A while back, (can’t seem to turn it up searching, feel free to claim credit downthread) one of the doper docs explained the function of the large and small intestines like so:
The small intestine absorbs nutrients into the body.
The large intestine dries the left over stuff into proper poo.
What I am interested in is the details of the drying process. How exactly does it do that? I’ve been involved with some industrial processes which involve sludge de-watering. It seems to require a fair bit pressure to get up to around 20% solids, and picking up after my dogs makes me think that the large intestine does at least that well if not a bit better.
I’m guessing this must be a filtration type process, possibly similar to a membrane filter (AKA reverse osmosis). If so, there needs to be a fair amount of pressure inside the intestine to overcome the osmotic pressure, as the poo is essentially a saturated solution. The intestine and the closures at both the input and output end would have to contain this pressure. From measuring how much pressure I can generate by blowing, 1-2 psi seems like about the limit I would consider reasonable. Industrial membrane systems I am familiar with require 10s of psi, and operate with very low solids content on the dirty side.
Second question:
Where does the removed water go? Just into the abdominal cavity? If so, what organs eventually collect it?
The small intestine absorbs about 80% of the water. The cells of the lining take in sodium from the gut contents and then pump the sodium into the intercellular space. The water in the gut then diffuses into the intercellular space by osmosis. The water is taken up by the capillaries in the intestine. The large intestine works the same way.
The lower intestine is also home to many of your intestinal flora which have an effect on your overall physiology. I work with some people looking into intestinal flora differences between ‘healthy’ people and those on a high-fat diet. One of the relevant open access articles they reference can be found here -
There is also evidence that the large intestine absorbs some of the nutrients created by those beneficial bacteria. Exactly what and how much gets absorbed depends on the critter we’re talking about. Horses have a massive and complicated lower intestine. The nutrition they get from the grass they eat comes largely from the bacteria living in their lower GI. Humans, being simple creaters, probably don’t absorb very much.
As detailed above, the water is removed by the cells setting up an osmotic gradient, not by pressure. However, the large intestine does squeeze the contents some and add mucous, for lubrication.
So is there any correlation between abdominal cramping (digestive system) and the large intestine? There are times when a cramping comes on requiring a rather swift walk to the toilet, followed by an explosive discharge, be it just gas, and/or sometimes solids and liquids. I’m not talking a stomach bug or food poisoning here.
There sometimes is a correlation between cramping, gas and the large intestine. This is the case in lactose intolerance where the rapid increase in intestinal bacteria coupled with raises in osmotic pressure lead to the painful cramps, gas and laxative effect.