Anatomy & Physiology Workbook For Dummies with Online Practice
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Basic anatomy may be fairly straightforward, but how the human body uses all those parts can present a smorgasbord of interesting discoveries. Here is just a peek at ten of the more intriguing aspects of what makes the body tick.

  • As the body’s internal defense network, the immune system sets blood and lymph against biological invaders seeking to colonize you. But its first line of defense is also the body’s largest organ: the skin.

    If your skin — and some of the “friendly” bacteria living there — didn’t secrete natural antibacterial compounds, you could wake up in the morning coated in microbial slime. As it is, though, most of the pathogens that land on you die quickly.

    [Credit: Illustration by Kathryn Born, MA]
    Credit: Illustration by Kathryn Born, MA
  • The skin isn’t the only thing working against the invading microbial hordes; the enzyme lysozyme found in human tears, saliva, and mucus is custom-made to disintegrate bacterial cell walls. (Remember: As eukaryotes, people’s cells don’t have walls).

  • Not that all microorganisms are bad. You have between 2 and 5 pounds of bacteria living inside you, much of it in the intestines. As scientists have begun to understand what that microbial life is up to, it has become clear that your internal “microbiome” is a big part of what keeps you healthy.

  • Humans dissolve into dust.

    Every human alive today spent about 30 minutes at the start as a single cell. Now, however, your body is making 25 million new cells every day, and you shed and regrow all of your outer skin cells about every 27 days.

    Beards are the fastest-growing hairs on the human body; if an average man never shaved or trimmed the growth, he would die with nearly 30 feet of beard hanging off his chin. The average human scalp has 100,000 hairs — blondes have more hair, on average, than dark-haired people do — and you lose between 40 and 100 strands of hair each day. Fingernails grow nearly four times faster than toenails do.

    That’s quite a bit of cellular production going on. Ever wonder what happens to those cells when they die? After all, you shed around one and a half pounds of dead skin every year.

    It’s dust — common household dust. Most of the dust you brush around in your house is dead skin. If you think that’s gritty, consider this: The ashes of a cremated body weigh around 9 pounds.

  • Besides being a digestive kick-starter, saliva prevents tooth decay and keeps your throat and mouth from drying out. It may not feel like it, but those six little glands make nearly half a gallon — about 1.5 liters — of saliva every day. Over the course of a lifetime, that’s enough to fill about two average-sized swimming pools.

  • Hard to stomach? Consider this: With hydrochloric acid inside that’s so corrosive it can dissolve wood and steel, your stomach should consume itself. But it doesn’t. Why? Because you make your own natural supply of antacid. The epithelial cells lining your stomach secrete a steady supply of bicarbonate that neutralizes stomach acid on contact.

    Be forewarned, though: That soothing bicarbonate produces carbon dioxide, among other things, as a by-product. That has to go somewhere. Some of it comes up in the form of a belch. And some of it contributes to the 17 ounces per day of flatulence that the average healthy human releases.

  • In one day, the average individual’s heart exerts enough power to lift a 1-ton weight more than 40 feet off the ground.

  • During that day’s pumping, the body’s red blood cells travel about 12,000 miles, or roughly half the distance around the Earth at the equator. They’ve got plenty of room to roam; if you laid every blood vessel in a body end-to-end, they would stretch roughly one-quarter of the way to the moon, or about 60,000 miles.

  • If that’s hard to believe, consider this: Every square inch of skin contains 20 feet of blood vessels. Tissue the size of a pinhead contains 2,000 to 3,000 capillaries. You certainly have plenty of blood to fill all that space; you have 2.5 trillion red blood cells, more or less, in your body at any given moment, and your bone marrow creates 100 billion new ones every single day.

    That’s so much blood that it would take 1.2 million mosquitoes each drinking their fill once to completely drain the average human of blood.

  • When people talk about sensation, most of the time they’re referring to the five primary senses — vision, hearing, taste, touch, and smell. But the notion of a “sixth sense” may be more than the stuff of science fiction.

    Vision combines senses for both light and color, and there is growing evidence that your proprioception — the ability to detect your relative position in space — may rely on your body’s ability to detect magnetic fields in much the same way migrating birds do. Blind people have been known to develop echolocation, or the ability to hear subtle changes in sound bouncing back from otherwise unseen objects. And stress sometimes causes people to experience time dilation.

    In fact, your senses are more subjective than you may like to admit. Things get even more complex when you consider the condition known as synesthesia, in which a person can “hear” color or “see” sound.

    The eyes, which can distinguish up to one million colors, routinely take in more information than the largest telescope ever created. The nose can identify and the brain subsequently can remember more than 50,000 smells. Static touch can discern an object about twice the diameter of an eyelash, while dynamic touch — dragging a finger along a surface — can detect bumps the size of a very large molecule.

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