The colon forms the largest part of the large intestine, the 'tube' through which digested food passes. That food is joined by digestive juices, including stomach acid and bile, on its route through the digestive system. That semi-fluid mass, known as chyme, isn't all just waste, though. A great many things happen to it on its way out of the body first.

The partially digested material that exits from the small intestine still contains a great deal of water, liquid that the body conserves naturally. The muscular and chemical action inside the colon helps extract that water from the chyme along its journey, a trip through the approximately 5 feet of colon inside the abdominal cavity.

Extracting that water isn't just a matter of efficiency, though. Proper water balance is vital to a huge variety of the chemical reactions in the body, as well as playing an essential role in internal temperature regulation. The colon does much more than just pack digested food into feces for elimination.

While that water is being extracted other important processes are taking place. There are over 700 species of bacteria that live in the 'gut', many of them playing a crucial role in digestion and health. Among other things, they help produce vitamins K and Biotin (a B-type vitamin).

The colon absorbs some of those products of the digestive processes of the bacteria themselves, then passes them back into the bloodstream. The blood then distributes those nutrients to tissues throughout the body. This is just one reason that antibiotics need to be taken with care. They can suppress not only harmful bacteria but the useful ones, too.

Those bacteria are also responsible for creating the gas (flatus) that sometimes upsets your stomach or fouls the air. As they consume undigested polysaccharides usually in the form of human-indigestible fiber we eat, they produce that gas. The gas itself is not, contrary to popular thinking, mostly methane or hydrogen sulphide ('egg' gas). It's chiefly nitrogen and carbon dioxide, with trace amounts of those other molecules, along with some hydrogen.

When those bacteria break down fiber they're producing nutrients not only for their own survival, but the vitamins mentioned above. At the same time, they generate compounds that provide nourishment for the cells that line the colon (the epithelium). They also contribute to creating lymphatic cells that form a key part of the immune system.

The chyme, now fully processed, continues to move down the colon by a slow process of muscular contraction called peristalsis. The python-like squeezing and relaxing moves the material ultimately down into the rectum, compacting it and extracting water to form feces. Note, the rectum is the six inch or so 'tube' just up from the anus, the opening. The feces are then squeezed out the anus in a bowel movement.

The whole process can take 10-12 hours and, as you can see, involves much more than simply pushing digested food waste through a 2 1/2 inch diameter tube. It's a necessary partner in total body health.