The lower gastrointestinal tract is the part of the digestive system that is responsible for the last part of food digestion and the expulsion of waste from the body. In this lesson, we’ll look at each part of the system and what functions each serves in the process of digestion.
The Lower Gastrointestinal Tract
When we last left Timmy, he was playing in his backyard pretending to be his favorite animal, a chameleon. Oh yeah, and he also ate a butterfly! We followed the path that the ingested butterfly took down Timmy’s upper gastrointestinal tract, from his mouth, to his pharynx, down his esophagus, and through his stomach. Now let’s continue on our journey through Timmy’s digestive system as we follow the path that the food takes through the lower gastrointestinal tract.
Digestion in the Duodenum
We know that liquefied food and gastric juice is released by the pyloric sphincter a little at a time from the stomach into the duodenum, which is the first section of small intestine where digesting food enters from the stomach. A lot of people think that this food is now digested into products that can be absorbed into the bloodstream, but that’s not the case at all, because full digestion of the food still requires a lot more work.As soon as the liquefied food and gastric juice enters the duodenum, the acid and partially digested food stimulates the pancreas to secrete bicarbonate, water, and many different digestive enzymes, which flow into the duodenum to mix with the gastric juice. You may remember that bicarbonate is a base and that bases neutralize acids. So when the bicarbonate secreted by the pancreas mixes with the gastric juice, it neutralizes the acid.
Meanwhile, digestive enzymes are secreted from the liver and pancreas as zymogens, which, you may remember, are inactive precursors of enzymes that require a change to be activated. These zymogens are activated by other enzymes in the duodenum and then start breaking carbohydrates into smaller sugars, and also breaking proteins into peptides and amino acids.Once the carbohydrate chains have been broken down into monosaccharides, they can be transported across the luminal membrane and into the epithelial cells of the small intestine. Likewise, proteins and peptides can’t be absorbed by the small intestine. But once they are broken down into their component amino acids, these are easily transported into the epithelial cells.
However, fats are a little more complicated.
You see, fats, which are also called lipids, are hydrophobic. They repel water and clump together with other lipids, because when they clump together they have less interaction with the water. This means that they are insoluble. They won’t go into solution and they aren’t available for soluble digestive enzymes to break them down.
Fortunately, our liver produces bile salts, which coat the lipids and keep them separated into tiny droplets that don’t clump together. These tiny, coated droplets give the digestive enzymes enough surface area to gain access to the lipids and break them down. One such digestive enzyme is lipase, which is an enzyme that breaks lipids down into monoglycerides and fatty acids. These monoglycerides and fatty acids can then be absorbed by the small intestine.
Although the liver produces bile, it is stored in the gall bladder, which then releases it into the duodenum when digestion is taking place.
Absorption in the Small Intestine
The small intestine can be broken down into three parts. We’ve already talked about the first part, the duodenum, which is the very first section of small intestine where the pancreas, liver, and gall bladder add their digestive enzymes, water, and bicarbonate to the digesting food. Following the duodenum are the jejunum and then the ileum.
These sections are specialized for the absorption of monosaccharides, amino acids, monoglycerides, fatty acids, and water. The epithelium is structured into large folds that increase surface area and absorption.And sticking out from the folds are finger-like projections, called villi, that further increase surface area and absorption.
Even the epithelial cells of the villi themselves are covered with tiny finger-like projections called microvilli, which, again, further increase surface area and absorption.Digesting food is moved through the small intestine by a process called peristalsis, which is the slow, rhythmic contraction of the smooth muscle in the intestinal wall. Peristalsis moves digested food through the small intestine, where most of the nutrients and water are absorbed, before it enters the large intestine.
The Large Intestine and Rectum
The large intestine, or colon, functions mainly to reabsorb most of the water that wasn’t reabsorbed by the small intestine.
Again, peristalsis moves what is left of the food through the large intestine, and finally into the rectum, which is the last part of the large intestine, where feces are stored before they are eliminated through the anus.
So, let’s review. The duodenum is the first section of small intestine where digesting food enters from the stomach. Contrary to popular belief, most digestion occurs in the small intestine, and not the stomach. Not only does the duodenum receive digesting food from the stomach, but it also receives water, bicarbonate, and digestive enzymes from the pancreas.
It receives bile from the gall bladder and a host of digestive enzymes from the liver. As the food is moved through the small intestine by peristalsis, carbohydrates are broken down into monosaccharides, proteins are broken down into amino acids, and lipids are broken down into fatty acids and monoglycerides.The monosaccharides, amino acids, fatty acids, and monoglycerides are then transported into the epithelial cells of the jejunum and ileum, which are the absorptive portions of the small intestine. The absorptive surface of the small intestine is arranged into large folds that increase surface area and absorption.
Sticking out from the folds are finger-like projections, called villi, that further increase surface area and absorption.Following the small intestine is the large intestine, or colon, which functions primarily to reabsorb most of the water that wasn’t reabsorbed by the small intestine. Again, peristalsis moves what is left of the food through the large intestine and finally into the rectum, which is the last part of the large intestine where feces are stored before they are eliminated through the anus.