The Role of the Large Intestine in Digestion

The large intestine is the last part of the digestive tract. It has three main regions – cecum, colon and rectum. These three regions form an almost rectangular “frame”, with the coils and loops of the small intestine inside it.

The cecum is a short pouch that links the small intestine to the colon and is about 5 feet long. Large intestine is connected to the small intestine through the ileocecal valve. Small intestine sends almost liquid digestive waste product to the large intestine.

Colon absorbs most of the water from this liquid product and converts it into more solid form that the body excretes as feces via the rectum and the anus.

Unlike the small intestine, the colon’s long muscle does not form a complete tube. Instead, the muscle is concentrated into three bands called taeniae coli. These run the length of the colon and form pouches along the length called haustra.

The muscular movement in the wall of the colon, mix and propel feces along the digestive tract towards the rectum. The movement of feces varies in rate, intensity and nature depending mainly on the stage of digestion.

There are three different types of motions that take place within the colon, they are known as segmentation, peristaltic contractions and mass movement. While peristalsis propels chyme in the forward direction, segmentation moves chyme in both directions. Fecal material travels very slowly in the colon, compared to the small intestine, to allow for water absorption within the colon walls.

Almost all the chemical breakdown of food is completed in the small intestine. And almost all the nutrients vital for the bodily functions are absorbed in the small intestine.

Almost digested liquefied food is called chyme. Chyme passes from the small intestine through the ileocecal valve into the cecum. From there it reaches the first part of the colon, the ascending colon.

The main function of the colon is to convert liquefied chyme into semi solid feces for storage and disposal. Sodium, chloride and water are absorbed through the lining of the colon into the blood, making the feces less watery.

Bicarbonate and potassium are secreted by colon to replace the sodium and chloride. There are also billions of friendly microorganisms that live within the colon. These bacteria live in symbiotic relationship with the body.

The innermost layer of the colon is the undulating mucosa. This layer has the goblet cells in the intestinal glands, which is responsible for secreting lubricating mucus. This mucus helps ease the passage of the feces.

The rectum is around 5 inches long and it is normally empty except just before and during the defecation. Below the rectum lies anal canal, which is around 1.5 inches long. Within the wall of the anal canal are two strong sets of muscles. They are internal and external anal sphincters.

During defecation, peristaltic contractions in the colon push feces into the rectum, which triggers the defecation reflex. Further contractions push the feces along, and the anal sphincters relax to allow them out of the body through the anus.

Billions of microorganisms, mainly bacteria, live in the large intestine. These are called the gut flora. These bacteria are harmless as long as their population is within a certain range and as long as they are confined within the large intestine.

These bacteria produce enzymes which break down certain food components, especially plant fiber cellulose. Humans cannot digest cellulose. This way, the bacteria can feed on the undigested fiber in fecal material and provide nutrients for the body and help reduce the amount of fecal material.

As part of their metabolism, bacteria produce vitamin K and B. They also produce hydrogen, carbon dioxide, hydrogen sulfide and methane gases. Most of these gases escape through the anus as flatulence. In addition, the gut flora also help control harmful microbes which may enter the digestive system. They also help the immune system in fighting disease by promoting formation of antibodies and the activity of lymphoid tissues in colonic lining.

At least one-third of the excreted fecal weight is gut flora. The gut flora and body exist in a mutually beneficial partnership.