What is poop made of?
About 3/4 of your average turd is made of water. Of course, this value is highly variable - the water content of diarrhea is much higher, and the amount of water in poop that has been retained (voluntarily or otherwise) is lower. Water is absorbed out of fecal material as it passes through the intestine, so the longer a turd resides inside before emerging, the drier it will be.
Of the remaining portion of the turd, about 1/3 is composed of dead bacteria. These microcorpses come from the intestinal garden of microorganisms that assist us in the digestion of our food. Another 1/3 of the turd mass is made of stuff that we find indigestible, like cellulose, for instance. This indigestible material is called "fiber," and is useful in getting the turd to move along through the intestine, perhaps because it provides traction. The remaining portion of the turd is a mixture of fats such as cholesterol, inorganic salts like phosphates, live bacteria, dead cells and mucus from the lining of the intestine, and protein.
Why does poop stink?
Poop stinks as a result of the products of bacterial action. Bacteria produce smelly, sulfur-rich organic compounds such as indole, skatole, and mercaptans, and the inorganic gas hydrogen sulfide. These are the same compounds that give farts their odor.
Why is poop brown?
The color comes mainly from bilirubin, a pigment that arises from the breakdown of red blood cells in the liver and bone marrow. The actual metabolic pathway of bilirubin and its byproducts in the body is very complicated, so we will simply say that a lot of it ends up in the intestine, where it is further modified by bacterial action. But the color itself comes from iron. Iron in hemoglobin in red blood cells gives blood its red color, and iron in the waste product bilirubin gives rise to its brown color.
A friend of mine just sent me the link to your website for fun. I really enjoyed the ‘Survival Guide for Taking a Dump at Work’. However, I believe that there is an error among your explanation for the color of feces. The brown color is a result of broken down red blood cells, but I don’t think that it’s a result of iron. Most of the iron that is released from the breakdown of heme is reabsorbed by cells in the bone marrow for incorporation into new red blood cells. That’s why we don’t have a large daily requirement for iron in our diets. Anyhow, what remains of the hemoglobin is then called biliverdin, which is converted into bilirubin and is excreted. This compound can build up in some people and cause jaundice. The colors of these molecules are due to their complex structures, which contain rings of carbon and nitrogen atoms. Intestinal bacteria further modify bilirubin to produce urobilinogen and stercobilinogen. If I remember correctly, the red color of blood is also due to the porphyrin ring of heme and not iron.
So, you’re absolutely correct to say that it’s a complicated explanation and it has to do with the breakdown of red blood cells, but I don’t think that iron has anything to do with the brown color of poop or the red color of blood.
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