Bodily fluids are not the first topic of choice in most people’s discussions. They can be gross and messy, but they can also indicate when there’s a problem within the body. No matter how unsavory we may find them, these fluids are essential to our health, and knowing they look the way they should could be the difference between spotting a major health issue and letting it linger on far too long. Whether it’s blood or urine, or one of the many others, here’s why our bodily fluids are colored the way they are.


Apart from menstruation, blood should pretty much stay inside the body. Sometimes we do catch a glimpse of our most important bodily fluid, though. Yet, if it’s flowing out of a cut or scrape, it’s always the same color. We’re so used to seeing blood in one color that “blood red” is even the name of a crayon — and it’s for good reason.

Blood gets its color from its abundance of (aptly named) red blood cells, which are responsible for carrying oxygen to tissues in the body. Each of these cells contains hemoglobin, a protein with an iron atom at its center. It’s the chemical bonds between these iron and oxygen molecules that, when reflecting light, make us perceive blood as red. This is why blood freshly pumped from the heart appears as a bright red, while oxygen-deficient blood traveling back to the heart appears a rustier, brownish red.

But why, then, do our veins appear blue? Another matter of light reflection is to blame. Our skin provides an extra layer for light wavelengths to pass through, and because of this, only blue wavelengths have the strength to reach our eyes instead of red.

Blood appearing bright red is oxygenated. Alden Chadwick (CC BY 2.0)


Mucus — appreciated by few and disgusted by many — is critical to keeping our bodies functioning the way they’re supposed to. Mucus is a slippery, gooey substance that covers many important membranes in our body, including the lungs, sinuses, and gastrointestinal tract. It normally just keeps these surfaces from drying out. But sometimes, just taking a look at the mucus we produce from our sinuses can clue us in on what’s going on inside our bodies.

When a person is healthy, mucus is generally clear. Most will notice, though, that the color changes when they catch a cold or feel themselves coming down with a flu-like virus. Yellow or green mucus develops when the immune system sends an army of white blood cells to the sinuses, where many symptoms of cold (like sneezing and congestion) manifest. White blood cells contain a greenish-colored enzyme, which causes mucus to take on a similar hue. The concentration of the substance also matters — large amounts of mucus will cause the color to appear more yellow or green than clear.

Mucus is also capable of turning red or brownish from blood. Only a small amount of blood can change the color of mucus, and this issue can arise from having an irritated nose from blowing or rubbing it too much — a common occurrence when suffering from a cold. Excessive drying during the winter months can also cause the cracked surface of the nose to produce blood. Both of these aren’t too serious, but if there is a large amount of blood in your mucus or it goes on for a while, you should see a doctor.


Everyone knows to avoid the yellow snow, and that’s because we know to associate it with urine. Urine gets its yellow color from a breakdown product of hemoglobin called urochrome. Healthy pee can range from nearly clear to a medium yellow, but anything much darker or lighter can be a sign of dehydration or too much water, respectively. Urine, unlike blood and mucus, is capable of displaying an array of colors.

Pink or red could be due to something as benign as eating beets with dinner, or something much more serious, such as bleeding caused by a kidney infection, UTI, or even mercury poisoning. Orange urine could be a sign of dehydration, but it could also signal a liver or bile duct condition. Blue or green urine — surely the strangest colors urine could be — is possibly the result of a rare genetic disorder, a certain bacteria, or food coloring. Either way, see a doctor if it persists.


Scientists aren’t 100 percent sure what causes slight color fluctuations in semen, but they have a pretty good idea. Semen is normally a whitish, transparent color, but can also be clear. Clearer sperm could be due to a lower consistency of sperm, which is what gives semen its whitish color. A man who frequently participates in sexual activity or masturbation will likely have clearer sperm than those who do not.

Anything outside of this range, however, could merit a visit to the doctor. Pink or reddish sperm could indicate inflammation or bleeding of the prostate or seminal vesicle. Yellow semen could be a sign of urine in the semen, jaundice, or oddly high levels of white blood cells. Most of the time these things aren’t a problem and will clear up in time, but asking a doctor never hurts.

Sperm are what gives semen its usual color. Pixabay Public Domain