The digestive tract consists of the digestive tract, a tube extending from the mouth to the anus, plus the associated organs, which secrete fluids into the digestive tract. The term gastrointestinal tract technically only refers to the stomach and intestine but is often used as a synonym for the digestive tract. The inside of the digestive tract is continuous with the outside environment, where it opens at the mouth and anus. Nutrients cross the wall of the digestive tract to enter the circulation.
The digestive tract consists of the oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, and anus. Accessory glands are associated with the digestive tract. The salivary glands empty into the oral cavity and the liver and pancreas are connected to the small intestine.
Various parts of the digestive tract are specialized for different functions, but nearly all parts consist of four layers or tunics the mucosa, sub mucosa, muscularis, and serosa or adventitia.
Digestive Tract Histology
The innermost tunic, the mucosa, consists of mucous epithelium, a loose connective tissue called the lamina propria, and a thin smooth muscle layer, the muscularis mucosa. The epithelium in the mouth, esophagus and anus resists abrasion, and epithelium in the stomach and intestine absorbs and secretes.
The sub mucosa lies just outside the mucosa. It is a thick layer of loose connective tissue containing nerves, blood vessels, and small glands. An extensive network of nerve cell processes forms a plexus (network). The plexus is innervated by autonomic nerves.
The muscularis which in most part of the digestive tube consists of an inner layer of circular smooth muscle and an outer layer of longitudinal smooth muscle. Another nerve plexus, also innervated by autonomic nerves, lies between the two muscle layers. Together the nerve plexuses of the sub mucosa and muscularis compose the enteric plexus. This plexus is extremely important in the control of movement and secretion within the tract.
The fourth, or outermost, layer of the digestive tract is either a serosa or an adventitia. Some regions of the digestive tract are covered by peritoneum and other regions are not. The peritoneum, which is a smooth epithelial layer, and its underlying connective tissue are referred to histologically as the serosa. In regions of the digestive tract not covered by peritoneum, the digestive tract is covered by a connective tissue layer called the adventitia, which is continuous with the surrounding connective tissue.
Peritoneum and Mesenteries
The body wall of the abdominal cavity and the abdominal organs are covered with serous membranes. The serous membrane that covers the organs is the visceral peritoneum, or serosa. The serous membrane that lines the wall of the abdominal cavity is the parietal peritoneum.
Many of the organs of the abdominal cavity are held in place by connective tissue sheets called mesenteries. The mesenteries consist of two layers of serous membranes with a thin layer of loose connective tissue between them. Specific mesenteries are given names. The mesentery connecting the lesser curvature of the stomach to the liver and diaphragm is the lesser omentum, and the mesentery connecting the greater curvature of the stomach to the transverse colon and posterior body wall is the greater omentum. The greater omentum is unusual in that it is a long, double fold of mesentery that extends inferiorly from the stomach before looping back to the transverse colon to create a cavity or pocket, called the omental bursa. Fat accumulates in the greater omentum, giving it the appearance of a fat-filled apron that covers the anterior surface of the abdominal viscera. Mesentery is a general term referring to the serous membrane attached to the abdominal organs. The term is also used specifically to refer to the mesentery that attaches the small intestine to the posterior abdominal wall. This mesentery is also called the mesentery proper.
Other abdominal organs lie against the abdominal wall, have no mesenteries, and are described as retroperitoneal. The retroperitoneal organs include the duodenum, pancreas, ascending colon, descending colon, rectum, kidneys, adrenal glands and urinary bladder.
The oral cavity, or mouth, is the first part of the digestive tract. It is bounded by the lips and cheeks and contains the teeth and tongue.
The lips are muscular structures, formed mostly by the orbicularis oris muscle. The outer surfaces of the lips are covered by skin. The keratinized stratified epithelium of the skin becomes thin at the margin of the lips. The color from the underlying blood vessels can be seen through the thin, transparent epithelium, giving the lips a reddish-pink appearance. At the internal margin of the lips, the epithelium is continuous with the moist stratified squamous epithelium of the mucosa in the oral cavity. The cheeks form the lateral walls of the oral cavity.
The buccinators muscles are located within the cheeks and flatten the cheeks against teeth. The lips and cheeks are important in the process of mastication, or chewing. They help manipulate the food within the mouth and hold the food in place while the teeth crush or tear it. Mastication begins the process of mechanical digestion, in which large food particles are broken down into smaller ones. The cheeks also help form words during the speech process.
The tongue is a large, muscular organ that occupies most of the oral cavity. The major attachment of the tongue is in the posterior part of the oral cavity. The anterior part of the tongue is relatively free. There is an anterior attachment to the floor of the mouth by a thin fold of tissue called the frenulum.
The tongue moves food in the mouth and, in cooperation with the lips and cheeks, holds the food in place during mastication. It also plays a major role in the process of swallowing. The tongue is a major sensory organ for taste, as well as being one of the major organs of speech.
There are 32 teeth in the normal adult mouth, located in the mandible and maxillae. The teeth can be divided into quadrants right upper, left upper, right lower, and left lower. In adults, each quadrant contains one central and one lateral incisor; one canine; first and second premolars; and first, second, and third molars. The third molars are called wisdom teeth because they usually appear in a person’s late teens or early twenties, when the person is old enough to have acquired some degree of wisdom.
The teeth of adults are permanent, or secondary, teeth. Most of them are replacements of the 2 primary, or deciduous, teeth.
Each tooth consists of a crown with one or more cusps, a neck and a root. The center of the tooth is a pulp cavity, which is filled with blood vessels, nerves and connective tissue, called pulp. The pulp cavity is surrounded by a living, cellular, bonelike tissue called dentin. The dentin of the tooth crown is covered by an extremely hard, acellular substance called enamel, which protects the tooth against abrasion and acids produced by bacteria in the mouth. The surface of the dentin in the root is covered with cementum, which helps anchor the tooth in the jaw.
The teeth are rooted within alveoli along the alveolar processes of the mandible and maxillae. The alveolar processes are covered by dense fibrous connective tissue and moist stratified squamous epithelium, referred to as the gingival, or gums. The teeth are held in place by periodontal ligaments, which are connective tissue fibers that extend from the alveolar walls and are embedded into the cementum.
Palate and Tonsils
The palate, or roof of the oral cavity, consists of two parts. The anterior part contains bone and is called the hard palate, whereas the posterior portion consists of skeletal muscle and connective tissue and is called the soft palate. The uvula is a posterior extension of the soft palate. The palate separates the oral cavity from the nasal cavity and prevents food from passing into the nasal cavity during chewing and swallowing.
The tonsils are located in the lateral posterior walls of the oral cavity, in the nasopharynx, and in the posterior surface of the tongue.
There are three pairs of salivary glands the parotid, submandibular, and sublingual glands. They produce saliva, which is a mixture of serous and mucous fluids. Saliva helps keep the oral cavity moist and contains enzymes that begin the process of chemical digestion. The salivary glands are compound alveolar glands. They have branching ducts with clusters of alveoli, resembling grapes, at the ends of the ducts.
The largest of the salivary glands, the parotid glands, are serous glands located just anterior to each ear. Parotid ducts enter the oral cavity adjacent to the second upper molars.
The submandibular glands produce more serous than mucous secretions. Each gland can be felt as a soft lump along the inferior border of the mandible. The submandibular ducts open into the oral cavity on each side of the frenulum of the tongue. In certain people, if the mouth is opened and the tip of the tongue is elevated, saliva can squirt out of the mouth from the ducts of these glands.
The sublingual glands, the smallest of the three paired salivary glands, produce primarily mucous secretions. They lie immediately below the mucous membrane in the floor of the oral cavity. Each sublingual gland has 10-12 small ducts opening onto the floor of the oral cavity.
The pharynx, or throat, which connects the mouth with the esophagus, consists of three parts the nasopharynx, oropharynx, and laryngopharynx. Normally only the oropharynx and laryngopharynx transmit food. The posterior walls of the oropharynx and laryngopharynx are formed by the superior, middle, and inferior pharyngeal constrictor muscles.
The esophagus is a muscular tube, lined with moist stratified squamous epithelium that extends from the pharynx to the stomach. It is about 25 centimeters (cm) long and lies anterior to the vertebrae and posterior to the trachea within the mediastenum. It passes through the diaphragm and ends at the stomach. Upper and lower esophageal sphincters, located at the upper and lower ends of the esophagus, respectively, regulate the movement of food into and out of the esophagus. The lower esophageal sphincter is sometimes called the cardiac sphincter. Numerous mucous glands produce thick, lubricating mucus that coats the inner surface of the esophagus.
The stomach is an enlarged segment of the digestive tract in the left superior part of the abdomen. The opening from the esophagus into the stomach is called the cardiac opening because it is near the heart. The region of the stomach around the cardiac opening is called the cardiac region. The most superior part of the stomach is the fundus. The largest part of the stomach is the body, which turns to the right, forming a greater curvature on the left, and a lesser curvature on the right. The opening from the stomach into the small intestine is the pyloric opening, which is surrounded by a relatively thick ring of smooth muscle called the pyloric sphincter. The region of the stomach near the pyloric opening is the pyloric region.
The muscular layer of the stomach is different from other regions of the digestive tract in that it consists of three layers an outer longitudinal layer, a middle circular layer, and an inner oblique layer. These muscular layers produce a churning action in the stomach, important in the digestive process. The sub mucosa and mucosa of the stomach are thrown into large folds called rugae when the stomach is empty. These folds allow the mucosa and sub mucosa to stretch, and the folds disappear as the stomach is filled.
The stomach is lined with simple columnar epithelium. The mucosal surface forms numerous, tube-like gastric pits, which are the openings for the gastric glands. The epithelial cells of the stomach can be divided into five groups. The first group consists of surface mucous cells on the inner surface of the stomach and lining the gastric pits. Those cells produce mucus which coats and protect the stomach lining. They are mucous neck cells, which produce mucous; parietal cells, which produce hydrochloric acids and intrinsic factors; endocrine cells, which produce regulatory hormones; and chief cells, which produce pepsinogen, a precursor of the protein-digesting enzyme pepsin.
The small intestine is about 6 meters long and consists of three partsthe duodenum, jejunum, and ileum. The duodenum is about 25 centimeter (the term duodenum means 12, suggesting that it is 12 inches long). The jejunum is about 2.5 meter long and makes up two-fifths of the total length of the small intestine. The ileum is about 3.5 meter long and makes up three-fifths of the small intestine.
The duodenum nearly completes a nearly an 18degree arc as it curves within the abdominal cavity. Part of the pancreas lies within this arc. The common bile duct from the liver and the pancreatic duct from the pancreas join each other and empty into the duodenum.
The small intestine is the major site of digestion and absorption of food, which are accomplished by the presence of a large surface area. The surface of the small intestine has three modifications that increase surface area about 600-foldcircular folds, villi, and microvilli. The mucosa and sub mucosa form a series of circular folds that run perpendicular to the long axis of the digestive tract. Tiny finger like projections of the mucosa forms numerous villi, which are 0.5-1.5 mm long. Most of the cells composing the surface of the villi have numerous cytoplasmic extensions, called microvilli. Each villus is covered by simple columnar epithelium. Within the loose connective tissue core of each villus is a blood capillary called lacteal. The blood capillary network and the lacteal are very important in transporting absorbed nutrients.
The mucosa of the small intestine is simple columnar epithelium with four major cell types: Absorptive cells, which have microvilli, produce digestive enzymes, and absorb digested food Goblet cells, which produce a protective mucus Granular cells, (Paneths cells), which may help protect the intestinal epithelium from bacteria; Endocrine cells, which produce regulatory hormones. The epithelial cells are produce within tubular glands of the mucosa, called intestinal glands, at the base of the villi. Granular and endocrine cells are located in the bottom of the glands. The sub mucosa of the duodenum contains mucous glands, called duodenal glands, which open into the base of the intestinal glands.
The duodenum, jejunum, and ileum are similar in structure except that there is a granular decrease in the diameter of the small intestine, in the thickness of the intestinal wall, in the number of circular folds, and in the number of villi as one progress through the small intestine. Lymph nodules are common along the entire length of the digestive tract. Clusters of lymph nodules, called Peyers patches, are numerous in the ileum. These lymphatic tissues in the intestine help protect the intestinal tract from harmful micro organisms.
The junction between the ileum and the large intestine is the ileocecal junction. It has a ring of smooth muscle, the ileocecal sphincter, and an ileocecal valve, which allows material contained in the intestine to move from the ileum to the large intestine, but not in the opposite direction.
Secretions of the Small Intestines
Secretions from the mucosa of the small intestine mainly contain mucus, ions and water. Intestinal secretions lubricate and protect the intestinal wall from the acidic chime and the action of the digestive enzymes. They also keep the chime in the small intestine in a liquid form to facilitate the digestive process. Most of the secretions entering the small intestine are produced by the intestinal mucosa, but the secretions of the liver and the pancreas also enter the small intestine and play important roles in the process of digestion.
The epithelial cells in the walls of the small intestine have enzymes bound to their free surfaces that play a significant role in the final steps of digestion. Peptidases break the peptide bonds in proteins to form amino acids. Disaccharidases break down dissacharides, such as maltose and isomaltose, into monosaccharide. The amino acids and monosaccharides can be absorbed by the intestinal epithelium.
Mucus is produced by duodenal glands and by goblet cells, which are dispersed throughout the epithelial lining of the entire small intestine and within intestinal glands. Hormones released from the intestinal mucosa stimulate liver and pancreatic secretions. Secretion by duodenal glands is stimulated by the vagus nerve, secretin release, and chemical or tactile irritation of the duodenal mucosa.
Movement of Small Intestines
Mixing and propulsion of chime are the primary mechanical events that occur in the small intestine. Peristaltic contractions proceed along the length of the intestine for variable distances and cause the chime to move along the small intestine. Segmental contractions are propagated for only short distances and function to mix intestinal contents.
The ileocecal sphincter at the juncture of the ileum and the large intestine remains mildly contracted most of the time, but peristaltic contractions reaching the ileocecal sphincter from the small intestine cause the sphincter to relax and allow movement of chime from the small intestine into the cecum. The ileocecal valve allows chime to move from the ileum into the large intestine, but tends to prevent movement from the large intestine back into the ileum.
Absorption in the Small Intestines
A major function of the small intestine is the absorption of nutrients. Most absorption occurs in the duodenum and jejunum, although some absorption also occurs in the ileum.
The liver weighs about 1.36 kilograms and is located in the right upper quadrant of the abdomen, tucked against the inferior surface of the diaphragm. The posterior surface of the liver is in contact with the right ribs 5-12. it is divided into two major lobes, the right and left lobes, separated by a connective tissue septum, the falciform ligament. Two smaller lobes, the caudate and quadrate, can be seen from an inferior view. Also seen from the inferior view is the porta, which is the gate through which blood vessels, ducts and nerves enter or exit the liver.
The liver receives blood from two sources. The hepatic artery brings oxygen-rich blood to the liver, which supplies liver cells with oxygen. The hepatic portal vein carries blood that is oxygen-poor but rich in absorbed nutrients and other substances from the digestive tract to the liver. Liver cells process nutrients and detoxify harmful substance from the blood. Blood exits the liver through hepatic veins, which empty into the inferior vena cava.
Many delicate connective tissue septa divide the liver into lobules with portal triads at the corners of the lobules. The portal triads contain three structuresthe hepatic artery, hepatic portal vein, and hepatic duct. Hepatic cords, formed by platelike groups of cells called hepatocytes, are located between the center and the margins of each lobule. The hepatic cords are separated from one another by blood channels called hepatic sinusoids. The sinusoid epithelium contains phagocytic cells that help remove foreign particles from the blood. Blood from the hepatic portal vein and the hepatic artery flows into the sinusoids and becomes mixed. The mixed blood flows towards the center of each lobule into a central vein. The central veins from all the lobes unite to form the hepatic veins, which carry blood out of the liver to the inferior vena cava.
A cleft-like lumen, the bile canaliculus, is between the cells of each hepatic cord. Bile, produced by the hepatocytes, flows through the bile canaliculi to the hepatic ducts in the portal triads. The hepatic ducts converge and empty into the right and left hepatic ducts, which transport bile out of the liver. The right and left hepatic ducts unite to form a single common hepatic duct. The common hepatic ducts is joined by the cystic duct from the gallbladder is a small sac on the inferior surface of the liver that stores and concentrates bile. The common bile duct joins the pancreatic duct and opens into the duodenum at the duodenal papilia. The opening into the duodenum is regulated by a sphincter.
The liver performs important digestive and excretory functions, store and processes nutrients, synthesizes new molecules, and detoxifies harmful chemicals.
The liver secretes about 70mL of bile each day. Bile contains no digestive enzymes, but it plays an important role in digestion by diluting and neutralizing stomach acid and by dramatically increasing the efficiency of fat digestion and absorption. Digestive enzyme cannot act efficient on large fat globules. Bile salts emulsify fats, breaking the fat globules into smaller droplets, much like the action of detergent in dish-water. The small droplets are more easily digested by the digestive enzymes. Bile also contains excretory products such as bile pigments, cholesterol and fats. Bilirubin is a bile pigment that results from the breakdown of hemoglobin.
Bile excretion by the liver is stimulated by secretin, which is released from the duodenum. Cholecystokinin stimulates the gall bladder to contract and release bile into the duodenum. Parasympathetic stimulation through the vagus nerve also stimulates bile secretion and release.
Most bile salts are reabsorbed in the ileum, and the blood carries them back to the liver, where they stimulate additional bile salts secretion and are once again secreted into the bile. The loss of bile salts in the feces is reduced by this recycling process.
The liver can remove sugar from the blood and store it in the form of glycogen. It can also store fat, vitamins, copper and iron. This storage function is usually short term.
The liver transforms some nutrients into more readily usable substances. Many ingested substances are harmful to the cells of the body. In addition, the body itself produces many by-products of metabolism that, if accumulated, are toxic. The liver is an important line of defense against many of those harmful substances. It detoxifies them by altering their structure, making their excretion easier. The liver can also produce its own unique new compounds. Many of the blood proteins, such as albumin, fibrinogen, globulins, and clotting factors, are synthesized in the liver and released into the circulation.
Pancreas is a fish-shaped spongy grayish-pink organ about 6 inches (15 cm) long that stretches across the back of the abdomen, behind the stomach. The head of the pancreas is on the right side of the abdomen and is connected to the duodenum (the first section of the small intestine). The narrow end of the pancreas, called the tail, extends to the left side of the body.
The pancreas makes pancreatic juices and hormones, including insulin. The pancreatic juices are enzymes that help digest food in the small intestine. Insulin controls the amount of sugar in the blood.
As pancreatic juices are made, they flow into the main pancreatic duct. This duct joins the common bile duct, which connects the pancreas to the liver and the gallbladder. The common bile duct, which carries bile (a fluid that helps digest fat) connects to the small intestine near the stomach.
The pancreas is thus a compound gland. It is compoundin the sense that it is composed of both exocrine and endocrine tissues. The exocrine function of the pancreas involves the synthesis and secretion of pancreatic juices. The endocrine function resides in the million or so cellular islands (the islets of Langerhans) embedded between the exocrine units of the pancreas. Beta cells of the islands secrete insulin, which helps control carbohydrate metabolism. Alpha cells of the islets secrete glucagon that counters the action of insulin.
The colon is made up of 6 parts all working collectively for a single purpose. Their purpose is ridding the body of toxins that have entered the body from food sources, environmental poisons, or toxins produced within the body. The colons role is to transfer nutrients into the bloodstream through the absorbent walls of the large intestine while pushing waste out of the body. In this process, digestive enzymes are released, water is absorbed by the stool, and a host of muscle groups and beneficial microorganisms work to maintain the digestive system.
The colon is approximately 4.5 feet long, 2.5 inches wide, and is a muscular tube composed of lymphatic tissue, blood vessels, connective tissue, and specialized muscles for carrying out the tasks of water absorption and waste removal. The tough outer covering of the colon protects the inner layer of the colon with circular muscles for propelling waste out of the body in an action called peristalsis. Under the outer muscular layer is a sub-mucous coat containing the lymphatic tissue, blood vessels, and connective tissue. The innermost lining is highly moist and sensitive, and contains the villi- or tiny structures providing blood to the colon.
The colon is actually just another name for the large intestine. The shorter of the two intestinal groups, the large intestine, consists of parts with various responsibilities. The names of these parts are the transverse colon, ascending colon, appendix, descending colon, sigmoid colon, and the rectum and anus.
The transverse, ascending, and descending colons are named for their physical locations within the digestive tract, and corresponding to the direction food takes as it encounters those sections. Within these parts of the colon, contractions from smooth muscle groups work food material back and forth to move waste through the colon and eventually, out of the body. The intestinal walls secrete alkaline mucus for lubricating the colon walls to ensure continued movement of the waste.
The ascending colon travels up along the right side of the body. Due to waste being forced upwards, the muscular contractions working against gravity are essential to keep the system running smoothly. The next section of the colon is termed the transverse colon due to it running across the body horizontally. Then, the descending colon turns downward and becomes the sigmoid colon, followed by the rectum and anus.
Ileocecal and Cecum Valves
The ileocecal valve is located where the small and large intestines meet. This valve is an opening between the small intestine and large intestine allowing contents to be transferred to the colon. The cecum follows this valve and is an opening to the large intestine.
Rectum and Anus
The rectum is about eight inches long and serves, basically, as a warehouse for poop. It hooks up with the sigmoid colon to the north and with the anal canal to the south.
The rectum has little shelves in it called transverse folds. These folds help keep stool in place until you’re ready to go to the bathroom. When you’re ready, stool enters the lower rectum, moves into the anal canal, and then passes through the anus on its way out.
The rectum intestinum acts as a temporary storage facility for feces. As the rectal walls expand due to the materials filling it from within, stretch receptors from the nervous system located in the rectal walls stimulate the desire to defecate. If the urge is not acted upon, the material in the rectum is often returned to the colon where more water is absorbed. If defecation is delayed for a prolonged period of time constipation and hardened feces results.
When the rectum becomes full, the increase in intrarectal pressure forces the walls of the anal canal apart, allowing the fecal matter to enter the canal. The rectum shortens as material is forced into the anal canal and peristaltic waves propel the feces out of the rectum. The internal and external sphincter allows the feces to be passed by muscles pulling the anus up over the exiting feces.