Rectum, Functions of the Large Intestine & Water Absorption

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  • 0:08 Large Intestine
  • 0:46 Goblet Cells
  • 1:32 Feces and Mass Movements
  • 2:35 Diarrhea and Constipation
  • 3:54 Defecation and Anal Sphincters
  • 5:46 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

The large intestine is the final processing area for digested food. In this lesson, you will learn how the large intestine removes water from undigested food and prepares for the elimination of feces through the anus.

Large Intestine

Food spends many hours traveling through the mouth, pharynx, esophagus, stomach and small intestine. In fact, by the time a food remnant reaches the end of the small intestine, it has been traveling for the better part of a day. Even though that might sound like a long time, the food remnant still has up to a day or two to go as it travels through your large intestine. The main things left to do are to dry out the undigested food remnant by absorbing water and eliminate the useless waste from the body. In this lesson, you will learn about the functions of the large intestine and how the body eliminates wastes from the digestive tract.

Goblet cells located in the large intestine secrete mucus
Large Intestine Closeup

Goblet Cells

Since most nutrient absorption was completed in the small intestine, we see that the inner mucosal lining of the large intestine is lacking the projections that increase nutrient absorption called villi. This gives the inner lining of the large intestine a somewhat smooth appearance when compared to the inner lining of the small intestine. However, the mucosa of the large intestine does contain a large number of goblet cells. Goblet cells are specialized epithelial cells that secrete mucus. Specifically, goblet cells are a variation of the simple columnar epithelial cells that we have encountered throughout the digestive tract. The name goblet refers to the cell's goblet-like shape. The top of the cell resembles a cup, and the bottom of the cell resembles a stem.

Feces and Mass Movements

The mucus produced by the goblet cells act as a lubricant to ease the passage of feces, which is the waste matter eliminated from the large intestine.

We saw in the esophagus and the small intestine that food was propelled by a process called peristalsis, which is a wavelike series of muscular contractions. Peristalsis also aids food propulsion in the large intestine; however, it's more sluggish here than what we saw in earlier parts of the digestive tract. Instead of peristalsis as the main form of food propulsion through the large intestine, we see something called mass movements. Mass movements are long, slow-moving but powerful contractile waves that move through the large intestine about three or four times a day. These periodic mass movements are mostly responsible for the propulsion of contents through your large intestine. It is interesting to note that bulk or fiber in your diet increases the strength of these contractions and softens feces allowing the large intestine to function more efficiently.

Location of the rectum and anal canal
Rectum Anal Canal Diagram

Diarrhea and Constipation

It is interesting to note that there are a fair amount of bacteria that reside in your large intestine. These bacteria actually serve a purpose and that is to metabolize some of the remaining nutrients that pass into your large intestine. However, this metabolism results in the release of gases, namely methane and hydrogen sulfide, which, as you might guess, is what contributes to the odor of feces. These bacteria also make some vitamin K as well as some B vitamins. These vitamins are absorbed by the large intestine along with some ions, but the chief product absorbed by your large intestine is water.

Water reabsorption is a main function of the large intestine. It can absorb 300 ml, or about a cup and a half, of water a day. This water removal dries out the feces. If food moves through the large intestine too quickly, it will not have sufficient time to absorb water, which will result in the passage of watery stool, or diarrhea. By contrast, when food residue remains in the large intestine for an extended period of time, too much water can be absorbed. This can result in constipation, which is defined as a hard stool or difficulty passing stool. Constipation may be due to a lack of fiber in the diet.

Defecation and Anal Sphincters

As feces travels through the large intestine and moves into the rectum, the wall is stretched and pressure within the rectum increases. This creates an urge to defecate. Defecation is the elimination of feces from the digestive tract. If defecation is to occur, the feces moves into the anal canal, which we previously learned is the final stop for feces before exiting the body. If the urge to defecate is denied, feces is prevented from descending into the anal canal by one of the two anal sphincters. We have encountered sphincters in a number of different areas within the digestive tract. A sphincter is simply a ring of muscle that controls an opening. The anal sphincters are defined as rings of muscle that controls the exit of feces through the anus. The anus is the opening at the end of the digestive tract through which feces leaves the body.

The two anal sphincters are involved in controlling defecation
Anal Sphincters Diagram

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