and particles that interact with the body. Colors, heat, texture, pressure, sounds: All these are symbols that represent portions of the environment. The essential nature of the universe is more akin to mathematics than stuff. The mind must forever perceive the world in the signage of the brain. 

“The heart is a wondrous organ and fully deserving of our praise and gratitude, but it is not invested even slightly in our emotional well-being. That’s a good thing. The heart has no time for distractions. It is the most singleminded thing within you. It has just one job to do, and it does it supremely well: It beats.”

Despite its reputation as a wellspring of emotion, especially love—the heart beats faster during sex and other moments of strong feeling—emotions originate elsewhere. The heart’s job is to oxygenate the body and transfer food derivatives and other biochemicals to the tissues, but sometimes intense emotional reactions require extra blood flow. Thus, the heart works in service to feelings, as needed, but those feelings come from the brain.

“Most hormones have a multiplicity of functions, which makes it harder to deconstruct their chemistry and riskier to tinker with it. Ghrelin, for instance, doesn’t just have a role in hunger, but also helps to control insulin levels and the release of growth hormone. Tampering with one function could destabilize the others.”

The body’s systems are complexly interrelated, and an artificial adjustment in one can cause others to veer away from their norms, in turn affecting other systems and so on in a series of recursive side effects. These interactions are one reason why taking multiple pharmaceuticals can cause more problems than cures. 

“Most of the best technology that exists on Earth is right here inside us. And everybody takes it almost completely for granted.”

The body is a marvel of engineering that squeezes a tremendous amount of equipment into small spaces. The wrist, for example, contains muscles, tendons, nerves, and blood vessels, yet must be highly flexible; the abdomen contains numerous organs tightly packed that must work together flawlessly. Cartilage serves many functions, including protecting the ends of long bones where they meet at the joints, its smoothness five times better than ice. 

“If someone invented a pill that could do for us all that a moderate amount of exercise achieves, it would instantly become the most successful drug in history.”

Moderate daily exercise improves health, extends lifespan, strengthens bones and muscles, improves mood, and makes easier the accomplishment of daily physical tasks. Up to a point, more vigorous exercise produces even more benefits.

“If fever is a defense mechanism, then any effort to suppress or eliminate it may be counterproductive. Allowing a fever to run its course (within limits, needless to say) could be the wisest thing.”

It’s important to cure the causes of illness and not merely the symptoms. Fevers make people feel miserable but they also help kill off invading microbes. The common practice for fevers is to take pain-reducing medications that suppress the increased temperature; doing so may increase the length of the illness.

“If you want to understand the immune system, you need to understand antibodies, lymphocytes, cytokines, chemokines, histamine, neutrophils, B-cells, T-cells, NK cells, macrophages, phagocytes, granulocytes, basophils, interferons, prostaglandins, pluripotent hematopoietic stem cells, and a great deal more—and I mean a great deal more.”

The few invaders, mostly microbial, that get past the skin must face a gauntlet of bodily immune organs, including lymph nodes, bones, spleen, thymus, and others. The body will also raise its temperature in a fever, cooking the invaders. Memory T-cells store information on previous infections and can quickly put down any future invasions. This form of immunity can be developed artificially with vaccines, which present the immune system with particles from dangerous pathogens that inform the T-cells what to look out for in the future. 

“Every time you breathe, you exhale some 25 sextillion (that’s 2.5 x 1022) molecules of oxygen—so many that with a day’s breathing you will in all likelihood inhale at least one molecule from the breaths of every person who has ever lived. And every person who lives from now until the sun burns out will from time to time breathe in a bit of you. At the atomic level, we are in a sense eternal.”

It’s hard to fathom how small are the atomic particles that make up the body and the things it ingests and discharges. That the lungs exhale 25 trillion trillion oxygen atoms only hints at how many are absorbed during an hour or a week. That a day of breathing introduces the body to oxygen previously inhaled by everyone who ever walked the Earth points to the thoroughness of the mixing of the atmosphere over time and the strange intimacy of its atoms with all living things.

“So many variables have been implicated in heart health—exercise and lifestyle, consumption of salt, alcohol, sugar, cholesterol, trans fats, saturated fats, unsaturated fats, and so on—that it is almost certainly a mistake to pin the blame decisively on any one component. A heart attack, as one doctor has put it, is ‘50% genetic and 50% cheeseburger.’”

Nutritional advice is fraught with difficulties. Most doctors are undereducated about nutrition, most studies fail to tease out completely all the variables that affect health besides what people eat, and nutrition scientists tend to fall into camps that ignore the other camp’s data. Advice shifts back and forth: Eat less fat, eat more fat, eat less salt, eat more salt, consume hydrogenated fats for health, avoid hydrogenated fats. The best advice may be simply to eat a diet that’s balanced and moderate.

“Inside, you are enormous. Your alimentary canal is about 40 feet long if you are an average-sized man, a bit less if you are a woman. The surface area of all that tubing is about half an acre.”

Like the brain, whose wrinkled cerebrum folds a tremendous amount of calculating power into a small space, the intestines cram dozens of feet of digestive tubing into an area less than two feet wide, with the insides of the small intestine’s tubing covered in villi, like tiny fingers with surfaces that add enormously to the total absorptive area of the intestines. Such efficiencies point up the extent to which a body must condense itself to function successfully in the world.

“Sleeping is the most mysterious thing we do. We know that it is vital; we just don’t know exactly why. We can’t say with any certainty what sleep is for, what is the right amount for maximum health and happiness, or why some people fall into it with ease while others struggle perpetually to attain it. We lose a third of our lives to it.”

Sleep provides benefits for both body and mind, but why creatures must sleep to accomplish those advantages is still unknown. Continuous lack of sleep is lethal yet leaves little sign of damage on the body. The persistent mysteries of sleep serve as reminders that, despite the enormous progress made in physiology in recent decades, science still has a tremendous amount to learn about the body.

“Sex may dilute our personal contribution to posterity, but it is great for the species. By mixing and matching genes, we get variety and that gives us safety and resilience. It makes it harder for diseases to sweep through whole populations. It also means that we can evolve. We can hold on to beneficial genes and discard ones that impede our collective happiness. Cloning gives you the same thing over and over. Sex gives you Einstein and Rembrandt—and a lot of dorks, too, of course.”

A few creatures, such as gecko lizards, make perfect clones of themselves through asexual reproduction, but most living things pass their genes to the next generation through sexual reproduction. This process introduces variation into the pool of genes of a species, helping to make those creatures more resilient when changes occur in their environment, from newly potent diseases and predators to shifts in temperature and rainfall.

“It has been suggested that children growing up today will be the first in modern history to live shorter, less healthy lives than those of their parents. We aren’t just eating ourselves into early graves, it seems, but breeding children to jump in alongside us.”

Despite tremendous strides in maternity care, parents increasingly lead less healthy lives, which puts their offspring at risk. It’s common, in Western countries, for 30% or more of births to be by elective Cesarean section, which deprives newborns of vital birth canal microbes that might otherwise protect them from major ailments later in life. Not only do modern parents avoid the pain and uncertainty of birth, but they also avoid the discomfort of exercise and the monotony of healthy eating.

“Pain is curiously mutable. It can be increased, attenuated, or even ignored by the brain depending on the situation. In extreme circumstances, it may not register at all.”

Many circumstances can alter the perception of pain. Worry or depression can make it worse, while soothing music, good food, and sex can diminish it. During moments of extreme preoccupation with other matters, as in the midst of battle, injuries may not be felt at all. 

“Medical science has never produced a more noble and selfless group of investigators than the pathologists and parasitologists who risked and all too often lost their lives in trying to conquer the most pernicious of the world’s diseases in the late 19th and early 20th centuries.”

Medical discoveries have often involved deliberate exposure to deadly diseases. History abounds with stories of men and women who risked their lives while trying to cure those diseases; many of them died in the attempt. Sadly, most of these people languish in obscurity while people benefit, blithely unaware of the sacrifices that keep them alive.

“We are born with the bodies of hunter-gatherers but pass our lives as couch potatoes.”

As if communicable diseases and genetic abnormalities weren’t enough, humans in Western society have become more sedentary, which causes their bodies to deteriorate. Junk food and lack of exercise gives people diabetes, heart disease, and other ailments, to the point where 70% of the causes of death today are preventable with lifestyle changes.

“Cancer is, appallingly, your own body doing its best to kill you. It is suicide without permission.”

A lot of things must go wrong for cancer cells to escape detection and attack the body. Live long enough, and the process is likely to begin. Strictly speaking, cancer cells aren’t trying to do anything but survive. Their runaway cell division, however, will lead to the death of the body and the cancer cells as well—effectively an assault on the cancer’s own source of survival—but they don’t “know” that. Even so, the irony, bad luck, unfairness, and relentlessness of the disease contribute to the fears surrounding cancer.

“Everyday attributes like empathy and common sense can be just as important as the most technologically sophisticated equipment.”

Many long-respected medical practices and treatments have turned out to be useless or harmful. High-tech procedures, machines, and drugs sometimes are less helpful than simple caring, listening, and sensible decision-making by doctors. A kind word or expression of concern can do wonders by relaxing and reassuring patients, which, among other benefits, calms their fight-or-flight sympathetic nervous system and somehow helps the immune system complete its work. 

“In 2011, an interesting milestone in human history was passed. For the first time, more people globally died from noncommunicable diseases like heart failure, stroke, and diabetes than from all infectious diseases combined. We live in an age in which we are killed, more often than not, by lifestyle. We are in effect choosing how we shall die, albeit without much reflection or insight.”

Modern technology has gotten so good at curing ancient scourges like plague, smallpox, and tuberculosis—while also producing large quantities of food, intoxicants, and labor-saving devices—that humans now suffer more fatal outcomes from self-indulgence than from the natural world’s lethal threats. The good news is that these new sources of mortality and morbidity are within people’s power to prevent through simple lifestyle changes.