How much of the daily insensible water loss is due to loss in stool?

a. 200 ml

Compared with the small intestine, the epithelium of the colon is relatively impermeable and requires a considerably longer period of time to absorb the salt, water, and carbohydrate presented to it. Between 500 and 1500 g of a semiliquid material enter the colon on a daily basis. Most of this substance is absorbed, yielding a stool output that weighs about 200 g. The reason for this efficient absorptive capacity, despite the relative impermeability of the colonic epithelium, relates to the close interrelation between epithelial transport and colonic motor activity. Transit through the large bowel takes about 10 times as long as that through the small intestine, usually requiring 2 or 3 days. Such slow passage provides optimal opportunity for the luminal contents to come into surface contact with the absorptive epithelium. Thus, sodium and water, which are extracted against high electrochemical and osmotic gradients, are efficiently absorbed. Further, the relatively static conditions of the colon allow the proliferation of vast numbers of anaerobic bacteria, which in turn break down carbohydrate to volatile fatty acids, which then are rapidly absorbed. The more rapid the colonic transit, the less efficient are these absorptive processes.