THE DIGESTIVE SYSTEM#The GI tract (gastrointestinal tract) The muscular alimentary canalMouth (Teeth & tongue)PharynxEsophagusStomachSmall intestineLarge intestineAnus

The accessory digestive organsSupply secretions contributing to the breakdown of foodSalivary glandsGallbladderLiverPancreas

#The Digestive ProcessIngestionTaking in food through the mouthPropulsion (movement of food) Swallowing Peristalsis propulsion by alternate contraction &relaxationMechanical digestionChewingChurning in stomachMixing by segmentationChemical digestionBy secreted enzymes: see laterAbsorptionTransport of digested end products into blood and lymph in wall of canal DefecationElimination of indigestible substances from body as feces

1.Mucosa (lining of lumen)a.Epithelium = varies with location: stratified squamous in the mouth, esophagus, and anus; simple columnar in the stomach and intestinesb.Lamina propria = loose FECTc.+/- Muscularis mucosae (thin) - smooth muscle2.SubmucosaLoose to moderately dense FECT Contains submucosal plexus3.Muscularis externaSmooth muscle along most of the gutSkeletal muscle near both ends of the gutUsually consists of inner circular and outer longitudinal layers4.Adventitia or SerosaAdventitia = loose FECT (on organ surfaces embedded in connective tissue) Serosa = loose FECT + mesothelium (on organ surfaces exposed to body cavities)

Typical Pattern of Layers in Digestive Tract Walls

Enteric Nervous System (ENS) Composed of two major intrinsic nerve plexuses both containing: sensory neurons, parasympathetic ganglionic neurons, sympathetic postganglionic fibers

Submucosal Plexus Found in submucosaInnervates the mucosa and submuscosa and regulates glands and smooth muscle of muscularis mucosaMyenteric plexus:Major nerve supply that controls GI tract mobilityFound in between the circular and liongitudinal layers of muscle in the muscularis mucosa and innervates it Muscle movements coordinated by ENS:Myenteric plexus can coordinate local responses independent of autonomic nervous system orInnervated primarily by parasympathetic division of ANS, when active, increases muscular activitySympathetic activity decreases muscular activity, constricts blood vessels hereThe Oral CavityFigure 246

Is bounded by lips, cheeks, palate, and tongue Has the oral orifice as its anterior openingIs continuous with the oropharynx posteriorlyOral MucosaLining of oral cavity has a stratified squamous epitheliumLayer of slightly keratinized cells covers only regions exposed to severe abrasion (gums, hard palate, dorsal tongue)Lining of cheeks, lips, and inferior surface of tongue is relatively thin, nonkeratinized, and delicateThe lamina propria (chorion) is a very thin layer of loose FECT occurs at the base of the epithelium in most areas. Most of the deeper connective tissue is moderately dense FECT or dense irregular FECT in most locations.

Oral Cavity StructuresLips (Labia)overly orbicularis orismucosa of each cheek is continuous with that of the lipsVermillion border or red border lies between highly keratinized skin of face and mucosa of mouth. It needs moistureCheeksOverly buccinator musclesVestibule: space between the cheeks (or lips) and the teethGingivae (Gums): ridges of oral mucosasurround base of each tooth on alveolar processes of maxillary bones and mandibleUvula: a dangling process that helps prevent food from entering pharynx prematurelyFauces: passageway between oral cavity and oropharynx

The TongueThe tongue is covered by oral mucosa. The epithelium on the upper surface is keratinized in places, particularly on the tips of filiform papillae. The chorion contains mucous glands og Weber.The tongue contains a core of skeletal muscle arranged in bundles. The bundles run in 3 planes at right angles to each other (i.e. in X, Y, and Z geometric axes) allowing movement of the tongue in almost any direction and allowing delicate controlled changes in the shape of the tongue.The upper surface of the tongue (the epithelium plus the adjacent lamina propria) forms numerous finger- like papillae which are classified into 3 categories.

Lingual papilae(1) Filiform Papillae(a) Pointed tip(b) Heavily keratinized epithelium(c) No taste buds(d) Most numerous type of papilla(2) Fungiform Papillae(a) Flattened apex(b) Moderately keratinized epithelium(c) Apical taste buds(d) Second most numerous type

(3) Circumvallate Papillae(a) Flattened apex(b) Moderately keratinized epithelium(c) Lateral taste buds in the papilla and in the surrounding structures(d) Surrounded by a groove with compound serous glands (von Ebner's glands) secreting into its base(e) Least numerous type of papilla

TeethThe tooth can be divided into two main areas: the crown and the root.Most of the hard tissue in teeth is dentine, a special calcified tissue, derived from mesenchyme. The dentine in the root is covered by a layer of cementum, calcified tissue derived from mesenchyme. The tooth is then connected to bone by the periodontal ligament, which has wide bundles of collagen fibres, and is embedded in bony ridge called alveolar ridge.Teeth is made up of three layers, enamel, dentine and a pulp cavity. The crown is protected by layer of enamel, a very hard, highly mineralised tissue, which is derived from ectoderm. Cementum, dentine and enamel differ from bone, in that they are not vascularised. Enamel also does not have collagen as its main consituent. It is made up of crystals or prisms of calcium phosphate. The centre of tooth is made up of a pulp cavity that extends down through the roots as a root canal. This region contains the nerve and blood supply to the tooth.Gums, or gingiva is the name for the oral mucosa that covers the tooth. At the gingival crevice (or sulcus), the cells in the epithelium of the gum adhere to the tooth enamel via a basement membrane. If bacteria calcify here, and accumulate, they can disrupt this seal, and the periodontal tissues can become infected and inflamed.

Structure of a toothDentine is made by odontoblasts, that lie on its inner border. Production of dentine is limited to the pulpal surface. About 90% of dentine is type I collagen, and about 70% of wet weight is hydroxyapatite.Odontoblasts are tall columnar secretory cells. Their secretory processes are embedded in the matrix, which is imprgnated with parallel dentine tubules. Dentine is laid down, and then calcified. Thus there is a thin layer of 'pre-dentine' which is not calcified between the dentine and odontoblasts.Enamel is made by ameloblasts, tall columnar ectodermally derived cells. It is produced before the tooth erupts. Each ameloblast has an elongated tip called a Tomes process, that secretes the organic matrix of an enamel rod. 96% of a tooth is mineralised. As the ameloblasts die when the tooth errupts, the enamel layer cannot be repaired.

The PharynxThe pharynx is the crossing of the respiratory and digestive pathways. It has three parts, named for the structure that is anterior to that part:nasopharynx is posterior to the nasal cavityoropharynx is posterior to the oral cavitylaryngopharynx is posterior to the larynxThe landmarks to officially demarcate these are the soft palate and epiglottis.

The pharynx- histological structureMucosaEpithelium- stratified squamous without keratinization in the oro= and laryngo=pharynx and respiratory in the nasopharynxLamina propria (chorion)- loose FECT with small mixt salivary glands and lymphoid infiltrates (see pharyngeal tonsil)Fibrous membrane- named the pharyngeal fascia- formes the skeleton of the pharinx and it is composed of collagen and ellastic fibers.The muscularis externa- striated muscle:Elevator longitudinal musclesConstrictor oblique muscles

The esophagus is a muscular tube that connects the pharynx with the stomach. It is the first portion of the digestive tract that has the four-layered structure: mucosa, submucosa, muscularis externa, adventitia. lumenESOPHAGUS#

Characteristic features of the esophagus relate to its function:--epithelium is stratified squamous non-keratinized, providing a moist surface resistant to friction, conducive to movement of swallowed boluses toward the stomach--muscularis externa transitions from skeletal (upper portion) to smooth muscle (middle and lower segments).this is a gradual transition, so the upper-middle region contains a mixture of skeletal and smooth muscle--a thick muscularis mucosa--esophageal glands for lubricationESOPHAGUS

#ESOPHAGUS

Mucosa contains nonkeratinized, stratified squamous epithelium and loose FECT chorionMucosa and submucosa both form large folds that extend the length of the esophagus and allow for expansionMuscularis mucosae consists of irregular layer of smooth muscleSubmucosa contains submucosal esophageal glands:produce mucous secretion which reduces friction between bolus and esophageal lining Muscularis externa:has usual inner circular and outer longitudinal layers Superiormost portion has skeletal muscle fibers, changes to smooth muscle inferiorly

#The upper portion of the esophagus, being continuous with the pharynx, contains skeletal muscle in the muscularis externa. This will transition into smooth muscle about 1/3 of the way toward the stomach, so that the muscularis externa in the lower portion of the esophagus is entirely smooth muscle.The muscularis mucosa has no such transition; it is always smooth muscle, from the cranial esophagus to the colon.

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The stomach is a dilated portion of the GI tract that accepts food from a meal and slowly releases it into the duodenum. It also is involved in food breakdown, producing pepsin and HCl. Numerous mucous cells protect the mucosal lining from these harsh agents.STOMACHAs you are aware, the stomach can be divided into four major regions:--cardia--fundus--body--pylorusHISTOLOGICALLY, we can divide the stomach into three parts:--cardia--fundus (which includes fundus and body)--pylorus

The cardia and the pylorus region are similar to each other, as are the body and fundus.

#Stomach-mucosaThe mucosa forms longitudinal folds or rugae.a.Epithelium(1) Mainly simple columnar; may be pseudostratified columnar in regions containing some types of enterochromaffin (APUD) cells (unicellular endocrine glandular epithelium)(2) Most cells of the lumen lining are mucous- secreting so the lining epithelium may be called a sheet gland(3) Enterochromaffin cells (or APUD cells) are small with basally located light cytoplasm and dark nuclei. At least some of these cells extend a sensory process to the luminal surface. Some may have acidophilic granules at the basal end.b.Lamina propria - loose FECTc.Muscularis mucosae - inner circular and outer longitudinal smooth muscle layers, frequently in bundles

The surface of the stomach is relatively smooth (i.e. it lacks villi). There are openings of the internal surface that lead to deep holes called gastric pits. The inferior portion of each pit is a narrowed isthmus. Projecting from the bottom of the pits are two or more gastric glands. The gland can be divided into a neck and fundus (base), terms which will be used to help identify predominant cell types in those regions.The glands are tightly packed, with little lamina propria between them.#The mucosa contains gastric glands (simple tubular glands) which are referred to as cardiac glands, fundic glands, or pyloric glands based on the gland structure and the region of the stomach in which the gland is normally found. Pure cardiac and pyloric glands would contain only mucus-secreting cells plus occasional enterochromaffin cells while fundic glands contain multiple exocrine cell types.

(a) Cardiac glands are branched tubular glands which occur in a narrow zone near the esophageal-stomach junction. They are usually shorter and less coiled than pyloric glands.

(b) Pyloric glands are located near the pyloric region of the stomach. These branched tubular glands are usually longer and more highly coiled than cardiac glands.

(c) Fundic glands occur throughout most of the stomach. These branched tubular glands contain three exocrine cell types plus numerous solitary endocrine cells.[1] Exocrine cell types[a] Chief cells (or zymogenic or peptic cells) -relatively columnar cells with basal nuclei, basophilic basal cytoplasm and acidophilic apical cytoplasm containing acidophilic secretory granules-serous secretory cells which secrete pepsinogen-located in the deeper 1/3 of the glands

[b] Parietal cells (or oxyntic cells) -spherical or triangular cells with central nuclei and canaliculi and with acidophilic cytoplasm-secrete HCl and intrinsic factor-occur singly or in small groups throughout the deeper 2/3 of the glands[c] Mucous cells -columnar cells with pale staining cytoplasm-mucous secretory cells-located throughout the glands but predominate in the upper 1/3 of the glands[2] Enteroendocrine cells (enterochromaffin cells)-different cells secrete a variety of locally acting hormones (mostly peptides or proteins such as secretin, cholecystokinin, secretin, and gastric inhibitory peptide) which coordinate gut functions. Some of the secretory granules may be acidophilic while others may not stain well.-some of these cells have the properties of enterochromaffin cells also called APUD (amine precursor uptake and decarboxylation) cells-most of these cells are small, basally located cells with light cytoplasm and moderately heterochromatic nuclei. A thin cytoplasmic process may extend to the apical surface of the epithelium.

2.Submucosa loose to moderately dense FECT3.Muscularis externa two or three (most common) layers of smooth musclea.Where two layers are present they are organized into inner circular and outer longitudinal layers.b.Where three layers are present they are organized into inner oblique, middle circular, and outer longitudinal layers.c.As is the case with other "pear-shaped" or oval organs, the muscle tends to run in bundles rather than in distinct layers.4.Serosa loose FECT and outer mesothelium-peritoneum

SI is longest part of dig. tubeDuodenum (short, 12 inches)fixed shape & positionMixing bowl for chyme Jejunum (2.5 m long) Most of digestion Ileum (longest at 3.5 m) Most of absorption, ends in Ileocecal valve slit valve into large intestine (colon) Small intestine- regionsSmall intestine- structureThe main functions of the small intestine are digestion, absorption of food and production of gastrointestinal hormones. The small intestine is 4-6 metres long in humans.To aid in digestion and absorption:the small intestine secretes enzymes and has mucous producing glands. The pancreas and liver also deliver their exocrine secretions into the duodenum.The mucosa is highly folded.large circular folds called plicae circulares (shown in the diagram to the right), most numerous in the upper part of the small intestinesmaller folds called villi, which are finger like mucosal projections, about 1mm long. the lining columnar epithelial cells have fine projections on their apical surfaces called microvilli. Together, these folds provide a huge surface area for absorption. Between the villi there are crypts, called crypts of Lieberkuhn, which extend down to the muscularis mucosae. These crypts are short glands.

Small intestine-mucosaThe mucosa of the small intestine is lined by a simple columnar epithelium which consists primarily of absorptive cells (enterocytes), with scattered goblet cells and occasionalenteroendocrine cells. In crypts, the epithelium also includes Paneth cells and stem cells.Absorptive cells (enterocytes) are responsible for absorbing nutrients from the intestinal lumen and transporting across the epithelium to the lamina propria, whence they diffuse into capillaries.The brush border of the intestinal epithelium not only facilitates absorption, it also provides a site of attachment for pathogens. Goblet cells secrete mucus to promote movement and effective diffusion of gut contents.Enteroendocrine cells secrete hormones to regulate secretion into the GI tract.Paneth cells, located at the bottoms of the crypts, secrete lysosomal enzymes and other factors into the crypt lumen. These agents presumably help protect the crypt epithelium with its vital stem cells.Stem cells line the walls of the crypts and continually replenish the intestinal epithelium, completely replacing all the absorptive and goblet cells approximately once every four days.Lamina propria of each villus is richly supplied with capillaries and also includes a single lacteal, for transporting absorbed nutrients. Lamina propria also includes thin strands of smooth muscle (presumably allowing some motility for individual villi, to encourage thorough fluid mixing at the absorptive surface) and numerous white blood cells. The muscularis mucosa of the small intestine forms a thin layer (only a few muscle fibers in thickness) beneath the deep ends of the crypts.

Small intestineThe submucosa of the small intestine is relatively unspecialized FECT, except in the duodenum where it is packed with the mucous-secreting Brunner's glands.Muscularis externa of the small intestine has the standard layers of inner circular and outer longitudinal smooth muscle, with ganglia of Auerbach's plexus scattered in between..Over most of the small intestine, the outer layer is a serosa attached to mesentery. The exception is the duodenum, which is retroperitoneal

Small intestine-particularitiesThe duodenum is readily distinguished from other regions of the small intestine by the presence of submucosal Brunner's glands, which may pack the submucosa so completely that the typical submucosal connective tissue is obscured. Brunner's glands provide abundant alkaline mucus to neutralize the acid contents entering from the stomach. Brunner's glands and the stomach's pyloric glands thus share a similar function. In fact, Brunner's glands look like pyloric glands that have enlarged and expanded into the submucosa of the duodenum. Villi are rather flatter in the duodenum than in the jejunum, and plicae are less frequent. The jejunum is the longest and most "typical" region of the small intestine. It has no conspicuous characteristics of its own, except for the absence of those features which characterize the duodenum and ileum (Brunner's glands and Peyer's patches, respectively). Villi are rather longer and more finger-like in the jejunum than in the duodenum, and plicae are usually apparent. The ileum has proportionally more goblet cells than more proximal sections of the small intestine. (The proportion of goblet cells generally increases as one progresses down the GI tract, with the highest proportion found in the lower tract.) The ileum also displays an increase in the amount of mucosal lymphoid tissue, which forms conspicuous clusters of lymph nodules, called Peyer's patches.

Duodenum-mucosa and Brunner glands

Ileum-Peyers patchesThe Large IntestineAlso called large bowel Horseshoe-shaped, about 1.5 meters long and 7.5 cm wideExtends from end of ileum to anusLies inferior to stomach and liverFrames the small intestineFunctionsReabsorption of water [the last 15-20%]Compaction of intestinal contents into fecesAbsorption of important vitamins produced by bacteriaStorage of fecal material prior to defecationParts of the Large IntestineCecum: the pouchlike first portionHas wormlike appendix projecting from it Colon: the largest portionRectum: the last 15 cm of digestive tractAnal canal

Ileocecal ValveAttaches the Ileum to the medial surface of cecuman expanded pouch receives material arriving from the ileumstores materials and begins compaction[opened by the gastoroileal reflex to receive material from S.I.]The AppendixAlso called vermiform appendixA slender, hollow appendage (about 9 cm long), dominated by lymphoid nodules (a lymphoid organ) Is attached to posteromedial surface of cecum

The ColonHas a larger diameter (this is why it is called large) and thinner wall than small intestine The wall of the colon forms a series of pocketlike pouches (haustra) giving it a sgmented appearanceHaustra permit expansion and elongation of colonColon Muscles 3 longitudinal bands of smooth muscle (taeniae coli) run along outer surfaces of colon deep to the serosa (similar to outer layer of muscularis externa)Muscle tone in taeniae coli creates the haustra Regions of the ColonAscending colonBegins at superior border of cecum Ascends along right lateral and posterior wall of peritoneal cavity to inferior surface of the liverTransverse colonCurves anteriorly from right colic flexureCrosses abdomen from right to leftIs supported by transverse mesocolonIs separated from anterior abdominal wall by greater omentumRegions of the ColonDescending colonProceeds inferiorly along left side:to the iliac fossa (inner surface of left ilium)Is retroperitoneal, firmly attached to abdominal wall Sigmoid colonS-shaped segment, about 15 cm longStarts at sigmoid flexureLies posterior to urinary bladderIs suspended from sigmoid mesocolonEmpties into rectum

The RectumForms last 15 cm of digestive tractIs an expandable organ for temporary storage of fecesMovement of fecal material into rectum triggers urge to defecate Anus Is exit of the anal canalHas keratinized epidermis like skin [anus][The rest of the rectum is columnar or nonkeratinized stratified squamous]Anal SphinctersInternal anal sphincter:circular muscle layer of muscularis externahas smooth muscle cells, not under voluntary controlExternal anal sphincter:encircles distal portion of anal canala ring of skeletal muscle fibers, under voluntary controlColon-mucosaThe mucosa appears smooth at the gross level because it has no villi. Numerous straight, tubular glands are present. They extend all the way to the muscularis mucosae. The glands and the surface are lined with simple columnar epithelium whose cell types are as described for the small intestine.Paneth cells are usually absent in the adult human and enteroendocrine cells are rare. Columnar absorptive cells and goblet cells are abundant. Goblet cells are more prevalent in the crypts than along the surface, and their number increases distally toward the rectum. The mucus facilitates the passage of the increasingly solid colonic contents, and covers bacteria and particulate matter. The absorptive cells have short, irregular microvilli, and although they secrete a glycocalyx, it has not been shown to contain digestive enzymes. The absorptive cells actively transport electrolytes. Water is also absorbed as it passively follows the electrolytes. As in the small intestine, undifferentiated cells are found at the base of the crypts.The lamina propria is highly cellular. It is particularly rich in lymphoid cells and and lymph nodules may interrupt the regular spacing of the crypts and extend into the submucosa (this is particularly evident in the appendix). The extensive development of GALT reflects the abundance and variety of microorganisms and noxious end products of metabolism. As in the small intestine, lymphatic vessels form a network around the muscularis mucosae. However, no lymph vessels extend into the lamina propria between colonic crypts. The muscularis mucosae has a circular and longitudinal smooth muscle layer.

Colon-wall layersThe submucosa is quite dense, similar to that of the small intestine.The muscularis externa consists of an inner circular and outer longitudinal layer. The inner circular layer is typical, but the outer longitudinal layer of the colon is very thin, except for three extremely thick longitudinal bands, called teniae coli. Bundles of muscle from the teniae coli penetrate the circular layer at irregular intervals. These discontinuities in the muscularis externa allow segments of the colon to contract independently. A contraction in one segment (2-5 cm) will peak over about 30 seconds, and disappear during the next 60. The lumen may almost be occluded during a contraction, allowing all the fecal matter to be in touch with the colon wall. The fecal matter is dug into and rolled - much like spading earth. The longitudinal layer contracts at the same time as the circular layer. Unstimulated portions between the contracting segments bulge outward, forming saccules or haustra. The next contraction would be in another area. All but 80 ml of the daily load of 450 ml of chyme are aborbed. Peristaltic movements in the colon (longitudinal layer) result in mass movements distally of colonic contents. They are not frequent (about once a day in a typical person).The entire transverse colon is covered with a serosa, whereas parts of the ascending and descending colon have an adventitia, which forms small pouches (appendices epiploicae) filled with fat tissue

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