What Does the Large Intestine Do?
The large intestine is part of the digestive system—or gastrointestinal (GI) system. The GI system is made up of the alimentary canal—the tract that runs from the mouth to the anus—and various organs that help with digestion, such as the liver. The large intestine is the last part of the alimentary canal. The large intestine and small intestine finish the digestive process that starts in the mouth and stomach. Here are details about the large intestine anatomy and what it does.
Large Intestine Anatomy
Anatomy is the structure of living things and their parts. The large intestine is approximately five feet long. This is about one-fifth of the entire length of the alimentary canal. Compared to the small intestine, the large intestine takes a much straighter path and is shorter. The “large” in large intestine comes from it being larger in diameter and broadness than the small intestine.
The large intestine has a muscular wall that contracts to move contents along. It’s inner lining has many goblet cells, which secrete mucus to keep things moving smoothly.
There are four main large intestine parts or sections:
- Cecum: This is the first two-inch part of the large intestine. It meets the small intestine in the lower right-hand corner of the abdomen. The appendix hangs from the cecum. Doctors do not know of any current purpose for the appendix.
- Colon: There are four sections of the colon, which runs from the cecum to the rectum. The first part—the ascending colon—goes up the right side of the abdomen from the cecum. The transverse colon runs across the upper abdomen on the front side. The third part is the descending colon, which drops down the left-hand side of the abdomen. The very end of the colon is the sigmoid colon. It’s a short S-shaped section that connects to the rectum.
- Rectum: This section is about 4.5 inches long and connects to the anal canal.
- Anal canal: This is the last 1- to 1.5-inch part of the large intestine that meets the anus.
Large Intestine Physiology
Physiology is the function of living things and their parts. The parts of the large intestine work together to accomplish two main jobs—absorbing water and forming solid waste, or feces.
No chemical digestion takes place in the large intestine. That process is complete when the small intestine passes its contents—or chyme—into the large intestine. The small intestine has absorbed most nutrients and 90% of the water from chyme. What’s left is undigested food, which is mainly fiber, and dead cells, salts, and dark bile pigments.
Each section plays a role in the overall large intestine function:
- Cecum: The cecum resembles a pouch. It receives and holds liquid contents from the small intestine. As the cecum fills with chyme, it distends, which stimulates muscles in the intestine wall. The cecum passes the liquid on to the colon when these muscles contract. This happens about every 25 minutes.
- Colon: The ascending and transverse colon sections absorb water—about 1 to 1.5 liters per day—and salts. Trillions of helpful bacteria live in the colon. Their actions help digest residual fiber and produce vitamins the colon absorbs. By the time contents enter the descending colon, they are solidifying into feces colored by bile pigments. The sigmoid colon contracts to move the feces into the rectum.
- Rectum: The rectum stores feces until the body can eliminate the waste.
- Anal canal: The anal canal passes feces out through the anus as a bowel movement.
The total time it takes for food to pass through the colon averages about 36 hours. Peristalsis propels contents through the entire digestive tract, including the large intestine. These wave-like contractions are called mass movements in the large intestine. They happen several times a day to push contents forward through the colon. These waves also help expel gas from the large intestine. When mass movements fill the rectum, you feel the need to have a bowel movement. This completes the job of the digestive tract.