What are some mind-blowing amazing facts about the human body?

Some real anatomy, and not bullshit sensationalized crap from buzzfeed:

The Palmaris Longus muscle is missing in approximately 15% of all people. To see if you have one, touch your thumb and pinky and flex your wrist. Palpate the center of your wrist. If you feel 2 tendons, you have a Palmaris longus (the other guy, close to the thumb is flexor carpi radialis). Here's a picture:

Another cool anatomical variation is that in ~20% of individuals, the right coronary artery and the posterior circumflex anastamose (join) to form the posterior intraventricular artery.
Your body secretes approximately 2 grams of IgA antibodies per day.
The biceps also supinates (turns the palm facing up) the forearm. To test, palpate your bicep and turn your palm facing up. You should feel it flex.

The kidneys are embryologically derived from the testes or ovaries. To test this out (easier in guys), have someone kick you in the testicles, and you should feel pain radiating to your lower posterior back (where the kidneys are). Credit to User for pointing this out.
The brachialis muscle is a stronger flexor of the elbow than the biceps.

Place your hands in this position:

Try and separate your thumb. Check. Now separate your pinky. Check. Now separate your index finger. Check. Now separate your ring finger. Trolled.

So why can't you? Your index finger and pinky have an additional set of extensor muscles (extensor indicies and extensor digiti minimi) that is independent the set of common extensors. Your ring finger and middle finger only has one. So when you engage and lock your middle finger in that position, that muscle is unavailable to your ring finger.

Your left kidney is higher up than your right kidney (because the liver is in the way!)
Men have two urinary sphincters whereas women only have one. The reason being is that the urinary tract in males serves a dual purpose: urination and ejaculation.

The ejaculatory ducts empty out into the prostate which is sandwiched between the external and internal urethral sphincters. The internal one is under involuntary control so that you don't end up ejaculating into your bladder (which is why you can't ejaculate and pee at the same time, and also why men find it hard to pee right after an orgasm). The external sphincter is under voluntary control and is what you use to hold your pee in.

There's this weird phenomenon called micturation syncope when you faint after you pee. Cause: Peeing, and especially when you're really straining to pee, stimulates the vagus nerve which slows down the heart, causing a drop in blood pressure.

You can maintain your blood sugar for approximately 2-3 days of fasting. After then, your glycogen stores run out and you become ketogenic — in fact in that state, a person will smell hungover without consuming any alcohol.
There are only two enzymes in the entire body that use vitamin B12 — methylmalonyl CoA mutase and methionine synthase.

The thumb is not a finger. So the next time you play two truths and a lie, say "I have eight fingers!".

The big toe, however, is a toe.