The lower intestine absorbs water from faecal matter, and uses that as one source of hydration for your body. I assume the bowel has various reasons for deciding when not to do this...and diarrhoea is the result.
I would like to know whether any factors control the amount of water the bowel chooses to absorb, or is it merely an on/off choice?
For example, does the bowel 'drink' as much water as it can when a person is dehydrated? If I glug two litres of water will my bowel not bother to absorb quite as much as other days? Will different people have different levels of absorption, giving some harder stools every day, compared to others?