Digestive system

Mr. Neuberger Digestive system

The digestive system is essentially a disassembly line • It’s primary purpose is to break nutrients down into forms that can be used by the body • Absorb them so they can be distributed to the tissues • Most of what we eat cannot be used in the form found in food, it must be broken down • Hamburger meat -> amino acids • Small units universal to all species Digestive Processes and General Anatomy

The Digestive System is the organ system that processes food, extracts nutrients from it, and eliminates the residue • 1. Ingestion- the selective intake of food • 2. Digestion- the mechanical and chemical breakdown of food into a form usable by the body • 3. Absorption- the uptake of nutrients into the blood and lymph • 4. Compaction-absorbing water and consolidating the indigestible residue into feces • 5. Defecation- the elimination of feces Digestive system functions

The digestive system has two anatomical subdivisions: • The Digestive Tract- muscular tube that extends from mouth to anus, measuring 9 m • Known also as the alimentary canal or gut • Includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine • Stomach and intestines form the gastrointestinal (GI) tract • The Accessory Organs are the teeth, tongue, salivary glands, liver, gallbladder, and pancreas General anatomy

The digestive tract is open to the environment at both ends • Most of the material in it has not entered any body tissue, considered to be external to the body until absorption occurs (epithelial cells of the alimentary canal) General anatomy

Most of the Digestive system follows this basic structural plan Mucosa Submucosa Muscularisexterna Serosa

The Mucosa (mucous membrane), lines the lumen • Consists of an inner epithelium • Loose connective tissue layer (Lamina Propria) • Thin layer of smooth muscle (Muscularis Mucosae) • Epithelium is simple columnar in most of the digestive tract, mouth, pharynx, esophagus, and anal canal differ • Subject to more abrasion- stratified squamous epithelium General anatomy

The Submucosa is the thickest layer of loose connective tissue, contains: • Blood vessels, lymphatic vessels, a nerve plexus, and mucus glands in some places • The Muscularisexterna consists of usually two layers of smooth muscle near the outer surface • Inner layer- encircle the tract • Outer layer- run longitudinally • In some areas the circular layer is thickened to form valves (sphincters)- regulate the passage of material through the digestive tract • Responsible for the motility that propels foods and residue General anatomy

The Serosa is composed of a thin layer of areolar tissue topped by a simple squamous mesothelium • Begins in the lower 3-4 cm of the esophagus and ends just before the rectum • The pharynx, most of the esophagus, and the rectum are surrounded by a fibrous connective tissue layer called the Adventitia- blends into the adjacent connective tissues of other organs General anatomy

Tongue movements, mastication (biting and grinding food), and the initial actions of swallowing employ skeletal muscles innervated by somatic motor fibers from 6 of the cranial nerves The salivary glands are innervated by sympathetic fibers from the superior cervical ganglion and parasympathetic fibers from the cranial nerves From the esophagus to the anal canal, most muscle is smooth muscle so it receives only autonomic innervation Sympathetic nervous system plays a lesser role, but in general it inhibits motility and secretion and keeps the GI sphincters contracted and closed, inhibits digestion Innervation

Even though the digestive tract receives such extensive innervation from the CNS, it can function independently even if the nerves are severed • The esophagus, stomach, and intestines have their own extensive nerve network • Enteric nervous system- thought to have over 100 million neurons, includes sensory neurons that monitor tension in the gut wall, and motor neurons that activate smooth muscle and gland cells of the gut innervation

Neurons of the ENS are distributed in two networks: • Submucosal (Meissner) plexus in the submucosa • Myenteric (Auerbach) plexus between the two layers of the muscularisexterna • Sensory nerve fibers monitor stretching of the GI wall and the chemical conditions in the lumen • These fibers carry signals to adjacent regions of the GI tract in short (myenteric) reflex arcs contained in the myenteric plexus, and to the central nervous system by way of long (vagovagal) reflex arcs, predominantly in the vagus nerves innervation

The digestive tract can be divided into 3 sections: Foregut, Midgut, and Hindgut • The Foregut- includes the mouth, pharynx, esophagus, stomach, and beginning of the duodenum • Above the diaphragm, it gives off several esophogeal arteries to the esophagus • Below the diaphragm, the foregut recieves blood from the branches of the celiac trunk Circulation

The Midgut begins at the opening of the bile duct and includes the rest of the duodenum, jejunum and ilium, and the large intestines (as far as 2/3 of the transverse colon) • It receives blood from the superior mesenteric artery circulation

The Hindgut includes the remainder of the large intestine, from the end of the transverse colon through the anal canal • All of the blood vessels that supply each section of the gut ultimately drain into the hepatic portal vein(enters the liver) circulation

The system of vessels connecting the lower digestive tract to the liver is the hepatic portal system • It routes all blood from the stomach and intestines through the liver before returning to the general circulation • It has 2 capillary systems • Capillaries in the small intestine receive digested nutrients, and the capillaries in the liver give these nutrients to the liver cells • Gives the liver a chance to process most nutrients and cleanse the intestinal blood of bacteria Circulation

When the stomach and intestines undergo the process of digesting food, they go through such strenuous contractions that they need freedom to move in the abdominal cavity • They are NOT tightly bound to the abdominal wall, but rather loosely suspended by connective tissue sheets called mesenteries • These also hold the abdominal viscera in their proper relationship to each other • Prevent the small intestine from becoming twisted and tangled by changes in the body position and by its own contractions • They also provide passage for the blood vessels and nerves that supply the digestive tract, and contain many lymph nodes and lymphatic vessels Relationship to the peritoneum

The parietal peritoneum is a serous membrane that lines the wall of the abdominal cavity • Along the posterior midline of the body, in turns inward and forms the dorsal mesentery, extends to the digestive tract • When in reaches an organ, ex. Stomach, it wraps around opposite sides of the organ, forming the serosa • In some cases, the two layers come together again and continue to another sheet of tissue called the ventral mesentery • The ventral mesentery may hang freely, or attach to the ventral abdominal wall or other organs Relationship to the peritoneum

Along the superior margin of the stomach, a ventral mesentery called the lesser omentumextends from the stomach to the liver • A fatty greater omentum hangs from the left inferior margin of the stomach and loosely covers the small intestine • The greater omentum has two layers, its inner layer helps form the mesocolon- anchors the transverse colon to the abdominal wall • Has healing properties • Intraperitoneal- when an organ lies within the peritoneal cavity • Retroperitoneal- when an organ lies outside the peritoneal cavity Relationship to the peritoneum

The mouth is also known as the oral cavity • Its functions include: ingestion, taste and other sensory responses to food, chewing, chemical digestion, swallowing, speech, and respiration • The mouth is enclosed by the lips, cheeks, tongue, and palate • Its anterior opening between the lips is the oral fissure and its posterior opening into the throat is called the fauces • It is lined with stratified squamous epithelium, and is keratinized in areas subject to high abrasion The mouth

The cheeks and lips retain food and push it through the teeth for mastication, and are essential for articulate speech and for actions such as suckling by infants • Their fleshiness is due to a combination of subcutaneous fat, the buccinator muscle of the cheeks, and the orbiculariousoris muscle of the lips • A median fold called the labial frenulum attaches each lip to the gum • The space between the cheeks, or lips and teeth is called the vestibule The cheek and lips

The lips are divided into 3 areas: • (1)- The cutaneous area is colored like the rest of the face and has hair follicles and sebaceous glands • (2)- The red area (vermillion), is the hairless region where the lips meet. It has unusually tall dermal papillae, which allow blood capillaries and nerve endings to come closer to the epidermal surface • (3)- The labial mucosa is the inner surface of the lip, facing the gums and teeth The cheek and lips

The tongue, although muscular and bulky, is an agile and sensitive organ with several functions: • It aids in food intake • It has sensory receptors for taste, texture, and temperature that are important in the acceptance or rejection of food • It compresses and breaks up food • It secretes mucus and enzymes • It compresses the chewed food into a soft mass, or bolus • It initiates swallowing • It is necessary for articulate speech The tongue

The surface of the tongue is covered with nonkaratinized stratified squamous epithelium and exhibits bumps and projections called lingual papillae • Taste buds • The anterior 2/3 of the tongue, the body, occupies the oral cavity; the posterior 1/3 occupies the oropharynx • The boundary between the body and root is marked by a V-shaped row of vallate papillae, behind that a groove called the terminal sulcus resides • A ventral median fold called the lingual frenulum attaches the body of the tongue to the floor of the mouth The tongue

The root of the tongue contains the lingual tonsils • Amidst the tongue muscles, are the serous and mucous lingual glands, which secrete a portion of the saliva • The tongue has both intrinsic and extrinsic muscles • The intrinsic ones produce the relatively subtle tongue movements of speech • The extrinsic ones produce the stronger movements of food manipulation The tongue

The palate separates the oral cavity from the nasal cavity, making it possible to breath and chew at the same time • The hard/bony palate is the anterior portion and has palatine ridges • The soft palate is posterior to this and contains skeletal muscle and glandular tissue • Contains the uvula as well which helps retain food until it is ready to be swallowed The palate- Quick overview

The teeth are collective called the dentition • Their purpose is to masticate or chew the food, breaking it down into smaller pieces • Makes the food easier to digest and process as well • Adults normally have 16 teeth on both their upper and lower jaws • Incisors- chisel-like, bite off food • Canines- puncture and shred food • Premolars and molars- crush and grind food The teeth- quick overview

Saliva moistens the mouth, digests a small amount of starch and fat, cleanses the teeth, inhibits bacterial growth, dissolves molecules so they can stimulate the taste buds, and moistens the food and binds particles together to aid in swallowing The salivary glands

Saliva is a solution of 97-99.5% water and the following solutes • Salivary amylase- begins starch digestion • Lingual lipase- activated by stomach acid, digests fats • Mucus- binds and lubricates food • Lysozyme- kills bacteria • Immunoglobin A- anitbody that inhibits bacterial growth • Electrolytes- The salivary glands

Two kinds of salivary glands, intrinsic and extrinsic • The intrinsic salivary glands are in indefinite number of small glands dispersed amid other oral tissues, lingual, labial, and buccal • Secrete saliva at a fairly constant rate, regardless of food • Salivation is controlled by groups of neurons called salivatory nuclei in the medulla oblongata and pons The salivary glands

The extrinsic salivary glands are three pairs of larger, more discrete organs located outside the oral mucosa, use ducts to get to oral cavity • 1. Parotid Gland- located beneath skin anterior to earlobe. Mumps is in inflammation of the parotid glands • 2. Submandibular Gland- located halfway along the body of the mandible, empties into the mouth by way of papilla on the side of the lingual frenulum • 3. Sublingual Gland- located on the floor of the mouth, empty into mouth posterior to the papilla of the submandibular duct The salivary glands

The pharynx has a deep layer of longitudinal muscle and a superficial layer of circular muscle • Superior, middle, and inferior pharyngeal constrictors • Help push food down into esophagus The pharynx

The esophagus is a straight muscular tube 25-30cm long, posterior to the trachea Its superior opening lies at the cricoid cartilage Passes through the diaphragm at the esophageal hiatus Its opening into the stomach is called the cardiac oriface (proximity to heart) Food pauses here briefly due to the lower esophageal sphincter(LES) (physiological, cannot be found in a cadaver) prevents stomach contents from regurgitating back into the esophagus The esophagus

The submucosa of the esophagus contains esophageal glands • Secrete lubricating mucus into the lumen • When the esophagus is empty, the mucosa and submucosa are folded into longitudinal ridges • The muscularisexterna is composed of skeletal muscle the first third of the esophagus, a smooth and skeletal mix the 2nd third, and the final third is all smooth muscle • Most of the esophagus is in the mediastinum, covered with adventitia The esophagus

Swallowing, or deglutition, is a complex action involving over 22 skeletal muscles in the mouth, pharynx, and esophagus, coordinated by the swallowing center • A pair of nuclei in the medulla oblongata • Coordinates a complex series of muscle contractions to produce swallowing without choking The esophagus

The stomach is a muscular sac in the upper left abdominal cavity immediately inferior to the diaphragm • Primary function is for food storage • Internal volume of 50ml empty, 1.5 after a typical meal, 4 L when extremely full • The stomach mechanically breaks up food particles, liquefies the food, and begins chemical digestion of proteins and a small amount of fat • Produces chyme- soupy or pasty mixture of semidigested food The stomach

The stomach is somewhat J-shaped, vertical in tall people, horizontal in short people • Lesser Curvature- extends from the esophagus to duodenum along the medial aspect • Greater Curvature- extends the longer distance from esophagus to duodenum on the lateral aspect The Stomach

The stomach is divided into 4 regions: • (1) Cardiac Region (Cardia)- small area within about 3 cm of the cardiac oriface • (2) Fundic Region (Fundus)- the dome-shaped portion superior to the esophageal attachment • (3) Body (Corpus)- makes up the greatest part of the stomach distal to the cardiac oriface • (4) Pyloric Region- slightly narrower pouch at the distal end, subdivided into a funnel-like antrum, and a narrower pyloric canal • The latter terminates at the pylorus, a narrow passage into the duodenum • Pyloric Sphincter- regulates the passage of chyme The stomach

The stomach wall has tissue layers similar to those of the esophagus • The surface of the mucosa is a simple columnar glandular epithelium • The surface cells are filled with mucin, after it is secreted, mucin swells with water and becomes mucus • Gastric rugae- longitudinal wrinkles when stomach is empty The stomach

The lamina propria is almost entirely occupied by tubular glands • The muscularisexterna has 3 layers, instead of 2- outer longitudinal, middle circular, and inner oblique layer • The gastric mucosa has depressions called gastric pits • 2 or 3 tubular glands open into the bottom of each gastric pit • In the cardiac region- cardiac glands • In the pyloric region- pyloric glands, gastric glands everywhere else The stomach

Cell types within the glands: • Mucus cells- secrete mucus, predominate in the cardia and pyloric glands • Regenerative (stem) cells, found in the base of the pit and neck of the gland, produce a continual supply of new cells • Parietal cells- secrete hydrochloric acid (HCl) • Chief cells- most numerous, secrete chymosin, lipase in infancy, pepsinogen throughout life • Enteroendocrine cells- secrete hormones and paracrine messengers that regulate digestion The stomach

The stomach is protected from self-digestion in 3 ways: • 1. Mucus Coat- a thick, highly alkaline mucus resists the action of acids and enzymes • 2. Tight Junctions- the epithelial cells are joined by tight junctions that prevent gastric juice from seeping between them • 3. Epithelial cell replacement- the stomach’s epithelial cells live only 3-6 days, they are replaced just as rapidly The stomach

The stomach “spits” about 3 mL of chyme at a time into the small intestinge • Nearly all chemical digestion and absorption occur here • Must have a large surface area • Its length is about 2.7-4.5 m • Divided into 3 sections: duodenum, jejunum, and ileum The small intestine

The duodenum constitutes the first 25 cm. Begins at the pyloric sphincter, ends at the duodenojejunal flexure, the first 12 cm of the duodenum is intraperitoneal, but the rest is retroperitoneal • Internally it has transverse to spiral ridges called circular folds, cause the chyme to flow on a spiral path along the mucosa, slowing its progress • Major duodenal papilla- where the bile and pancreatic ducts empty into the intestine • Minor duodenal papilla- receives an accessory pancreatic duct The small intestine

The jejunum is the next 2.5 m, or the first 40% of the small intestine beyond the duodenum Most digestion and nutrient absorption occur here Its wall is relatively thick and muscular Has a rich blood supply that give it a red color The small intestine

The ileum forms the last 3.6 m or 60% of the post-dudenal small intestine • Its wall is thinner, less muscular, and less vascular • Its circular folds are smaller and more sparse, and are lacking on the distal end • Peyer patches- prominent lymphatic nodules in clusters • The end of the small intestine it the ileocecal junction, where the ileum joins the cecum of the large intestine • Ileocecal valve- protrudes into the cecum and regulates the passage of food residue into the large intestine • Both the jejunum and ileum are intraperitoneal The small intestine

Tissue layers have been modified appropriately for digestion and absorption • The muscularisexterna has a thick inner circular layer and thinner outer longitudinal layer • Effective digestion and nutrient absorption require that the small intestine have a large internal surface area • Three folds: circular folds, villi, and microvilli • The villi are largest in the duodenum and become progressively smaller in the more distal regions of the small intestine • Covered with 2 epithelial cells- most importantly- absorptive cells The small intestine

The core of a villus is filled with areolar tissue of the lamina propria • Embedded in this tissue are an ateriole, a bed of blood capillaries, a venule, and a lymphatic capilarry called a leacteal • Blood capillaries absorb most nutrients • Each villus has a fuzzy brush border of microvilli, increases surface area • Duodenal glands- secrete and abundance of alkaline mucus, neutralizes stomach acid and shields the mucosa from its corrosive effects The small intestine

Each absorptive cell of a villus has a brushy border of microvilli • Increases surface area, also contains brush border enzymes, help out with the final stages of chemical digestion. Contact digestion • Intestinal cysts- pores that open into tubular glands • Upper half contains absorptive and goblet cells, lower half contains epithelial cells- 3-6 day lifespan • Paneth cells- secrete lysozyme and other defense proteins that resist bacterial invasion of the mucosa • Duodenal glands- secrete and abundance of alkaline mucus which neutralize the stomach acid to shield the mucosa from its corrosive effects The small intestine

The large intestine receives about 500 mL of indigestible food residue per day • Reduces it to about 150 mL of feces by absorbing water and salts, and eliminates the feces by defecation • The large intestine measures about 1.5 m long • Begins with the cecum, a pouch in the lower right abdominal quadrant inferior to the ileocecal valve • The appendix is an extension off the cecum, contains dense populations of immune cells • Animal adaptation The large Intestine

Digestive system