Digestive System Digestive Tractthekingdomofkamor.weebly.com/.../3/6/26364219/complete_digestiv… · The digestive tract begins at the oral cavity and performs 1) analysis, 2) mechanical

12/9/14 11:57 AMDigestive System

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Digestive System Digestive Tract The digestive tract is organized around a long digestive tract that begins at the mouth and ends at theanus. The tissue in this tract can be divided into four layers:

Mucosa 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 resistmechanical 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 supportfor 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 twothin concentric layers, an inner circular layer and outer longitudinal layer. Contraction of themuscularis mucosae agitates the epithelium and alters its shape.

Submucosa The submucosa is the connective tissue that surrounds the mucosa. Large blood vessels andlymphatics and occasional exocrine glands are found in this layer. A network of nerve fibers andneurons 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 anouter longitudinal layer of smooth muscle. The layers sandwich a myenteric plexus (Auerbach’splexus) 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 theneurons of the myenteric plexus. Regions along the tract where the circular layer is thickened form sphincters that control the flow ofmaterials from one part of the tract to the next.

Serosa Where the digestive tract is free to move within the abdominal cavity it is surrounded by a serousmembrane called the serosa. It places where the tract is firmly attached to surrounding structures theattaching connective tissue is called adventitia.

Movement of Digestive Materials Materials move in the digestive tract by the following two types of movement:

Peristalsis Peristalsis is the coordinated contraction of the muscularis that moves a bolus (mass of ingesta) alongthe length of the digestive tract.

Segmentation Segmentation is coordinated contraction of the muscularis that fragments and churns digestedmaterials with intestinal secretions there is no net movement in either direction.

Peritoneum The serosa is also called the visceral peritoneum and is continuous with the parietal peritoneum. Theperitoneum lines the peritoneal cavity within the abdominal cavity. Abdominal organs have the followingrelationships with the peritoneum:

Intraperitoneal organs These organs lie completely within the peritoneal cavity and are covered on all sides by visceralperitoneum, e.g., stomach, liver, ileum.



Retroperitoneal organs These organs are outside the peritoneal cavity and only their anterior surface is covered by visceralperitoneum, e.g., kidneys, ureters, aorta.

Secondarily retroperitoneal organs These organs are similar to retroperitoneal organs in their relationship to the peritoneal cavity butacquire their position during development when the visceral peritoneum on their posterior side fuseswith the parietal peritoneum.

Mesenteries The intraperitoneal organs are suspended in the peritoneal cavity by fused double-sheets of peritonealmembrane called mesenteries. Mesenteries provide a conduit for blood vessels, nerves and lymphatics tothe 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 mesocolon – suspends the transverse colon.

Sigmoid mesocolon – suspends the sigmoid colon. (The ascending and descending colon becomesecondarily retroperitoneal.)

Greater omentum – A mesentery containing a thick layer of adipose tissue forms a pouch that extendsfrom 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.

Tongue The tongue is divided into a body (in the oral cavity) and a root (in the pharynx). The v-shaped lineformed by the circumvallate papillae indicate the boundary between the body and the root. The superiorsurface 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 oralfloor. The tongue contains intrinsic muscles that alter the shape of the tongue and extrinsic muscles thatpositions 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 thevestibule near the second upper molar by the parotid duct.

2. Sublingual salivary glands – are under the floor of the mouth and are drained by numeroussublingual 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 mucinswhich are glycoproteins that enhance the lubricating qualities of saliva. Saliva also helps to control oralbacterial populations.

Teeth 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 extendinto the dentin from cells in the pulp cavity. Highly vascular connective tissue within the pulp cavityreceives blood and sensation through blood vessels and nerves that enter the root at the apical foramenand travel through the root canal. The tooth is anchored to the bony socket of the alveolar process by collagen fibers of theperiodontal ligament. A bony substance called cementum covers the dentin of the root and the fibersof 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 coveredby 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 forcrushing, 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 eachside. The primary teeth are replaced with the secondary or permanent dentition consisting of twoincisors, 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.

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

Esophagus The esophagus is a muscular tube approximately 1 foot long that transports food and liquids from thepharynx 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.

Stomach 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 stomachhas a lesser curvature and the lateral surface has a greater curvature and the anterior and posteriorsurfaces 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 withthe 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 curvingsegment of the J.

4. Pylorus – is the curve of the J. A muscular pyloric sphincter regulates the release of chyme intothe 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 curvaturesto form two prominent mesenteries:

Greater omentum – forms a pouch that hangs like an apron between the anterior abdominal wall andthe 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 muscularismucosae 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 thatproduce a carpet of mucus. Gastric pits open on the surface of the mucosa and are connected to tubulargastric glands at their base. The neck is the place where the gastric glands and gastric pits join and themucosal 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 forabsorption of vitamin B 12.

2. Chief cells – secrete pepsinogen that is converted to the active proteolytic enzyme pepsin by theacids of the stomach.

3. Enteroendocrine cells – G cells produce the hormone gastrin that stimulates gastric juicesecretion and gastric motility.



Small Intestines The small intestines is where digestion and absorption of nutrients primarily occurs. The surface area ofthe 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:

Duodenum This initial segment is the shortest (about 25 cm or 10 inches long) and the widest segment. Exceptfor 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 copiousmucus 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.

Jejunum The jejunum begins where the duodenum abruptly bends and becomes intraperitoneal. The jejunum isabout 2.5 m (8 ft) long. The bulk of chemical digestion and nutrient absorption occurs here. Isolatedlymphoid nodules are present in the lamina propria.

Ileum The last segment of the small intestines is about 3.5 m (12 ft) long and ends where it joins the cecumat the sphincter called the ileocecal valve. Both the jejunum and ileum are supported by a mesenterycalled the mesentery proper. Lymphoid nodules are more numerous in the lamina propria of theileum 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 epithelialcells of the villi have microvilli on their apical surface. Goblet cells that secrete mucins are found amongthe 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 theintestinal glands continually divide and migrate onto the villi where they are eventually shed at the tips ofthe villi. The lamina propria of the small intestines have lymphatic vessels called lacteals that transport absorbedlipids to the circulation.



Large Intestines The large intestines is about 1.5 m (5 ft) long and frames the small intestines. Its major functionsinclude: 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:

Cecum The cecum is an expanded pouch that receives ingesta from the ileum through the ileocecal valve inits medial wall. The appendix is attached to the cecum and contains numerous lymphoid nodules.Colon 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 outersurface of the colon underneath the serosa.

3. Fatty appendices of the colon (epiploic appendages) – are pockets of adipose tissue containedwithin 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 ofthe peritoneal cavity, respectively. The transverse and sigmoid colons are intraperitoneal and aresuspended by mysentery.

Rectum The rectum is the last 15 cm (6 in) of the large intestines. It temporarily stores feces prior toexcretion. The last portion of the rectum is the anal canal which ends at the anus where the epitheliumchanges from simple columnar to stratified squamous. Distention of veins in the lamina propria andsubmucosa 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 taeniaecoli.



Accesory Digestive Organs Liver The liver is the largest visceral organ and has more than 200 different functions that fall in one of threecategories:

1. Metabolic regulation – for example, regulation of circulating levels of carbohydrates, lipids andamino acids.

2. Hematological regulation – liver cells synthesize plasma proteins and phagocytic cells remove oldor damaged red blood cells.

3. Synthesis and secretion of bile – bile helps neutralize acidic chyme from the stomach and enablesdigestion 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 rightand 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. Thesegments 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 andhepatic portal vein which delivers blood containing nutrients from the intestines. The hepatic veins drainblood 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 arearranged 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, phagocyticKupffer 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 toform the common hepatic duct. The common hepatic duct fuses with the cystic duct to form thecommon bile duct.



Gall Bladder The gall bladder is a hollow pear-shaped, muscular organ that stores and concentrates bile. Betweenmeals, bile secreted by the liver enters the gall bladder through the cystic duct. Under the stimulation thatoccurs during a meal, bile is ejected from the gall bladder into the cystic duct which fuses with thecommon hepatic duct to form the common bile duct which opens into the duodenum at the duodenalpapilla.

Pancreas The pancreas is secondarily retroperitoneal and is bound to the posterior wall of the abdominal cavity. Itcan be divided into a broad head that lies within the loop of the duodenum, a body, and a blunt roundedtail that lies near the spleen.

The pancreas is primarily an exocrine organ producing digestive enzymes and buffers and is secondarilyan endocrine organ. The pancreatic exocrine secretions are delivered to the duodenum by a largepancreatic duct which joints the common bile duct at the duodenal ampulla. A small accessorypancreatic duct may branch from it and empty its secretion separately at the lesser duodenal papilla.

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Digestive System Digestive Tractthekingdomofkamor.weebly.com/.../3/6/26364219/complete_digestiv… · The digestive tract begins at the oral cavity and performs 1) analysis, 2) mechanical