• Large Intestine
  • Colon
    • Cecum
    • Ascending Colon
    • Transverse Colon
    • Descending Colon
  • Rectum
  • Anus

The large intestine is the end of the GI tract. This part of the gut starts with the colon and ends at the anus.

• The intestines include both the small and large intestine. The colon -the main workstation of the large intestine, is about 1/5 (20 %) of the total length of the entire span of both small and large intestines.
• The colon is illustrated above, in art provided by NIH (click the image to learn more). The rectum and anus are also shown, but not labelled.
  • You can see that the large intestine stretches across the belly, it starts at the so-called ascending colon, where the small intestine (not shown) ends in the large bowel. This large bowel then goes up (ascends) the right side of the abdomen. The appendix is that little worm-shaped blind sac (in the picture) that juts out towards the midline near where the ascending colon starts. After the colon ascends up to the top of the abdomen, it turns and goes across the abdomen as the transverse colon, and then descends on the left side of the abdomen as the descending colon. All this ascending and descending is not built into the colon itself, which is like lengths of sausage with a hollow core, its lumen. Instead, the colon is hung in nets of connective tissue and by the shiny sheets that line the abdomen, the peritoneum. The rectum is shown going straight down in the mid-line to the anus, where the gastrointestinal tract meets the skin, and the outside world.
• The large bowel's appearance and physical characteristics are easy to understand if the basic concepts of its functions and purpose in the body is kept in mind. That's is more or less all about what happens to its contents from its beginning to its end.
• Those contents start out as the loose, watery caustic chyme that exits the end of the small intestine, and end up as the (hopefully) well-formed solid feces that exit the anus, and the body, as solid waste.

The large bowel is focused on water and electrolyte balance, and "waste management". It's in the large bowel that water is absorbed and stool is formed. There's also some detoxification that goes on in the colon, and much of that is done by the friendly bacteria that colonize the lining of this tract. In a sense, proper ecology is important fot the function of the large bowel, because overgrowth of the wrong sorts of bacteria lead to abnormal stool production and can lead to a problem with toxins.

Just as the entire gastrointestinal tract is normally a one-way processing system for food, so the large bowel (the tract at the GI tract's end), is also a one-way system. One that takes the contents of small intestine and processes it into stool, which is expelled out of the body. As lowly as that function seems to be, many wise people have remarked on how - if it is not performed correctly, the entire body suffers.

• There's an old medical student joke that involes a contest between the mouth and anus; the brain is asked-which one matters more? Well, the brain chooses the mouth, and the disappointed anus suggests a trial - let the body do without either for a while, and then decide. Both are sealed, and guess which one the brain begs to open first?

Inane as this little story is, it makes an undeniable point. Despite the lack of dignity accorded the colon, rectum, and anus in most cultures; the lower digestive tract is quite as vital to health and comfort as those other portions of the body that are ordinarily held in great respect. The subjective attitude towards parts of our bodies is not irrelevant in clinical medicine. When it comes to disease and disorders of disdained parts of the body like the anus, or abnormalities in such things as feces, shame sometimes accounts for late diagnosis and neglect; which in turn accounts for illnesses that might have been cured -or at least relieved, causing needless death and disability .

So, exactly what happens here?

Well, "here" is a tract that is several feet long, and along its length, the nearly totally digested food ends up becoming...feces. The very end of the large intestine, the anus, is a control point for their expulsion. The rectum is primarily a storage depot. The colon processes the contents of the small intestine into stool.

• What exits the last part of the small intestines is what enters the colon; and that's: what got eaten, plus a bunch of digestive juices, minus most of the nutrients.
• This stuff, the contents of the small intestine's last portion (like practically everything in biology) has a name, chyme. It's a watery liquid that's got some still-active enzymes in it, it's stained dark from bile and it's a bit caustic because of its basic pH.
  • When people have surgery to either permanently or temporarily allow their GI tract to end at the small intestine's end, it's called an ileostomy. That can be a difficult thing to manage because of the nature of chyme. Chyme's very irritating to the skin because of the enzymes, and since chyme is a big volume of watery stuff,it means that the collecting bag has to be pretty much continually monitored and changed frequently.
• Now, everybody knows what what the feces that comes out of their backside look like, and everybody knows, too- that there's some variation to those stools dpending on diet and other factors. But it's fair to say, that when a person is healthy and eating well, stools are dark colored and well-formed, and that they are solid and not liquid.
• The main change between chyme and stool takes place in the colon.

Sometimes the various parts of the large intestine are considered separately, particularly the last part, the rectum; but all can be properly called the large bowel. Even the very last part of the gastrointestinal tract, the anus, is part of the lower bowels. It ends at the skin, the outer part of the body, where there are voluntary muscles in a circular arrangement called a sphincter.

The cecum and vermiform process (the appendix), with their arteries (in red). In the illustration by ADAM at the top right of this page, the cecum and appendix are not labelleled, but they are shown. The cecum is the broad start of the ascending colon, and the appendix is the little "u" that hangs off it's left.

• Henry Gray: The Large Intestine in The Anatomy of the Human Body.1918

• The Large Intestine: Introduction and Index