Turdopoiesis

by Elio
 

When it comes to consistency, we are somewhere between cows and goats. I am talking of course about the texture of our feces, which in health are neither runny nor very hard. When it comes to the speed of evacuation, we are also in-between: we poop at about 2 centimeters per second, ele­phants at 6 cm/sec and dogs at 1 cm/sec.
 

Scatology aside, why should we care about feces? Of the innumerable reasons one can propose, one of significance to microbiologists is that, as we have noted before here, what is called the in­testinal microbiome is based on the analysis of feces. In other words, the material studied is ex­ter­nal and no longer part of the intestine. But it's sure easy to sample.
 

The distinction between the feces and the intestinal context lies principally in the properties of an ultra-thin layer of mucus that lines the walls of the large intestine. The mucus layer is as thin as hu­man hair, so much so that it is hard to measure. Despite this, mucus is very slippery, being more than 100 times less viscous than feces.
 

Figure 1. The large intestine. Source

A few more pertinent facts: The intestinal contents ex­tend down from the length of the colon to the rectum and, in humans, varies from one to four pounds. We daily eliminate on average about 130 g of feces. If, as is be­lieved, the number of bacteria is about the same or grea­ter than that of human cells, every bowl movement makes us at least temporarily more human.
 

About ¾ of the feces is water. The solids are mainly organic compounds, about 25-50% of the mass being microbial. For the hydrodynamics of defecation, see here. The brown color of feces is due to ster­cobilin and urobilin, the products of bacterial degradation of the bile pigment, bilirubin. The cha­racteristic odor comes from gases produced by bacteria, mainly skatole, mercaptans, and hydro­gen sulfide.
 

What are the main functions of the large intestine? There are four: absorbing water and electro­lytes to condense the intestinal contents into feces, allowing bacteria to produce needed vitamins plus to decompose some of the remaining indigestible material, and propelling feces southwards toward the rectum. Most nutrients and up to 90% of the water were already absorbed by the small intestine. The process of water absorption in the large intestine is not unlike that in the small intestine, a response to osmotic gradient established by sodium ions traveling from the lu­men through the epithelial cells. Some vitamins required by humans, among others, K, biotin, B6, B12, and folic acid, are synthetized by colonic bacteria. These also produce short china chain fatty acids, (SCFA) mainly propionic and butyric, the latter being a major energy source for the intestinal epi­thelia cells. However, in humans, SCFAs are not a major energy source, as they are in horses and other mammals with an oversized cecum (a bit like having a rumen at the other end).
 

Humans are brought up to pay particular attention to their digestion, especially to its end stages. Given the subject's complexities, it seems well worth it.