The digestive system consists of the gastrointestinal tract, or alimentary canal, and accessory digestive organs. Food enters the tract at the mouth and is processed by the organs. The contents that leave the system at the end of the tract are primarily undigested food. Your goals for learning are To identify the organs and sphincters of the gastrointestinal tract. To describe the structure of the wall of the gastrointestinal tract. To identify the accessory organs of the digestive system. To state a simplified function for each organ. Here's what you need to know That mucous membranes line body cavities that are open to the outside. The histological makeup of mucous membranes. The types of epithelial tissues and their functions. The histological organization of the four-layered wall in the GI tract is similar from the stomach to the last portion of the anal canal. Throughout the tract, modifications of the histological organization serve regional functions. Let’s begin with the small intestine. We will illustrate the histological organization using the wall of the small intestine. The villi, microvilli, and intestinal crypts are specific modifications of the small intestine. We will describe other regional modifications as we move through the tract. Let’s begin with the mucosa. An epithelium, made up of columnar cells, forms the inner or luminal boundary of the tract. The lamina propria, a connective tissue layer, underlies the epithelium and carries blood and lymph vessels. A thin sheet of smooth muscle, the muscularis mucosae, forms the deep boundary of this layer. Functions of the GI epithelium include Secretions of substances used in digestion Absorption of the digested products Goblet cells are found in the epithelium. Goblet cells secrete mucus. Other epithelial cells secrete fluids, such as acid in the stomach and water and salts in the small intestine. Secretory products are released into the lumen. Then, there are enteroendocrine cells. The hormones of the GI system are produced and secreted by individual mucosal cells called enteroendocrine cells. They are present over a large area. Stimuli for release are in the lumen or near the basal surface of the cells. Note that hormones are released into the interstitial fluid to enter capillaries of the lamina propria. Hormones are not released into the lumen of the GI tract. Nutrients are absorbed across a single layer of cells into blood capillaries or lymph vessels called lacteals. As we leave the mucosa, note that there are stem cells present throughout the GI tract that divide continuously to replace epithelial cells that are regularly shed. A new epithelium is produced every three to six days. Now, let’s discuss the muscularis mucosae. This thin muscle, with inner circular fibers and outer longitudinal fibers, moves the mucosa. These movements alter mucosal folds and move the villi to aid digestion and absorption. Then, there is the submucosa. The submucosa is loose connective tissue that carries blood and lymph vessels. At its deep boundary lies a network of intrinsic neurons, called the submucosal plexus that serves the digestive system. Next is the muscularis externa. The muscular portion of the wall contains two layers of smooth muscle. The innermost layer is circular while the outer layer is longitudinal. These muscles produce waves of muscular contraction known as peristalsis, and oscillatory movements of the intestines known as segmentation. Tucked between the smooth muscle layers lies another network of intrinsic neurons called the myenteric plexus. It is richly interconnected with the submucosal plexus. Together they form the enteric nervous system that serves the gut. Finally, there is the serosa. A serous membrane covers the external surface of the digestive tract that lies inside the peritoneal cavity. Its presence reduces friction as gastrointestinal organs fill and empty during digestion. The gastrointestinal tract is a muscular tube that begins at the mouth, or the oral cavity, and ends at the anus. Accessory organs include the teeth, tongue, gallbladder, and digestive glands. Let’s begin with the mouth. Food enters the GI tract through the mouth. Mechanical action of teeth and tongue break food apart and mix it with saliva. A chewed portion of food called a bolus is separated and swallowed. The epithelium, lining the mouth and pharynx, is stratified squamous. It provides protection against abrasion and high temperatures. The epithelium covering the hard palette and dorsal surface of the tongue is keratinized. Then, there is the esophagus. The esophagus has no digestive or absorptive functions. It is simply a conduit that conveys food from the pharynx to the stomach. The esophagus passes through the diaphragm at the ­esophageal hiatus. The upper third of the esophagus is striated muscle, the middle third is a mixture of striated and smooth muscle, and the lower third is smooth muscle. Like the mouth and pharynx, the lining is stratified squamous epithelium. Now, let’s move to the stomach. The stomach expands to store ingested food. Its movements pulverize lumps and mix stomach secretions with the food. Acidic gastric juice digests cells, tissues and some macromolecules. Partially digested food is called chyme. The stomach has four regions; the cardia, fundus, body, and pyloric region. The antrum is the largest part of the pyloric region and the pylorus is the constricted terminal portion. The empty stomach is flat with a volume of about 50 ml. Its interior is thrown into folds called rugae. The rugae flatten out as the fundus and body distend to accommodate a meal. The stomach can hold about 1 litre of food with little change in internal pressure. Pressure rises as the stomach distends further to accommodate upto 4 litres of food. In addition to the circular and longitudinal muscle layers of the GI tract, the stomach has an inner oblique layer. Muscles are thin in the fundus and body and produce only weak contractions. Gastric muscles increase in thickness in the pyloric region. Strong contractions in the pyloric region are important for mixing ingested food with gastric juice and emptying chyme into the small intestine. Now we’ll move to the small intestine. Most digestion and absorption occur in the small intestine. The stomach delivers chyme to the small intestine in little squirts. The rate of delivery matches the processing capacity of the intestine. Juices in the small intestine neutralize the acidic chyme, restore normal osmolarity, and continue digestion of macromolecules. Breakdown products of macromolecules are absorbed across the intestinal epithelium into blood or lymph. The small intestine has three regions: duodenum, jejunum, and ileum. The duodenum is just 8 to 11 inches long. The total length of jejunum and ileum is 8 to 13 feet, the jejunum slightly shorter than the ileum. Next, let’s see the internal structures of the small intestine. The interior of the small intestine contains many permanent circular folds called plicae circulares. The duodenum has few folds. Its function is to neutralize chyme and render it ice osmotic. The number of folds is greatest in the jejunum where most absorption takes place. Plicae circulares increase the absorptive surface area of this organ. The plicae are covered with finger-like projections of the mucosa called villi,which also increase the absorptive surface area. Villi are longest in the jejunum, and shortest in the terminal ileum. Let’s learn more about villi. The columnar epithelial cells of the small intestine have many tiny projections on their luminal surface called microvilli that further increase the absorptive surface area. The surface is called a brush border because it resembles the bristles of a hairbrush. Let’s learn about the immune functions of the small intestine. The GI tract provides an entryway into the body for pathogens, as well as for food and drink. Cells throughout the upper GI tract secrete antimicrobial agents, and gastric acid renders chyme nearly sterile. Some intestinal villus cells secrete both antibacterial enzymes and immunoglobulins. In the distal ileum, there are lymphoid nodules in the connective tissue beneath the epithelium. Normal bacteria from the large intestine may enter the small intestine. Lymphocytes in the nodules prevent these bacteria from entering the bloodstream. Next, we’ll study the large intestine. The small intestine delivers chyme to the large intestine. Water and salts are absorbed from the chyme as it progresses through the large intestine. Bacteria normally present produce some vitamins that are also absorbed. The large intestine has subdivisions including: the cecum, appendix, colon (including the ascending, transverse, descending, and sigmoid colon), rectum, and anal canal. The outer longitudinal layer of the muscular wall is reduced to three bands called teniae coli. Contractions of the inner circular layer of muscle form the sac-like impermanent haustra. In the distal anal canal, the simple columnar epithelium that began in the stomach changes to stratified squamous. It protects underlying tissues from abrasion. The anus is the outlet of the GI tract. The feces eliminated from the GI tract are primarily indigestible food combined with bacteria, inorganic material, and sloughed off epithelial cells. Sphincters separate the organs of the GI tract. They control the entrance of food into the tract, control the progression of material along the tract, prevent reflux of material, and control the elimination of feces. This is the upper esophageal sphincter. The upper esophageal sphincter is a thick layer of circular muscle at the superior end of the esophagus. This sphincter is skeletal muscle, and when constricted it prevents air from entering the esophagus. Next is the lower esophageal sphincter. The lower esophageal sphincter, or gastroesophageal sphincter, is at the gastroesophageal junction. No thickened circular muscle is present. A zone of increased pressure, called a physiological sphincter, functions like a circular muscle. When constricted, the lower esophageal sphincter prevents reflux of gastric contents into the esophagus. This is the pyloric sphincter. The pylorus has thick muscular walls. The greatly thickened circular muscle is called the pyloric sphincter. It controls the rate of gastric emptying, hence the name pyloric, a Greek word meaning “gatekeeper.” This is the ileocecal sphincter. The ileocecal sphincter, or valve, separates the terminal ileum from the cecum. Distension of the terminal ileum opens the sphincter to permit the flow of chyme into the large intestine. Distension of the colon closes the sphincter preventing reflux into the ileum. Finally, the anal sphincters. There are two anal sphincter muscles. The circular smooth muscle of the anal canal thickens to form the internal anal sphincter. It is under involuntary control. Skeletal muscle surrounds the anal canal to form the external anal sphincter which is controlled voluntarily. Relaxation of both anal sphincters permits defecation. The salivary glands, liver, and pancreas produce secretions that are discharged into the lumen of the GI tract. These are the accessory glands. The salivary glands produce saliva that moistens ingested food, and cleanses and protects the mouth. The major digestive function of the liver is the production of bile. It is produced continuously by hepatocytes, and is stored and concentrated in the gallbladder. Bile is necessary for the digestion and absorption of fat. The exocrine pancreas produces digestive juice that contains enzymes to digest all major foodstuffs. It also produces bicarbonate to neutralize acidic chyme from the stomach. Here’s the summary of what we have covered The digestive system consists of a muscular tube and accessory organs. The wall of the GI tract from stomach to anus has four layers. The stomach stores and digests food. Most digestion and absorption occurs in the small intestine. Accessory glands produce juices that support digestion.