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GoodGoodGoody t1_irbk0dg wrote

Reply to comment by BallardRex in What happens when a bruise heals? by DwagonFyre

Excreted as waste: my understanding is that poo is brown from dead red blood cells tinting it.

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Lopsided_Chicken400 t1_ircmv52 wrote

It's not dead RBCs that make feces brown. It's a byproduct of hemoglobin catabolism called stercobilin.

Dying RBCs are removed from circulation by the spleen, and the hemoglobin inside them is split into its heme and globin components. The iron is then removed from the heme group. The remainder of the heme is a yellow pigment called bilirubin. Bilirubin is then put into the bloodstream and makes it's way to the liver. Liver cells remove bilirubin from circulation and package it into bile. Bile is released from the liver and gallbladder into the small intestine to aid in the digestion of fats. The bilirubin then works its way through the rest of the GI tract. In the colon, bilirubin is converted by bacteria in other pigments. One of those pigments, stercobilin, is brown.

And that is why feces are brown. It's also why the feces of a person with liver failure or a bile duct obstruction will be gray/white and fatty.

Edit: bike -> bile

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UseaJoystick t1_ircurlr wrote

The human body is fascinating. Recycling dead RBCs into Iron for presumably new RBCs, and the remainder into bile for enhanced digestion. The body really knows how to use everything to its fullest extent.

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Lopsided_Chicken400 t1_ird4i9l wrote

The bilirubin in the bile isn't the part that aids in digestion. Bile is a mixture of quite a few things. Some of those things, like bilirubin, are wastes that the body is eliminating in the feces. Other components are functional.

One of the functional components of bile is bile salts. Bile salts are produced in the liver and they aid in digestion by emulsifying fats. Emulsification is important because it makes it easier for the enzymes that digest the fat (lipases from the pancreas, mainly) to access it.

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Kirk_Kerman t1_ird3bru wrote

They're called salvage pathways. It's a lot less energetically expensive to recycle.

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