Chapter 25: Anatomy of the Digestive System

Dr. Kim Wilson

Digestive System

Consists of two parts:

1. Gastrointestinal tract – a tube from mouth to anus

• Mouth• Pharynx• Esophagus• Stomach• Small and large intestine

2. Accessory organs of digestion – teeth, tongue, salivary glands, liver, gall bladder, pancreas

Digestive System

Functions of the Digestive System

• Ingestion – Taking in food and liquids • Secretion – Water, enzymes, acid or

buffers secreted into the gastrointestinal tract

• Mixing and propulsion – Food is churned and moved along the GIT by peristalsis.

• Digestion – Food is mechanically and chemically broken into smaller particles

• Absorption – Passage of digested substances into GIT wall and then into blood & lymph

• Defecation – Elimination of undigested or unabsorbed waste in the feces.

Walls of the Digestive Tract

Consists of 4 layers of tissue:

1. Mucosa• Innermost layer made up of the mucous epithelium,

lamina propria and the muscularis mucosa

2. Submucosa• Contains many glands, blood vessels and

parasympathetic nerves that forms the submucosal (Meissner’s) plexus

3. Muscularis• Inner layer of circular muscle and outer layer of

longitudinal smooth muscle• Contains Auerbach (myenteric) plexus between the 2

muscle layers

4. Serosa• Outermost layer, same as the visceral peritoneum• Connected to the parietal peritoneum via the

mesentery

Oral (Buccal) Cavity• Structure of the mouth

1. Lips• Externally covered by skin,

internally by a mucous membrane

• Keep food in the mouth• Sense temperature and

texture of food• Needed to form speech

sounds

2. Cheeks• Lateral boundaries of the

oral cavity• Formed largely by the

buccinator muscle lined with mucous membrane

Oral (Buccal) Cavity

3. Hard and Soft Palate• Hard palate: palatine and

maxillary bones• Soft palate: muscular arch

separating the mouth from the nose• Uvula projects off the

soft palate

Oral (Buccal) Cavity

4. Tongue• 3 parts: root, tip, and body • Skeletal muscle covered by mucous membrane• Covered with papillae which contain taste buds• Lingual frenulum helps anchor the tongue to the

floor of the mouth• Rich supply of blood vessels allows for quick

absorption (sublingual medications)• Intrinsic muscles: originate and insert in the

mouth, used for mastication and speech• Extrinsic muscles: insert into the tongue, but

originate from the hyoid or skull bones, used for swallowing (deglutition) and speech

Papillae• Rough elevations; cover upper surface and sides of tongue• 3 types

• VALLATE• Large and mushroomlike• Form inverted "V" on posterior portion of tongue

• FUNGIFORM• Located on sides and tip of tongue

• FILIFORM• Small and white in appearance (very numerous)• Located on anterior 2/3 of tongue

Circumvallate Papillae

Oral (Buccal) Cavity

5. Salivary Glands• Parotid glands

• Anterior and inferior to the ear• Produce watery saliva containing enzymes; open into the

mouth via the Stenson ducts• Inflammation = mumps

• Submandibular glands• At the mandibular angle• Produce saliva containing enzymes and mucus• Wharton ducts open into the mouth on either side of the

frenulum• Sublingual glands

• In front of the submandibular glands• Drained by ducts of Rivinus• Produce mucus saliva

Teeth• Involved in mastication and speech• Crown: Exposed portion of the tooth• Neck: enameled part of tooth below

gum line• Root: anchors the tooth into the

periodontal membrane• Enamel: Hard, protective outer

covering• Dentin: living, cellular, calcified

tissue within the root, dentin is covered by cellular bone-like structure that helps hold tooth in the socket.

• Pulp cavity within the dentin, filled with blood vessels, nerves, and connective tissue

• Periodontal ligaments: hold tooth in socket.

• Cementum: Anchors the root • Gingiva: dense, fibrous C.T.

covered by stratified squamous epithelium.

Teeth

• Two sets• Primary,

deciduous, milk: Childhood (20)

• Permanent or secondary: Adult (32)

• Types• Incisors, canines,

premolars and molars

Pharynx and Esophagus

• Swallowing moves the food mass, or bolus, from the mouth into the pahrynx

• Esophagus: a muscular tube which connects the pharynx to the stomach– Lies posterior to the trachea– Pierces through the diaphragm– 1st segment of the digestive tube where the 4 layers

of tissue can be observed– Upper 1/3: voluntary, striated muscle– Middle1/3: involuntary, mixed– Lower 1/3: Involuntary, smooth– Upper and lower esophageal sphincters

• Lower AKA cardiac sphincter– Stretching of the esophageal hiatus may allow

upward bulging of the stomach and lower end of the esophagus; hiatal hernia

– GERD: stomach acid flows back up through the LES

Stomach Size and Divisions• Size and Position

– Enlarged after a meal and collapses as the food is pushed through– Holds 1- 1.5 L– Under the liver and the diaphragm, most left of midline

• Divisions– Fundus, body, and pyloris– Cardiac portion: where esophagus meets the stomach– LES or cardiac sphincter controls movement of food from the esophagus into the stomach– Pyloric sphincter controls movement from the stomach into the small intestine

Stomach Wall

• Gastric mucosa– Epithelial lining of the stomach

made up of rugae folds and gastric pits

– Gastric glands, which secrete HCl and digestive enzymes, are found within the pits

• Contain 3 major cell types:

1. Chief cells – secrete pepsin

2. Parietal cells – secrete HCl and Intrinsic Factor

3. Endocrine cells – produce ghrelin and gastrin

• Gastric muscle– 3 layers of muscle: longitudinal,

circular and oblique• Increases contraction and

mixing ability

Stomach Muscles• Cardiac sphincter

• Located between esophagus and stomach (at entrance)• Controls materials entering stomach

• Pyloric sphincter• Located between stomach (pylorus) and small intestine (at exit)• Controls materials exiting stomach

Functions of the Stomach

1. Food reservoir – food is stored until it can be digested

2. Secrete gastric juice for digestion of food

3. Churns and mixes food

4. Secretes intrinsic factor important in B12 uptake

5. Small amount of absorption; drugs, water, alcohol, some fats

6. Secretes gastrin ( regulates digestive functions) and ghrelin (increases apetite)

7. HCl destroys pathogens

Small Intestine

• 3 divisions: duodenum, jejunum, ileum• Wall of the small intestine

– Made up of circular plicae containing villi projections

• Each villus contains an arteriole, a venule and lacteal

• Microvilli line each villus creating a brush border

• Goblet cells – mucus• Endocrine cells – hormones

– Intestinal crypts of Lieberkuhn are sites of mitosis where new enterocytes are formed

• New cells are pushed up and the old ones shed

• Paneth cells are located at the base of each crypt and produce enzymes to inhibit bacterial growth

Large Intestine• Diameter is greater than the small

intestine, but it is much shorter (5 ft vs 20 ft)

• Divisions– Cecum: blind pouch off the first part of

the LI, located in the LRQ• Ileocecal valve allows material to

move from SI to LI– Colon

• Ascending colon moves up the right side of the abdomen

• Transverse colon moves horizontally across the abdomen from the hepatic flexure to the splenic flexure

• Descending colon moves down the left side of the abdomen

• Sigmoid colon extends below the colon and connects to the rectum

Greater and Lesser Omentum• Greater Omentum

• An extension of visceral peritoneum from the greater curvature of the stomach

• Hangs over the intestines in a double fold (“Lace Apron”)• Function: Protection – In inflammation, the greater omentum envelops

the inflamed area and “walls it off” from the rest of the abdomen• Lesser Omentum

• An extension of visceral peritoneum from the lesser curvature of the stomach to the liver (Holds stomach to liver)

Large Intestine cont.

• Divisions cont.• Rectum

• Last part of the colon which terminates at the anus; contains the internal (involuntary) and external (voluntary) anal sphincters

Large Intestine• Modifications of the walls of the LI

– Intestinal mucus glands lubricate feces– Longitudinal muscle group produce taeniae coli– Circular muscle group produces haustra

Veniform Appendix• Accessory organ whose function is not fully understood• Possible breeding ground for nonpathogenic intestinal bacteria• Contains lymph tissue• Inflammation is known as appendicitis• McBurney’s point: area in the RLQ which is extremely tender to touch with

appendicitis

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Peritoneum and Mesenteries

• Peritoneum – large sheet of serous membrane• Visceral: Covers organs• Parietal: Covers interior surface of

body wall• Retroperitoneal: Certain organs

covered by peritoneum on only one surface and are considered behind the peritoneum; e.g., kidneys, pancreas, duodenum

• Mesenteries: two layers of peritoneum with thin layer of loose C.T. between• Routes by which vessels and

nerves pass from body wall to organs

• Greater omentum: connects greater curvature of the stomach to the transverse colon.

• Lesser omentum: connects lesser curvature of the stomach and the proximal part of the duodenum to the liver and diaphragm.

• Transverse mesocolon, sigmoid mesocolon, mesoappendix.

Liver

• Largest organ in the body (3 – 4 pounds)• Lies immediately beneath the diaphragm, within the right hypochondrium• 2 lobes separated by the falciform ligament

– Right lobe (5/6 of the liver)• Right lobe proper, caudate lobe and quadrate lobe

– Left lobe (1/6 of the liver)• Each lobe is separated into lobules and supported by a capsule of Glisson

– A central vein extends through each lobule– Hepatic cells, sinusoids, bile canaliculi, arteries and veins also make up the

lobules

Hepatic Lobules - "Structural Units of the Liver"

• Each lobe is divided into lobules by blood vessels and fibrous tissue

• Each lobule is a tiny cylinder that contains 5 ‑ 6 sides

Liver – Hepatic Lobule Function

• Hepatic lobule function– Blood enters lobule from

hepatic artery– Blood oxygenates hepatocytes– Sinusoids contain phagocytic

Kupffer cells– Blood continues along the

sinusoids to the central vein– Central veins lead to the main

hepatic veins which drain into the inferior vena cava

– Bile formed by hepatocyes passes through the canaliculi to join bile ducts

Liver – Bile Ducts• Bile Ducts

• Small bile ducts join to form the right and left hepatic ducts which join to form the common hepatic duct

• The common hepatic duct merges with the cystic duct from the gallbladder to form the common bile duct

• Bile is emptied into the SI at the duodenum via the major duodenal papilla

Functions of the Liver

• Liver cells detoxify substances• Liver cells secrete bile which aids in fat

digestion• Liver cells help metabolize proteins, fats

and carbohydrates• Liver cells store iron, vitamins A, B12 and

D• Produces plasma proteins and serves as a

site of hematopoeisis during fetal development

Gallbladder

• Lies underneath the liver• Cholecystitis – GB

inflammation• Cholelithiasis – gallstone

formation• Cholecytectomy – GB

removal

Gallbladder Functions

• The GB stores bile and concentrates it• During fat digestion, the GB contracts and ejects bile into the

duodenum• Jaundice results when an obstruction of bile flow occurs

• Bile cannot be lost through the feces and enters the blood, creating a yellowish skin hue

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Pancreas• Pancreas both endocrine and

exocrine• Head, body and tail• Endocrine: pancreatic islets.

Produce insulin, glucose, and somatostatin

• Exocrine: groups acini (grape-like cluster) form lobules separated by septa, produce digestive enzymes

• Intercalated ducts lead to intralobular ducts lead to interlobular ducts lead to the pancreatic duct.

• Pancreatic duct joins common bile duct and enters duodenum at the hepatopancreatic ampulla controlled by the hepatopancreatic ampullar sphincter

Functions of the Pancreas

• Functions of the pancreas• Acinar cells secrete

digestive enzymes• Beta cells secrete

insulin• Alpha cells secrete

glucagon