The Body: A Guide for Occupants

Children who suffered from diabetes, a dysfunction of blood sugar regulation, always died painfully until 1920. That year, researchers figured out that the human pancreas, which produces digestive juices, also produces insulin, which controls blood sugar but is largely missing in juvenile diabetes victims. Insulin was quickly manufactured in quantity and delivered to patients, saving their lives. The results “may be rated the first great triumph of medical science” (139). 

Juvenile diabetes, or Type 1, is caused by a genetic flaw plus a possible environmental trigger, such as a microbe. Type 2, which begins later in life, is partly inherited but largely a result of lifestyle factors, including faulty diet and lack of exercise. The number of Type 2 victims has skyrocketed in recent decades—90% of diabetes victims today are of the Type 2 variety—but Type 1 cases are increasing as well, for unknown reasons. 

Chemicals like insulin are hormones, messengers produced in one location that cause actions in another. As many as 100 have been discovered; most are produced in the endocrine glands, including the pituitary, pineal, hypothalamus, thyroid, parathyroid, thymus, pancreas, adrenals, and testes and ovaries. Each has powerful effects on the body. The pituitary, a bean-sized organ in the brain, regulates growth, sex, excitement, immune response, low blood sugar, and other functions. An overactive pituitary caused one person to grow to nearly nine feet in height. 

Endocrinology got its start with Thomas Addison, one of the “Three Greats,” London doctors active in the 1830s. Addison made an extensive study of anemia, appendicitis, and the adrenal glands. He defined a number of ailments, including Addison’s disease, a deterioration of the adrenals that plagued U.S. president Kennedy. 

The first of many hormones found to be produced outside endocrine glands was leptin, discovered in fat cells in 1995. Lack of circulating leptin makes people hungry, but an abundance merely causes puberty to happen earlier. Humans evolved in environments of scarcity, which delayed anatomical development. A feast is hard to turn down, and people’s daily feasting speeds up development and puts fat on their hips. 

Most hormones have multiple tasks, so that tinkering with the levels of a single hormone can have multiple effects. This complexity makes hormonal therapies tricky to apply. Hormone injections can have effects opposite from those of the natural versions. Naturally secreted oxytocin, for example, causes people to bond and be more affectionate; when administered artificially, it sometimes causes aggression. 

Testosterone gives men deep voices and facial hair, increases their sex drive, and makes them aggressive and more prone to risk taking. The hormone also is produced in women but in smaller amounts that increase their libido but otherwise “mercifully leaves their common sense undisturbed” (149). 

The liver is the largest gland and the busiest one in the body, handling 500 different functions, including detoxification, energy conversion and glucose management; manufacture of hormones, proteins, and bile; and vitamin storage. A damaged liver can regrow itself. The liver is subject to many maladies, including fatty liver disease, common to alcoholics but also found in about one-third of adults. Two million Americans have hepatitis C, which damages the liver without symptoms. 

The pancreas, located behind the stomach, is roughly the size and shape of a banana and produces about a quart a day of pancreatic juice that contains insulin and other blood regulators, along with digestive liquids. Just under the rib cage on the left is the spleen, a fist-sized organ that helps with immune response and holds a reservoir of blood for when muscles have a sudden need for more. 

The gallbladder, beneath the liver, stores bile, a digestive substance formerly called gall and produced by the liver for use in the digestive tract, where it helps especially to break down fats. Stones can form in the gallbladder—one-fourth of adults have them, usually without symptoms—and can block ducts, causing life-threatening crises. 

As humans age, stones also can form in their kidneys, which are located just under the rear of the rib cage. The kidneys filter out wastes, regulate salt content and blood chemistry, and help control blood pressure. The kidneys process enough liquid each day to fill a bathtub. By age 70, kidneys have lost half their effectiveness; fortunately, there are two of them. Diabetes, obesity, and high blood pressure can lead to kidney failure, a disease that has grown 70% since 1990. 

Stones can also form in the urinary bladder, which otherwise holds more than two cups of urine in a man; less in a woman. Diarist Samuel Pepys, who suffered from bladder stones, in 1658 underwent an operation to remove a stone nearly the size of a tennis ball. Despite no antibiotics or pain medicine, Pepys survived. He kept the stone in a lacquered box as a memento. 

The body packs a great number of organs elegantly into a limited space.

When opened during surgery, they glisten and throb with life; the innards of a cadaver, though, appear listless. Medical schools receive more donated bodies than they can handle; each is treated carefully and with ceremonial respect within clean rooms where no photography is allowed, and most of each body is returned to the donor family for funeral services. 

In previous centuries, there weren’t enough cadavers available for dissection, despite the availability of executed criminals, and medical schools took to stealing bodies from cemeteries. In the early 1800s, England allowed schools to retrieve bodies of those who died in the poorhouse. 

In 1858, Henry Gray published the first authoritative book on the human body, updated and published to this day as Gray’s Anatomy. The first edition’s 363 meticulous illustrations were done by medical student Henry Vandyke Carter; the book’s success was largely due to those drawings, but Carter got little credit. 

The body has 206 bones, more or less—there are rare exceptions—and more than half are in the hands and feet alone. Bones give structure to the body, help us move about, protect our innards and brain, make new blood cells, and help us hear via the tiny bones of the inner ear. They also produce a hormone, osteocalcin, that helps regulate fertility, glucose, moods, and memory. 

Bones are light yet stronger than concrete. They thicken with use: Athletes, for example, may have stronger bones in their pitching and serving arms. Ligaments connect bones together, while tendons attach muscles to bones. Cartilage, smoother than ice, coats the ends of bones where they meet, so they can move against each other in the joints without grinding. Flexible cartilage also helps shape the ears and nose, among other body parts. Unlike bone, cartilage has no blood supply and doesn’t heal properly after damage. 

The skeleton makes up 8.5% of the body, while the 600 muscles are 40% of the body. A dozen muscles shift the eyes while reading; 10 move the thumb. The heart and tongue are muscles. In the hand, 17 muscles operate 29 bones held together with 123 ligaments. 

The human hand contains three muscles not found anywhere else in nature; these muscles make humans more dexterous and contribute to a more refined civilization. The thumb stands apart from its four finger neighbors and helps them grasp things. 

The feet, with dozens of bones, muscles, ligaments, and tendons that carry people through 200 million steps in a lifetime, must absorb the shock of walking, use their arches to add spring to the step, and push off toward the next step, the toes sometimes grabbing and digging in to do so. Human feet, spinal discs, and hips are relatively new to the upright stance and imperfectly adapted to it; backaches and sore feet are the price people pay. Hip replacements due to worn out cartilage generally failed until doctors began swapping the top of the femur, or upper leg bone, for a titanium knob and changing out the hip socket for plastic. 

Of the 250 species of primates, only humans walk upright. Walking conferred advantages to early humans—carrying and throwing, scanning the environs—but also made them more vulnerable to predators. Bipedal walking is tricky because 90% of the time only one foot is in contact with the ground. Human bones and muscles permit a graceful stride; chimpanzees can only waddle. 

Also unique to the human primate is running. Though much slower than four-legged beasts, humans can jog for miles, exhausting prey that perform short sprints. Shoulders evolved to make efficient the hurling of rocks and spears, enabling hunting from a distance but also adding dislocated shoulders to the list of human ailments. 

Walking confers health benefits, but this wasn’t known until a late-1940s British study compared double-decker bus drivers, who were always seated, with bus conductors, who were always on the move and climbing the stairs, and found that drivers had twice as many heart attacks. Many studies since then have confirmed that regular exercise adds years of life. Exercise improves bone strength, helps prevent disease, regulates mood, and wards off senility, yet only 20% of people get even moderate amounts of exercise. The average American walks about a third of a mile per day, while modern hunter-gatherers average 19 miles. 

Because humans are designed to exercise a lot and eat when they find food, the modern sedentary, feasting lifestyle causes weight gain. Most U.S. adults are overweight and a third are obese; children are becoming overweight as well, and half will be obese as adults. U.S. health care costs for all these extra pounds is $150 billion. Other wealthy countries struggle similarly with weight issues. 

Sitting for six hours or more increases mortality by 20% for men and 40% for women, and exercise doesn’t undo that damage. Exercise also is a slow way to reduce weight: Losing one pound requires seven hours of jogging. Standing or walking around, however, burns an extra 107 to 180 calories per hour; over time, this seems to account for much of the difference between fat and thin people. 

A mouse’s heart beats 600 times a minute; an elephant, just 30 times. Regardless of size, all animals average about 800 million heartbeats per lifetime, including humans until recently, when advances in medicine permitted people to live long enough to average 1.6 billion heartbeats. 

Mammals are warm blooded, which enables them to move about vigorously day and night in warm or cold temperatures. Reptiles are cold blooded and must warm up in the sun before becoming active. Mammals require 30 times as much energy to stay warm, and their body maintains an internal temperature within one or two degrees of 98.6 regardless of temperature or exertion levels, vital to protecting the brain. Sweating, along with shivering and diverting blood flow to vital organs, are important parts of that process. 

In tests, a marathoner ran portions of a marathon at 131° F and his core body temperature barely changed by one degree. One English experimenter in 1775 withstood a chamber heated to 210° F for three minutes. During illness, sometimes the body generates a fever by raising its temperature a few degrees; this slows viral reproduction by more than 99%. Even a normal temperature impedes microbial invasion. 

Much of what medicine knows about the inner workings of the body was discovered by Harvard physiologist Walter Bradford Cannon, who became the leading expert on the autonomic nervous system that controls blood flow, breathing, and digestion. He also studied the body’s adrenaline response to emergencies, giving it the moniker “fight or flight” (189). 

Adenosine triphosphate (ATP) is the energy molecule of the body. It captures energy from food, stores it like a battery, then releases it to a body cell as needed. In the process, it becomes a smaller molecule but then receives another unit of incoming energy, resumes its ATP form, and the process begins again, in an ongoing equilibrium. Enough ATP is recycled each day in the human body to equal the weight of that body. 

Only 4% of the surface of Earth is habitable for humans; the rest is either too hot or cold, too wet, or at too high an altitude to permit survival. Some miners in Chile work at 19,000 feet but sleep 1,500 feet down the mountain. At high altitude, more blood cells are generated in the body, but this thickens the blood and can cause circulation problems and force the heart to work harder. Airliners flying at 35,000 feet have cabins pressurized to 4,900 to 7,900 feet; a sudden loss of pressure will cause passengers and crew to suffer hypoxia and become incapacitated within 10 seconds. 

Most humans can’t survive a fall of more than 25 feet, but now and then a person will fall out of a plane and survive by landing on a snowbank or some other lucky location. On occasion, a toddler has wandered outside in winter and found hours later, its heart stopped, but been revived without ill effect. Not so lucky are children left in cars that reach 130° F; their underdeveloped sweat glands can’t process the heat.

During World War II, German and Japanese scientists subjected war prisoners to deadly experiments, hoping to learn the limits of human endurance. The victims were subjected to extremes of heat and cold, bomb blasts, diseases, toxins, and nerve agents. As many as 250,000 civilians died in the Japanese experiments, but, at war’s end, most of the Japanese doctors escaped Allied punishment in return for sharing what they had learned.