The types of contraction in the gut differ depending on the region and the type of food which has been eaten. Some contractions cause onward movement of the food, others cause mixing and grinding. The esophagus, stomach, small intestine, and large intestine are the main regions of the GI tract. They are separated from each other by special muscles, called sphincters, which regulate the movement of ingested material from one part to another. Each part of the GI tract has a unique function to perform in digestion, and each has a distinct type of motility and sensation.
Esophagus and Stomach Digestion begins in the mouth where food is chewed, mixed with saliva, and swallowed. The esophagus propels food from the mouth to the stomach. The stomach is large enough to temporarily store the food eaten at each meal. Solid food is gradually broken down by powerful muscle contractions in the lower end of the stomach. This muscular activity produces small food particles suitable to enter the small bowel, where processes of nutrient absorption begin.
Different types of food empty from the stomach at different rates; for example, fatty foods take longer to leave the stomach than other foods.
Beverages are handled differently by the stomach, emptying more quickly into the small bowel and not requiring break-down into smaller particles. Normally, most of an average-sized meal has left the stomach after about 2 hours. In the stomach the food stimulates the release of digestive juices secretions like hydrochloric acid and digestive enzymes that chemically further break down and mix with the food. The mixture is referred to as chyme.
In the small intestine, the muscular contractions occur irregularly, varying in strength and type. Here also, the different nutrients in food affect the type of contractions generated.
After an average sized meal, the contractions continue for several hours, mixing the food and moving it along the intestine. These types of contractions last until most of the meal residues enter the large intestine.
Different foods travel at different rates along the small intestine; for example, foods high in fat travel more slowly than fiber-rich foods. After most of the food has left the small intestine, a different pattern of contractions appears. Bursts of powerful contraction, occurring about every 90 minutes during fasting and particularly at night, progress slowly down the intestine. The small intestine has 3 segments:. Duodenum — The chyme first enters into the duodenum where it is exposed to secretions that aid digestion.
The secretions include bile salts, enzymes, and bicarbonate. Pancreatic enzymes help digest carbohydrates and fats. Bicarbonate from the pancreas neutralizes the acid from the stomach.
Jejunum — The chyme is then further transited down into the second or middle part of the small intestine, the jejunum. Ileum — The ileum is the last section of the small intestine and leads to the large intestine or colon. The abdominal muscles contract straining , causing the intra-abdominal pressure to increase. The perineal wall is lowered and causes the anorectal angle to decrease from 90 degrees to less than 15 degrees almost straight , and the external anal sphincter relaxes.
The rectum now contracts and shortens in peristaltic waves, thus forcing fecal material out of the rectum and down through the anal canal. The internal and external anal sphincters, along with the puborectalis muscle, allow the feces to be passed by pulling the anus up and over the exiting feces in shortening and contracting actions.
Privacy Policy. Skip to main content. Digestive System. Search for:. The Large Intestine. Anatomy of the Large Intestine The large intestine absorbs water from the remaining indigestible food matter and compacts feces prior to defecation.
Learning Objectives Describe the function and anatomy of the large intestine. Key Takeaways Key Points The large intestine starts in the right iliac region of the pelvis, just at or below the right waist, where it is joined to the bottom end of the small intestine. It is about 4. It contains the least lymphoid tissue, and it is a part of mucosa-associated lymphoid tissue that gives it an important role in immunity.
On the surface of the large intestine, three bands of longitudinal muscle fibers called taeniae coli, each about 0. They start at the base of the appendix and extend from the cecum to the rectum. Key Terms appendix : An inner organ without any known use that can become inflamed. Differences Between Large and Small Intestine The large intestine differs in physical form from the small intestine in several ways. Additional Structures The appendix is attached to its inferior surface of the cecum.
Histology of the Large Intestine The large intestine has taeniae coli and invaginations the intestinal glands , unlike the small intestines. Learning Objectives Describe the histology of the large intestine. The bands of longitudinal muscle fibers start at the base of the appendix and extend from the cecum to the rectum. Both the small intestine and the large intestine have goblet cells, but they are abundant in the large intestine.
Key Terms goblet cell : columnar epithelium : Epithelial cells whose heights are at least four times their width.
Bacterial Flora The largest bacteria ecosystem in the human body is in the large intestine, where it plays a variety of important roles. Key Takeaways Key Points The large intestine absorbs some of the products formed by the bacteria that inhabit this region, such as short-chain fatty acids that are metabolized from undigested polysaccharides fiber. Other bacterial products of undigested polysaccharide fermentation include gas flatus , which consists primarily of nitrogen and carbon dioxide.
Factors that disrupt the microorganism population of the large intestine include antibiotics, stress, and parasites. Key Terms passive diffusion : The net movement of material from an area of high concentration to an area of low concentration without any energy input.
Digestive Processes of the Large Intestine In the large intestine, a host of microorganisms known as gut flora help digest the remaining food matter and create vitamins. Learning Objectives Summarize the digestive processes of the gut flora of the large intestine. Key Takeaways Key Points The large intestine takes about 16 hours to finish the remaining processes of the digestive system. The colon absorbs vitamins created by the colonic bacteria.
Gut flora consists of microorganisms that live in the digestive tracts of animals; the digestive tract is the largest reservoir of human flora. The colon compacts feces and stores fecal matter in the rectum until it can be defecated.
The gut flora performs many useful functions, such as fermenting unused energy substrates, training the immune system, preventing the growth of pathogenic bacteria, regulating the development of the gut, producing vitamins for the host, and producing hormones to direct the host to store fats. Key Terms saccharolytic : The breakdown of carbohydrates for energy. Gut Flora Gut flora consists of microorganisms that live in the digestive tracts of animals—the gut is the largest reservoir of human flora.
Though people can survive without gut flora, the microorganisms perform a host of useful functions, such as: Fermenting unused energy substrates.
Training the immune system. Preventing growth of harmful, pathogenic bacteria. Regulating the development of the gut. Producing vitamins for the host such as biotin and vitamin K. Producing hormones to direct the host to store fats. Gut Flora and Specialized Digestion. Absorption and Feces Formation in the Large Intestine The large intestine absorbs water from the chyme and stores feces until they can be defecated. Learning Objectives Describe the process of absorption and feces formation in the large intestine.
Key Takeaways Key Points Partially digested food passes from the small intestine to the large intestine or colon. Within the colon, digestion is retained long enough to allow fermentation via gut bacteria that break down some of the substances that remain after processing in the small intestine. The large intestine houses over species of bacteria that metabolize polysaccharides into short-chain fatty acids that produce large amounts of vitamins —especially vitamin K and biotin—and gas.
The normal flora of bacteria in the large intestine is essential in the development of certain tissues, including the cecum and lymphatics.
Key Terms feces : Digested waste material that is discharged from the bowels; excrement. Defecation Reflex Defecation is a combination of voluntary and involuntary processes that create enough force to remove waste material from the digestive system.
Learning Objectives Describe the defecation reflex. Key Takeaways Key Points The rectal ampulla acts as a temporary storage facility for the unneeded digestive material.
A sufficient increase in fecal material in the rectum causes stretch receptors from the nervous system that are located in the rectal walls to trigger the contraction of rectal muscles, the relaxation of the internal anal sphincter, and an initial contraction of the skeletal muscle of the external sphincter. If defecation is delayed for a prolonged period, the fecal matter may harden and autolyze, and result in constipation.
Once the voluntary signal to defecate is sent back from the brain, the ano-rectal angle decreases, becoming almost straight, and the external anal sphincter relaxes. When a person swallows, food pushes into the esophagus, the muscular tube that carries food and liquids from the mouth to the stomach.
Once swallowing begins, it becomes involuntary and proceeds under the control of the esophagus and brain. The lower esophageal sphincter, a ringlike muscle at the junction of the esophagus and stomach, controls the passage of food and liquid between the esophagus and stomach.
As food approaches the closed sphincter, the muscle relaxes and lets food pass through to the stomach. The stomach stores swallowed food and liquid, mixes the food and liquid with digestive juice it produces, and slowly empties its contents, called chyme, into the small intestine. The muscle of the upper part of the stomach relaxes to accept large volumes of swallowed material from the esophagus.
The muscle of the lower part of the stomach mixes the food and liquid with digestive juice. The muscles of the small intestine mix food with digestive juices from the pancreas, liver, and intestine and push the mixture forward to help with further digestion.
The walls of the small intestine absorb the digested nutrients into the bloodstream.
0コメント