Digestive System


Digestive Tract
     The digestive tract is organized around a long digestive tract that begins at the mouth and ends at the anus. The tissue in this tract can be divided into four layers:
     The mucosa is an example of a mucous membrane that consists of:
  Mucosal epithelium
  The type of epithelium varies along the tract depending upon function. It is stratified to resist mechanical abrasion and stress and simple in places where there is secretion or absorption. Plicae (sing. plica), of folds, are sometimes present to increase surface area.
  Lamina propria
  This is the underlying layer of areolar connective tissue. It contains blood vessels, nerve endings, lymphatic vessels, and cells of the immune system. It provides mechanical and metabolic support for the epithelium and protection as part of the immune system.
  Muscularis mucosae
  The outer boundary of the mucosa is often marked by a band of smooth muscle arranged in two thin concentric layers, an inner circular layer and outer longitudinal layer. Contraction of the muscularis mucosae agitates the epithelium and alters its shape.
     The submucosa is the connective tissue that surrounds the mucosa. Large blood vessels and lymphatics and occasional exocrine glands are found in this layer. A network of nerve fibers and neurons cell bodies called the submucosal plexus (Meissner’s plexus) is present in this layer.
  Muscularis Externa
     This is a muscular layer that surrounds the submucosa. It consists of an inner circular layer and an outer longitudinal layer of smooth muscle. The layers sandwich a myenteric plexus (Auerbach’s plexus) that contains nerve fibers and parasympathetic ganglia. These muscle layers mechanically process and propel the contents of the digestive tract by coordinated movements controlled by the neurons of the myenteric plexus.
     Regions along the tract where the circular layer is thickened form sphincters that control the flow of materials from one part of the tract to the next.
     Where the digestive tract is free to move within the abdominal cavity it is surrounded by a serous membrane called the serosa. It places where the tract is firmly attached to surrounding structures the attaching connective tissue is called adventitia.


Movement of Digestive Materials
     Materials move in the digestive tract by the following two types of movement:
  Peristalsis is the coordinated contraction of the muscularis that moves a bolus (mass of ingesta) along the length of the digestive tract.
  Segmentation is coordinated contraction of the muscularis that fragments and churns digested materials  with intestinal secretions there is no net movement in either direction.


     The serosa is also called the visceral peritoneum and is continuous with the parietal peritoneum. The peritoneum lines the peritoneal cavity within the abdominal cavity. Abdominal organs have the following relationships with the peritoneum:
  Intraperitoneal organs
  These organs lie completely within the peritoneal cavity and are covered on all sides by visceral peritoneum, e.g., stomach, liver, ileum.
  Retroperitoneal organs
  These organs are outside the peritoneal cavity and only their anterior surface is covered by visceral peritoneum, e.g., kidneys, ureters, aorta.
  Secondarily retroperitoneal organs
  These organs are similar to retroperitoneal organs in their relationship to the peritoneal cavity but acquire their position during development when the visceral peritoneum on their posterior side fuses with the parietal peritoneum.


     The intraperitoneal organs are suspended in the peritoneal cavity by fused double-sheets of peritoneal membrane called mesenteries. Mesenteries provide a conduit for blood vessels, nerves and lymphatics to the digestive tract and stabilize the position of organs.
     Mesenteries include:
  Mesentery proper – The mesentery that suspends the small intestines from the dorsal body wall.
  Mesocolon – The mesentery attached to the colon that includes:
  Transverse colon – suspends the transverse colon.
  Sigmoid colon – suspends the sigmoid colon. (The ascending and descending colon become secondarily retroperitoneal.)
  Greater omentum – A mesentery containing a thick layer of adipose tissue forms a pouch that extends from the greater curvature of stomach to the transverse colon.
  Lesser omentum – A mesentery between the lesser curvature of the stomach and the liver.


Oral Cavity
     The digestive tract begins at the oral cavity and performs 1) analysis, 2) mechanical processing, 3) lubrication, and 4) digestion. The oral cavity is lined by oral mucosa which has stratified squamous epithelium that protects the mucosa from abrasion.
    The mucosal surfaces within the oral cavity include:
  Cheeks – form the lateral walls of the oral cavity.
  Labia (sing. labium) – the inner lining of the lips.
  Gingivae – the mucosa that covers the alveolar processes and surrounds the neck of each tooth.
  Hard and soft palate – form the roof of the oral cavity.
  Uvula – tissue that dangles from the posterior margin of the soft palate.
  Tongue – dominates the floor of the oral cavity.
     Spaces within the oral cavity include:
  Vestibule – is the space between lips, cheeks and teeth.
  Fauces – is the passage between the oral cavity and the pharynx.


     The tongue is divided into a body (in the oral cavity) and a root (in the pharynx). The v-shaped line formed by the circumvallate papillae indicate the boundary between the body and the root. The superior surface of the tongue is called the dorsum.
     The lingual frenulum is a thin fold of mucous membrane that holds the body of the tongue to the oral floor.
     The tongue contains intrinsic muscles that alter the shape of the tongue and extrinsic muscles that positions the tongue in the oral cavity.


Salivary Glands
     There are three pairs of salivary glands that secret saliva into the oral cavity:
   1. Parotid salivary glands – located anterior to the outer ear, produces secretions that empties into the vestibule near the second upper molar by the parotid duct.
  2. Sublingual salivary glands – are under the floor of the mouth and are drained by numerous sublingual ducts.
  3. Submandibular salivary glands – located on the medial side of the mandible under the mylohyoid line. Submandibular ducts drain secretions through an opening on either side of the lingual frenulum.
     The saliva contains salivary amylase which begins digestion of complex carbohydrates and mucins which are glycoproteins that enhance the lubricating qualities of saliva. Saliva also helps to control oral bacterial populations.


     Mastication, or chewing, is performed by the teeth.
  Tooth Anatomy
     The bulk of the tooth is formed by a bony substance called dentin. Cytoplasmic processes extend into the dentin from cells in the pulp cavity. Highly vascular connective tissue within the pulp cavity receives blood and sensation through blood vessels and nerves that enter the root at the apical foramen and travel through the root canal.
     The tooth is anchored to the bony socket of the alveolar process by collagen fibers of the periodontal ligament. A bony substance called cementum covers the dentin of the root and the fibers of the periodontal ligament are anchored in cementum.
     The crown is the visible portion of the tooth above the gingivae. The dentin of the crown is covered by enamel, the hardest material in the body. The neck is the boundary between the crown and the root.
  Types of teeth
     There are four types of teeth:
  Incisors – blade-like teeth for clipping and cutting.
  Cuspids (Canines) – conical with a pointed tip for tearing and slashing.
  Bicuspids (Premolars) – have one or two roots and flattened crowns with prominent ridges for crushing, mashing and grinding.
  Molars – have three or more roots and broader crowns with ridges for crushing and grinding.
  Dental Succession
     Twenty primary or deciduous teeth consist of two incisors, one cuspid and two molars on each side. The primary teeth are replaced with the secondary or permanent dentition consisting of two incisors, one cuspid, two bicuspids, and three molars on each side.
  Dental Frame of Reference
     The various surfaces of the teeth are designated as follows:
  Labial or buccal surface is the surface that faces the lips or cheeks.
  Palatal or lingual surface is the inner surface facing the tongue.
  Mesial surface is the medial or anterior surface.
  Distal surface is the lateral or posterior surface.
  Occlusal surface is the surface that comes into contact with the opposing tooth during chewing.


     The pharynx serves as a common passageway for food, liquids and air.


     The esophagus is a muscular tube approximately 1 foot long that transports food and liquids from the pharynx to the stomach. Some of the distinctive histological features of the esophagus are:
  The epithelium of the mucosa is stratified squamous to resist abrasion.
  The submucosa has esophageal glands that produce mucus for lubrication.
  The muscularis externa consists of skeletal muscle in its upper third, mixed skeletal and smooth muscle in its middle third, and smooth muscle in its lower third.
  The outer layer of the esophagus is adventitia that anchors the esophagus against the dorsal body wall.


     The functions of the stomach include:
  1. bulk storage of food
  2. mechanical breakdown of food
  3. chemical digestion of food
  Anatomy of the Stomach
     The stomach is an expanded J-shaped segment of the digestive tract. The medial surface of the stomach has a lesser curvature and the lateral surface has a greater curvature and the anterior and posterior surfaces are smooth and rounded.
     The stomach can be divided into 4 regions:
  1. Cardia – is the region of the stomach immediately surrounding the junction of the esophagus with the stomach.
  2. Fundus – is the region of the stomach superior to the gastroesophageal junction.
  3. Body – is the largest region of the stomach and lies between the fundus and the lower curving segment of the J.
  4. Pylorus – is the curve of the J. A muscular pyloric sphincter regulates the release of chyme into the duodenum.
     In the contracted stomach the mucosa is thrown into folds called rugae.  As the stomach expands the rugae flatten out to allow the stomach’s expansion.
  Mesenteries of Stomach
     The visceral peritoneum of the anterior and posterior surfaces fuse at the lesser and greater curvatures to form two prominent mesenteries:
  Greater omentum – forms a pouch that hangs like an apron between the anterior abdominal wall and the abdominal viscera.
  Lesser omentum – is the mesentery between the lesser curvature and the liver.
  Musculature of the Stomach
     Both the muscularis mucosae and muscularis externa have extra muscle layers. The muscularis mucosae has an extra outer circular layer and the muscularis externa has an extra inner oblique layer.
  Histology of the Stomach
      The mucosal epithelium of the stomach is simple columnar and consists of mucus-secreting cells that produce a carpet of mucus. Gastric pits open on the surface of the mucosa and are connected to tubular gastric glands at their base. The neck is the place where the gastric glands and gastric pits join and the mucosal neck cells at this location continually divide to replace the cells that are lost.
  Gastric Secretory Cells
     The gastric glands produce gastric juice and have three secretory cells:
  1. Parietal cells – secrete hydrochloric acid and intrinsic factor. Intrinsic factor is necessary for absorption of vitamin B 12.
  2. Chief cells – secrete pepsinogen that is converted to the active proteolytic enzyme pepsin by the acids of the stomach.
  3. Enteroendocrine cells – G cells produce the hormone gastrin that stimulates gastric juice secretion and gastric motility.


Small Intestines
     The small intestines is where digestion and absorption of nutrients primarily occurs. The surface area of the intestinal mucosa is increased by transverse folds called plicae circulares. The intestines is about 6 m (about 20 ft in length) and is divided into three regions:
  This initial segment is the shortest (about 25 cm or 10 inches long) and the widest segment. Except for a small segment where it is attached to the pylorus of the stomach, it is secondarily retroperitoneal. It receives chyme from the stomach and digestive secretions from the pancreas and the liver.
  The submucosa of the duodenum contains duodenal (Brunner’s) glands that produce copious mucus containing bicarbonate ion that rapidly changes the pH of the chyme coming from the stomach and protects the lining of the duodenum from its initial acidity.
  The jejunum begins where the duodenum abruptly bends and becomes intraperitoneal. The jejunum is about 2.5 m (8 ft) long. The bulk of chemical digestion and nutrient absorption occurs here. Isolated lymphoid nodules are present in the lamina propria.
  The last segment of the small intestines is about 3.5 m (12 ft) long and ends where it joins the cecum at the sphincter called the ileocecal valve. Both the jejunum and ileum are supported by a mesentery called the mesentery proper. Lymphoid nodules are more numerous in the lamina propria of the ileum and may form aggregated lymphoid nodules (Peyer’s patches).
  Histology of the Small Intestines
     The mucosa of the small intestines has finger-like projections called villi (sing. villus). The epithelial cells of the villi have microvilli on their apical surface. Goblet cells that secrete mucins are found among the epithelial cells. The plicae circulares, villi and microvilli increase the surface area enormously.
     Tubular intestinal glands are found between the villi extending into the lamina propria. Cells in the intestinal glands continually divide and migrate onto the villi where they are eventually shed at the tips of the villi.
     The lamina propria of the small intestines have lymphatic vessels called lacteals that transport absorbed lipids to the circulation.


Large Intestines
     The large intestines is about 1.5 m (5 ft) long and frames the small intestines. Its major functions include:
  1. Reabsorption of water, electrolytes and compaction of feces.
  2. Absorption of vitamins.
  3. Storage of fecal material.
     The large intestines can be divided into three parts:
     The cecum is an expanded pouch that receives ingesta from the ileum through the ileocecal valve in its medial wall. The appendix is attached to the cecum and contains numerous lymphoid nodules.
     The colon makes up most of the large intestines and has several distinctive features:
  1. Haustra (sing. hastrum) – are pouches that permit distention and elongation.
  2. Taeniae coli – are three separate longitudinal ribbons of smooth muscle visible in the outer surface of the colon underneath the serosa.
  3. Fatty appendices of the colon (epiploic appendages) – are pockets of adipose tissue contained within pouch-like extensions of the serosa.
  Regions of the colon
     The colon can be divided into four regions, 1) ascending colon, 2) transverse colon, 3) descending colon, and 4) sigmoid colon.
     The ascending and descending colons are secondarily retroperitoneal on the right and left sides of the peritoneal cavity, respectively. The transverse and sigmoid colons are intraperitoneal and are suspended by mysentery.
     The rectum is the last 15 cm (6 in) of the large intestines. It temporarily stores feces prior to excretion. The last portion of the rectum is the anal canal which ends at the anus where the epithelium changes from simple columnar to stratified squamous. Distention of veins in the lamina propria and submucosa of the anal canal  are called hemorrhoids.
  Histology of Colon
     Distinctive histological features of the colon include:
  1. Absence of villi.
  2. Goblet cells are abundant
  3. Intestinal glands are deep and contain numerous goblet cells
  4. Presence of large, numerous lymphoid nodules

5. Longitudinal layer of muscularis externa have three thickened bands corresponding to the taeniae coli.


Accesory Digestive Organs
     The liver is the largest visceral organ and has more than 200 different functions that fall in one of three categories:
  1. Metabolic regulation – for example, regulation of circulating levels of carbohydrates, lipids and amino acids.
  2. Hematological regulation – liver cells synthesize plasma proteins and phagocytic cells remove old or damaged red blood cells.
  3. Synthesis and secretion of bile – bile helps neutralize acidic chyme from the stomach and enables digestion of lipids in the small intestines.
  Anatomy of the liver
     On the anterior surface of the liver the falciform ligament, a ventral mesentery, separates the right and left lobes of the liver. The round ligament is found in the inferior margin of the falciform ligament. The liver is attached to the diaphragm by the coronary ligament.
     The lobes of the liver can be divided into segments based on the blood supply and secretory ducts. The segments have clinical significance, particularly for surgery.
  Blood supply to the liver
     The liver has two sources of blood: hepatic artery proper which delivers oxygenated blood and hepatic portal vein which delivers blood containing nutrients from the intestines. The hepatic veins drain blood from the liver and delivers it to the inferior vena cava.
  Histological organization of the liver
     The parenchyma of the liver is divided by connective tissue into liver lobules. The hepatocytes are arranged in plates one cell thick that are arranged like the spokes of the wheel around the central vein. The spaces between the plates contain sinusoids that have in addition to endothelial cells, phagocytic Kupffer cells.
     Blood enters the lobule through the portal areas at the corners of the lobules that contain:
  1. Branches of the hepatic portal vein
  2. Branches of the hepatic artery proper
  3. Branches of the bile duct
     Blood that is drained by the central vein drains into hepatic veins that drain into the inferior vena cava.
  Bile secretion
     Bile is secreted into channels formed by opposing hepatocyte cell membranes called bile canaliculi. The bile is collected by bile ductules that connect to the bile ducts of the portal areas.
     The right and left ducts collect the bile secreted by their respective liver lobes. These ducts combine to form the common hepatic duct. The common hepatic duct fuses with the cystic duct to form the common bile duct.
Gall Bladder
     The gall bladder is a hollow pear-shaped, muscular organ that stores and concentrates bile. Between meals, bile secreted by the liver enters the gall bladder through the cystic duct. Under the stimulation that occurs during a meal, bile is ejected from the gall bladder into the cystic duct which fuses with the common hepatic duct to form the common bile duct which opens into the duodenum at the duodenal papilla.


     The pancreas is secondarily retroperitoneal and is bound to the posterior wall of the abdominal cavity. It can be divided into a broad head that lies within the loop of the duodenum, a body, and a blunt rounded tail that lies near the spleen.
     The pancreas is primarily an exocrine organ producing digestive enzymes and buffers and is secondarily an endocrine organ. The pancreatic exocrine secretions are delivered to the duodenum by a large pancreatic duct which joints the common bile duct at the duodenal ampulla. A small accessory pancreatic duct may branch from it and empty its secretion separately at the lesser duodenal papilla.