Blood, sweat, tears? Classic bodily fluids. And then there’s mucus, spit, semen, and urine—well known to juvenile minds everywhere. But blood, for instance, only makes up 9% of your total bodily fluid. What else is oozing around inside you?
1. Interstitial Fluid
Interstitial Fluid, AKA tissue fluid, works in conjunction with lymph and plasma (the liquid part of your blood) to maintain your body’s internal pressure and make sure your organs and other fluids properly interact. Most interstitial fluids have a specific job and a specific name, like peritoneal fluid, which lubricates everything inside the abdomen, or pleural fluid, which coats the lungs to allow them to do that whole oxygen exchange thing so you can live. And you were giving air all the credit.
Lymph is pale and doesn’t look like much, but it plays a huge role in keeping you from dying. Several roles, in fact. It runs throughout your entire body, identifies bacteria and viruses, carries them to your lymph nodes to activate the immune system, then meets up with your blood stream to deliver important items like proteins and infection-fighting white blood cells. It delivers fats from your digestive system to wherever your body needs them (fatty lymph is called chyle), and toxins to the excretory system for disposal. And all the while it helps interstitial fluid maintain that fluid balance. Imbalanced fluids result in swelling, like in the ankles of pregnant ladies.
If plasma is blood without the red blood cells, serum is plasma without the clotting agents like fibrin and platelets. Of its many roles, it’s best known as “the stuff inside a blister.” Blood and lymph both contain serum, but all three exist separately in the body, and serve different purposes at different times. But serum is the most basic of the three, which kind of explains its all-purpose, old-timey name.
4. Aqueous and Vitreous Humors
And speaking of old-timey names—yes, you have humors. But these aren’t the “black,” “yellow,” “phlegm” and “blood” humors, the imbalance of which ancient docs claimed were the source of all malady. These fluids are humors in name only, and are necessary to eye function: The aqueous humor is a clear fluid that brings nutrients to the lens area of your eye, so that you’re not—you know—looking at blood all the time. The vitreous humor (or vitreous body) is the gelatinous goop that fills the inside of your eyeball.
Bile is another term misused by physicians of yesteryear—the black and yellow humors were called “yellow bile” and “black bile,” too much of which would make a person overly idealistic or melancholy, respectively. Real bile, or gall, is produced by the liver, held in the gallbladder (Greco-Romans were right about that part) and it squirts into the beginning of the small intestine when you eat. Its salts act kind of like soap, breaking down fats into more absorbable/digestible bits. Along with urine, bile helps the body excrete most of its toxins and other undesirables.
6. Cerebrospinal fluid
Cerebrospinal fluid—or CSF to the cool kids in the know—surrounds the spinal cord and brain, and fills the hollow spaces all up inside them. Not only does it cushion your all-important central nervous system from potential trauma, it also removes cell waste, and delivers hormones that help the brain and spinal cord communicate with the rest of the body.
Endolymph is not really lymph in the aforementioned sense. It’s the fluid in your inner ear that helps you keep your balance. Like the bubble in a level, your endolymph holds its place in relation to gravity, while your body moves. Small hairs in the ear detect the position of the endolymph, and tell the brain how best to maintain your equilibrium, so you don’t wind up hurling that...other...bodily fluid.
Sebum is a fancy word for skin oil—that stuff you have way too much of when you’re going through puberty. Secreted by sebaceous glands, sebum is odorless, but can collect zit-causing bacteria, which stinks as it decomposes.
Milk only occurs in some humans of course, but the mammary glands that produce it are similar to other glands in the body. Scientists believe they probably evolved from either sweat glands or the aforementioned sebaceous glands. What you’ve heard is true: Men have the right hardware for lactation, and if the necessary hormones happen to be present, they, too, can nurse their young.
10. Female Ejaculate
Female Ejaculate is another lady secretion that has the scientific community pretty baffled. Studies strongly suggest that the fluid most closely resembles prostate plasma, and may originate in lady glands analogous to the male prostate.
Cytosol, or intracellular fluid, is the fluid inside a given cell—unlike all the other fluids we’ve mentioned here, which work outside and around cells. Remember those cell cross-sections you had to draw in middle school biology? Cytosol is the gunk in which all the other parts of the cell are suspended. You may never lay eyes on your intracellular fluid, but it makes up a whole third of your total fluid volume.
Cerumen is just the fancy name for earwax, a combination of sebaceous and modified sweat-like secretions that protect the ear canal from potentially harmful foreign bodies. There are actually two kinds of earwax: the brown, sticky “wet” kind, and the grey, flaky “dry” kind. Turns out there's a genetic difference between the two, identifiable in a single DNA base pair. Wet-type cerumen is the dominant type, more common in people of European and African heritage. Dry-type cerumen is recessive, and more common in people of Asian and Native American decent. Tracing the earwax gene has recently given researchers a window into the history of human distribution.