large intestine
Meaning of large intestine: Little food value is left by this stage; the work here is reclaiming water and packing what remains into stool.
Definition of large intestine
The large intestine is the final section of the digestive tract, running from the end of the small intestine to the anus. It has four parts: the cecum, the colon, the rectum and the anal canal. Liquid residue arrives from the small intestine, gives up water and salts along the colon, and is held as stool in the rectum until it passes.
What are its parts?
Material leaving the small intestine passes through a one way valve into the cecum, a pouch in the lower right of the abdomen with the appendix attached to it. From there it travels up the right side, across the top of the abdomen, down the left side and through an S shaped bend, the four named stretches of the colon. The rectum stores what remains until it is passed through the anal canal. The whole tube is shorter than the small intestine but noticeably wider, and its wall is gathered into a row of pouches by three ribbon like bands of muscle running its length.
How is its job different from the small intestine's?
| Feature | Small intestine | Large intestine |
|---|---|---|
| Main task | Absorbing nutrients from food | Reclaiming water and salts |
| Lining | Carpeted with finger like villi | Smooth, with mucus producing glands |
| Contents | Liquid and still nutrient rich | Gradually thickening residue |
| Resident bacteria | Relatively few | Enormous and varied population |
A large volume of fluid reaches the cecum each day, most of it digestive juice the body secreted rather than anything that was drunk. Nearly all of it is taken back across the colon lining, along with sodium and other salts. When that recovery fails, as it does in many gut infections, the residue reaches the rectum still liquid, which is what diarrhea is.
What do the bacteria there do?
The colon holds a dense population of bacteria, far larger than anywhere else in the body. They ferment the fiber that human enzymes cannot break down, and the short chain fatty acids released are absorbed; one of them is the preferred fuel of the cells lining the colon itself. The same organisms make small amounts of vitamin K and several B vitamins, and the gas that fermentation produces is a normal byproduct. They also occupy the ground, competing with organisms that would cause disease, which is part of why a course of antibiotics can be followed by an infection the resident population would otherwise have held in check.
Also known as
Used in a sentence
Colonoscopy allows direct visualization of the lining of the large intestine.
Good to know
The large intestine mainly absorbs water and forms stool, while nutrient absorption occurs primarily in the small intestine, a functional distinction that is often confused between the two organs.
Word origin
Where “large intestine” comes from
Intestine comes from Latin intestinum, meaning gut, from intus, meaning within; large distinguishes it from the narrower small intestine.