Why Urine is Yellow

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A urine sample.

Admit it. You’ve always wondered why. Why is urine yellow? What chemical or chemicals are involved? Consider the primary red blood cell component, “heme.”

Heme & Porphyrin

Dorland’s Medical Dictionary for Health Consumers, 2007, informs us heme is “an iron compound of protoporphyrin which constitutes the pigment portion or protein-free part of the hemoglobin molecule.” Heme is the colorful portion (red) of our blood. Notice, in particular, the partial word in the definition, porphyrin.

Recycling & Trash

Red blood cells become old and need to be replaced. What happens to the old red blood cells? They are, first, consumed by macrophages in the blood. For the person who has his or her body intact, much of this occurs within the spleen. It also occurs in other parts of the body. It should be noted that most of the iron is extracted for reuse, and does not remain in any product that is excreted.

The Spleen & Bile

Heme contains four pyrrole-like, nitrogen-containing rings, connected together into a porphyrin ring (recall the reference above). The heme portion is converted into bilirubin—an open structure containing these rings. This metabolism usually occurs within the spleen, and so the bilirubin is part of what is called “bile.”


The Final Act – Bilirubin Yields Urine

Bilirubin is further broken down into urobilin. The process involves the liver. A small amount of this lightly tinted waste is passed out in the urine, and this is why we identify its color as urine yellow.

So now you know. And guess what? You are transparent. Which means, I can read your mind. You are wondering why feces are brown. Am I right or am I right?

Note: You might also enjoy Why Dogs Pee on Tires


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