Medical school is a unique four-year window into the secrets held by these big sacks of carbon we all live inside. Most people aren’t given the language or tools to dive into those secrets, which is a shame, because some of the stuff they teach you in medical school is truly mind-blowing. It is a gift. So that is the goal of this little project, The Body Magic—to share some of that gift, or at least the juiciest parts.
I am also making these to remind myself (and maybe other people too) that the body is SO incredible—that a trillion microscopic events had to work perfectly, and continue to work perfectly, for you to exist as a person on this planet right now. And understanding more about the human body only makes it that much more magical!
That knuckle pop?
It’s just the sound of gas bubbles collapsing from increased space in the joint. It takes some time for gas bubbles to accumulate again once they’ve popped, which is why you can’t crack the same knuckle again right away.
People used to think that knuckle cracking led to arthritis over time, but then a doctor experimented on himself by cracking the knuckles of only one hand over the course of his life. He took periodic X-rays and found no difference in his two hands after decades of cracking. That study (and some bigger ones) showed that knuckle cracking is totally harmless (much like farting)—just maybe a little annoying to those around you (ditto). So carry on cracking!
You know when you stay in the bath too long and your fingers shrivel up?
That’s actually a sensory response! Your nervous system has detected that you have been wet for a very long time, so it helps you out by shrinking your blood vessels so that your hands can grip things better. Like if you are a car and your hands are your tires, then your body is morphing your fingers into tire treads.
Scientists think this was an evolutionary development, but they’re not sure if other animals’ bodies adapted the same way. Thank you, beautiful human body, for allowing me to grab wet fruit and run barefoot in the rain!
A developing fetus doesn’t use its lungs to breathe.
Instead, it relies on oxygen-rich blood from the mother delivered via the placenta. Because its heart doesn’t want to pump blood to non-functional lungs, oxygenated blood bypasses the lungs by sneaking through a hole between the pulmonary artery and the aorta. When a baby is born and takes its first breath, the hole begins to close, and for most babies it closes by the time they’re a couple of days old.
So you had already weathered some heartbreak by the time you were born! Good to get some practice in early.
Most of the joints in our body we think of as joints—like our knees—contain synovial fluid.
Synovial fluid is made up of lubricin, which helps keep joints frictionless by lubing them up, and hyaluronic acid, which makes the synovial fluid viscous when it is compressed and helps protect joints from damage.
Synovial fluid also provides nutrients to, and removes waste from, your cartilage. Mechanically compressing your synovial fluid—like by walking or running—helps speed up that nutrient exchange. So with every step you take, you’re squishing good stuff into your connective tissue! Good job.
Your skin is divided into 30 sections called dermatomes.
Each dermatome is supplied by nerves branching from a single spinal root housed in the vertebrae of your spine. These nerves transmit sensations like pain, touch, and temperature. Since the feelings map is more or less the same for everybody, you can use the map to figure out the root of someone’s feelings.
For example, if someone feels tingling down the back of their arm and numbness in their middle finger, you might be suspicious about a pinched nerve arising from the seventh cervical vertebra (C7), because that’s the vertebra that corresponds to those places on the body map. There is so much hidden logic to this carbon-bag of flesh and blood!
Your cornea is the transparent surface of your eye.
Your cornea covers your anterior chamber (a space in your eyeball filled with watery fluid), your iris (the colored part), and your pupil (the black hole in the center). Your cornea has to be transparent so that light can enter your eye and allow you to see, so it can’t get its oxygen from clunky blood vessels like most parts of your body. Instead, oxygen from the air dissolves in your tears and diffuses throughout your cornea—and the waste product of your eye, carbon dioxide, diffuses out.
Unfortunately the amount of oxygen diffused through your tears is only enough to keep the cornea healthy, so you can’t hold your breath, cry a lot, and expect your eyeballs to breathe for you. But the cornea is the fastest healing tissue in the body—scratch your cornea, and it will usually heal in 1-3 days, thanks in part to the oxygen from your tears. Sounds to me like crying is pretty good for you.
There are two kinds of cells in your retina (a layer in the back of your eyeball that responds to light): rods and cones.
The center of your retina is filled with cones, which are very sensitive to color and resolution, but not so sensitive to light and motion. The outer parts of your retina are filled with rods, which are the opposite: not so sensitive to color and resolution, but very sensitive to light and motion. This is why it is easier to see dim stars (and shooting stars!) with your peripheral vision, and if you look directly at stars, with all your fancy color-sensing cones, the stars seem to disappear.
So in conclusion: when someone shouts “over there!” during a meteor shower, don’t turn your head—just move your eyes!
You are 90-99% bacteria! Yes, you!
Our bodies contain 10 times as many bacteria as human cells, and most of those little bugs live in our gut. Altogether, they have over 3 million genes, while we human creatures only have about 23,000. Which means that we are walking around mostly...not human.
By about 20 weeks of pregnancy, a female fetus has all of its oocytes, or immature egg cells (about 6-7 million, in fact).
So when your mother was developing in your grandmother, the egg cell that would eventually become you already existed inside of her! Or, put another way, if you are a biological mother, your mother once carried the egg cell that would become her grandchild. ~whoah~
Most of the oocytes in a fetus die off, leaving only about 1-2 million present at birth. Even more die off in childhood, leaving only 300,000-400,000 at puberty, and once menstrual cycles start, women lose about 1,000 oocytes a month, or 30-35 per day.
One last fun egg fact: The egg cell is the largest cell in the human body. It is about the width of a strand of hair—you can see it without a microscope!
Bones are constantly reshaping themselves in response to how much stress is placed on them.
On earth, weight-bearing exercise like walking, running, or basketball helps maintain bone mass. But in space, where astronauts are weightless, bones don’t have to support the body against gravity, so they do less work and thus weaken over time, especially in areas like the hips, legs, and spine. Astronauts lose about 1-2% of their bone mineral density every month they are in space, a phenomenon known as “spaceflight osteopenia.” Special nutrition and workout protocols—like wearing bungee harnesses on treadmills—can slow the progression of bone loss, but not entirely eliminate it. So traveling to outer space doesn’t just take guts — it takes some bone as well…(sorry).