Do you know enough basic anatomy to keep yourself healthy? It’s time to find out.

About 17 percent of men in the U.K. don’t know they have a prostate. Only one in two knows where the prostate is, and over 90 percent don’t actually know what it does. And lest you think we’re picking on men, another poll found that half of young women can’t locate the vagina.

It’s not that these people don’t know their own bodies at all—presumably most of them manage to have sex and perform all the standard bodily functions. It’s that our anatomical education is sorely lacking. People often aren’t taught what part of the genitals are called the vagina, or what a prostate might do, much less where the gallbladder happens to sit.

But not knowing something as simple as the rough location of your organs matters more than you think. A basic understanding of anatomy can help you make better healthcare decisions, especially if your doctor is the type to sling seemingly esoteric jargon. Physicians are so fluent in their own terminology that they often forget to translate for patients, so those who are able to follow along on their own will be better informed. And doctors don’t have to be especially unreasonable in their expectations in order to confuse patients (see: vaginas).

Unfortunately, we’re woefully under-informed. Researchers at the Lancaster University’s medical school tested a group of volunteers to see how solid their anatomy knowledge was. They selected 21 body parts that they reasoned most people would have heard of, based on which organs and muscles are commonly referenced on TV or other popular media. Then they asked each volunteer to identify where each body part was on a simple diagram.

The only anatomical part that 100 percent of people got right was the brain. Only one in five knew where the spleen went, and just one in four could identify the gallbladder’s home. And laugh all you want, but the average person probably couldn’t tell you where the spleen is. Just see for yourself. We put together a labeled diagram of all the body parts that the participants were asked to identify. Go ahead and write down numbers 1-21 and see if you can match the numbers to the organs. And keep in mind that for the actual study, volunteers were given a totally blank body map and asked to show where each part went, which is way harder.

Harder than it seemed, huh? And if you couldn’t even put the thyroid or the cruciate ligament on the map, how much do you really know about them? Because they’re two incredibly common anatomical structures to have problems with at some point in your life. Here’s some info on all the body parts that you probably couldn’t place:

1. Achilles tendon

This is probably the one you’re most familiar with, because of the related phrase “Achilles’ heel.” And—you guessed it—the Achilles tendon is in the heel. It attaches your calf muscles to the heel bone, which is why if you tear it, you generally can’t walk.

2. Adrenals

One of the least well-identified spots in the study, the adrenal glands are not—as you probably thought—in the neck. They’re actually hanging out just above your kidneys, where they secrete hormones and steroids (not the kind guys use to get jacked) to help regulate your complicated endocrine system.

3. Appendix

People were split 50-50 on this one, and to be fair the appendix can sometimes be in a weird location. Mostly you should know that it’s attached where your large and small intestines meet, and that it’s not clear whether it’s truly vestigial. It may still play a role in maintaining your gut flora and your immune system. But if it becomes infected and you need an appendectomy, you’ll be just fine without it.

4. Bladder

You should be able to figure this one out, if only based on the discomfort you feel when you try to hold in your pee. The bladder is a muscular sac that sits between your kidneys (where your urine is made) and your urethra, which is the tube that your pee comes out of. Muscles in the pelvic floor keep the bladder closed, but these tend to weaken as people age. This is why some adults experience incontinence. It’s also why some have to pee more frequently after having children—because shoving a seven-pound infant out of your vagina can really screw with your pelvic muscles.

5. Biceps

Significantly more men than women could identify the biceps, and you’ll never guess why. Men are taught more about muscle locations because lifting is a stereotypically male pursuit. But listen up non-dudes: if you want those Michelle Obama arms, you gotta pick up something heavier than a three-pound dumbbell. That being said, your biceps aren’t a very functional muscle group, and they’re mostly only worked by doing curls, a.k.a. the ultimate vanity lift.

6. Brain

This is the one you really have to get right. Your brain takes up most of the space in your skull, which is why when you get a head injury you need to worry about a concussion. A concussion occurs when the brain slams into your skull; to some degree it’s always just perched gently inside of there waiting to slosh itself around, and is liable to get damaged when the squishy gray matter presses against hard bone.

7. Cornea

Locating this one would be harder if we gave you an eye diagram. Zoomed out to a view of the whole body, almost everyone could identify the cornea—it’s the clear part of your eye that covers your iris and pupil. It’s also part of how your eye focuses, so problems with its transparency or structure can seriously impair your vision. Fun fact: because your cornea has to be see-through, it doesn’t have any blood vessels, so oxygen and other nutrients that those cells need have to dissolve in your tears, diffusing from the surface of your eyeball into the cornea.

8. Cruciate ligament

This one might sound unfamiliar until you realize that one of your cruciate ligaments is your ACL (the anterior cruciate ligament). They’re the ligaments in your knees that hold your femur and tibia together, while still allowing your knee to flex.

9. Diaphragm

Not the birth control method. One of the least recognized body parts, your diaphragm is actually one of the organs you can use consciously—you just don’t realize you’re using it until you take a few singing lessons. The diaphragm is how you inhale. It’s basically a big muscle that separates the upper part of your chest from your lower body cavity. As it contracts (picture it being pulled down towards your stomach), the space in your chest increases, causing your lungs to expand and air to flow in. When it relaxes, the space decreases and your lungs force air out (and you’re welcome for making you very aware of your own breathing right now).

10. Gallbladder

Ah, the gallbladder. Like your urinary bladder, the gall-variety stores gross internal fluids, though in this case that fluid is bile. Bile helps your small intestines to digest fat, so the gallbladder releases small amounts as needed. If you have a dysfunctional gallbladder, your body might not be able to break down fatty foods.

11. Hamstrings

The hamstrings are not strings of any kind—they’re the very large muscles on the backs of your thighs. The name might come from the fact that in quadrupeds (animals that walk on all fours), hamstrings are the tendons running along the backs of the legs. Humans also have hamstring tendons, we just also call those muscles ‘hamstrings’ because we like to be confusing with our jargon. Your hammys are incredibly strong, and together with your glutes (that’s your butt) can lift and squat enormous amounts of weight.

12. Heart

The heart fell somewhere in the middle of the group in terms of identifiability, hopefully because of the confusion about which side it sits on. To clear that up: it’s your left side. Except if you’re one of the 1 in 10,000 people with situs inversus, a disorder in which the organs are all on the opposite side from what’s normal. Problems with the heart often don’t feel like they’re in that area, because the brain can misinterpret where the pain signal is coming from. In comparison to your skin and external body parts, your internal organs have very few sensory nerve endings. So sometimes as the signal comes back through the spinal cord, the brain mistakenly concludes that a different, more nerve-dense part of the body is in pain. Women having heart attacks tend to experience discomfort in the chest, while men complain of arm or shoulder pain.

13. Kidneys

Those lil beans proved fairly difficult to locate. They sit in the middle of your back, filtering your blood day in and day out. A complicated series of membranes selectively allow certain molecules to pass through, which allows the kidneys to remove toxic components and put them into your pee. Filtered blood goes back into the circulatory system, while the urine heads towards the bladder. If one kidney fails, you can generally live without it. If two fail, you can try to get a transplant, and in the meantime go on dialysis, which is essentially an external form of filtration that performs basic kidney functions on your behalf.

14. Liver

You probably thought this was lower down, didn’t you? Most people couldn’t locate the liver, much less tell you that it’s actually a gland. Glands are organs that produce chemicals (like hormones) in order to release them into the bloodstream. It makes bile to help digest food, secretes hormones, synthesizes proteins, and filters out certain toxins. Livers can grow back to some degree if they’re damaged, but chronic damage will eventually cause the liver to fail altogether. A couple types of liver dialysis are being tested to help those patients.

15. Lungs

One in five people missed this one. Maybe they didn’t realize how high up they are? In any case, your lungs are how your body acquires oxygen and gets rid of carbon dioxide. Air gets pulled into all these tiny little pockets, called alveoli, where a ton of blood vessels are waiting close to the surface to trap oxygen molecules and release CO2.

16. Pancreas

This was one of the hardest organs to identify. The pancreas sits surprisingly high in the body, where it produces all kinds of important hormones, including insulin. You may be familiar with insulin as the primary hormone involved in diabetes, which is a disease where people can’t properly regulate their insulin (or in some cases, lack it entirely). The pancreas also helps out with digestion by neutralizing stomach acid as food moves into the intestines and secreting digestive enzymes.

17. Quadriceps

Again, more men than women knew this one. Your quads are so-called because four of them combine to form the entire front and sides of the thigh. They’re what you work on the leg press at the gym, as well as in squats (though if you’re doing squats to proper depth, they shouldn’t be the primary driver). Quads are also crucial for running and walking, since they control the knee joint.

18. Spleen

Here we are at the least identifiable organ. It can probably blame its obscurity on its lack of singular role. The organ is mostly a blood filter, but it also helps regulate the immune system, reserves blood cells, recycles hemoglobin, and creates antibodies, among other things. You probably mostly hear about it in cancer patients, who can sometimes get an enlarged spleen, or in car accidents, when the spleen can rupture. Other than that, you’ll probably never think about your spleen again.

19. Stomach

The stomach was surprisingly hard to locate, again, probably because it’s oddly high up. Your esophagus is just not as long as you think, people. This should go without saying, but your stomach helps digest your food using acid and enzymes—though it doesn’t do as much work as you might think. We owe a lot of our stomach-related knowledge to a man named Alexis St. Martin, who unfortunately acquired a hole in his abdominal wall when he was shot on June 6th, 1822, and who served the rest of his days as a glorified guinea pig for an Army doctor. Dr. William Beaumont spent years inserting various items into the hole and removing them hours or days later to see how digestion had progressed. Gross? Borderline unethical? Crucial to the study of gastric anatomy? Yes, to all of the above.

20. Thyroid

This is what you were thinking of when you tried to place the adrenal glands in the neck. Your thyroid is an endocrine gland, which means it secretes hormones. Specifically, the thyroid helps regulate your metabolism. A hyperactive thyroid generally causes weight loss as it speeds up the metabolism, but lest any dieters get wistful, it also comes with a host of unpleasant problems like heart palpitations, anxiety, chest pain, diarrhea, and hair loss. A hyp-o-active thyroid, on the other hand, causes weight gain, fatigue, constipation, and abnormally slow heart rate. It’s often caused by a lack of iodine in the diet—which is why we now iodize almost all of our salt.

21. Triceps

And finally, the triceps. This three-headed muscle sits at the back of your arm, and is how you’re able to straighten the limb. Triceps are far more useful than biceps, since you use them (along with your lats) to push yourself up off the ground or to, say, get out of a swimming pool. To do that kind of muscle-up, you need tricep strength.

This article has been updated to correct which organ dialysis is used for currently. As of 2017 we only have dialysis for kidney failure—liver dialysis is still only in the testing phase.