Chapter 24 The Digestive System BIO 211 Lab Instructor Dr. Gollwitzer

Today in class we will: Discuss the gastrointestinal tract and identify its components Identify the accessory organs associated with the digestive tract The layers that make up the digestive tract wall and each layer’s characteristics Mucosa Submucosa Muscularis externa Serosa Begin our discussion of the gastrointestinal tract components in more detail Oral cavity Pharynx Esophagus

Digestive System Gastrointestinal (GI) tract = continuous, muscular tube from mouth to anus Oral cavity (mouth) Pharynx Esophagus Stomach Small intestine Large intestine Anus

Digestive System Accessory digestive organs = any digestive organ attached to the GI tract Teeth Tongue Glandular organs Salivary glands Pancreas Liver Gall Bladder

Figure 24-1

Histological Organization 4 Major layers to wall Mucosa Submucosa Muscularis externa Serosa

Figure 24-3

Mucosa Inner lining of digestive tract A mucous membrane Consists of Epithelium - moistened by glandular secretions Lamina propria = areolar CT “filler”

Mucosa: Digestive Epithelia Depend on location, function, stresses Simple or stratified Columnar or squamous

Mucosa: Digestive Epithelia Nonkeratinized stratified squamous epithelium Where mechanical stress most severe In oral cavity, pharynx, esophagus, rectum Life span = 2-3 days Simple columnar epithelium Where absorption and secretion occur (with villi and goblet cells) Stomach, small intestine, most of large intestine Life span = 6 days

Mucosa: Lamina Propria Layer of areolar (loose connective) tissue Contains Blood vessels, sensory nerve endings, lymphatic vessels, lymphoid tissue Mucosal glands and glandular secretions Muscularis mucosae/interna Band of smooth muscle and elastic fibers Smooth muscle arranged in 2 concentric layers Inner, circular layer (around lumen) Outer, longitudinal layer Contractions alter shape of lumen and move epithelial folds

Mucosa: Glandular Structures Have secretory function Associated with simple columnar cells and/or mucous-secreting cells Goblet cells (exocrine)  mucus Enteroendocrine cells  hormones (e.g., G cells  gastrin) Coordinate activities of digestive tract and accessory glands e.g., chief cells, G cells, parietal cells

Submucosa Layer of dense, irregular connective tissue Around muscularis mucosae Contains Large blood vessels and lymphatic vessels Exocrine glands  buffers and enzymes into lumen Submucosal plexus contains nerve fibers and neurons

Muscularis Externa Dominated by smooth muscle cells Inner, circular layer Outer, longitudinal layer Important role in mechanical processing and movement of materials along tract Also contains Lymphoid nodules (Peyer’s patches) Masses of lymphoid tissue; have lymphocytes that protect small intestine from bacteria that are normal inhabitants of large intestine Myenteric plexus/plexus of Auerbach Network of neurons located between circular and longitudinal muscle layers Movements coordinated by enteric nervous system (part of ANS)

Serosa A serous membrane = lines sealed, internal subdivisions of ventral body cavity Covers muscularis externa of most of digestive tract except oral cavity, pharynx, esophagus, and rectum, which have adventitia (a dense collagen fibrous sheath)

Figure 24-3

Figure 24-3

Oral (Buccal) Cavity Space within mouth Lined by oral mucosa (nonkeratinized stratified squamous epithelium) Vestibule = space between cheeks or lips and teeth Labial frenulum = fold of mucosa; connects lip to gum Cheeks = lateral walls of oral cavity Mucosae supported by fat pads and buccinator muscles Labia (lips) Gingivae (gums) Ridges that surround base of each tooth On alveolar processes of maxillary bones and mandible

Oral (Buccal) Cavity Palates Tongue Hard palate – formed by maxillary bones (anterior) and palatine bones (posterior) Soft palate – fleshy part posterior to hard palate Formed from skeletal muscle Posterior margin supports uvula = dangling process that helps prevent food from entering pharynx prematurely Tongue Muscular organ attached to floor of oral cavity Dorsum (superior) surface covered with lingual papillae (location of taste buds) Lingual frenulum – thin fold of mucous membrane that attaches tongue to floor of oral cavity

Figure 24-6

Pharynx Throat area posterior to nasal and oral cavities Shared by respiratory and digestive systems Extends between internal nares and entrances to larynx and esophagus Nasopharynx Posterior portion or nasal cavity Separated from oral cavity by soft palate Contains Pharyngeal tonsils and opening to auditory (eustachian) tube Oropharynx Posterior portion of oral cavity Between soft palate and base of tongue Fauces = opening between oral cavity and oropharynx Laryngopharynx Inferior part Between hyoid bone and entrances to esophagus and larynx

Esophagus Hollow, muscular tube From posterior laryngopharynx to stomach Descends through thoracic cavity posterior to trachea Enters abdominopelvic cavity through esophageal hiatus in diaphragm

Esophageal Histology Muscularis externa transitions from Adventitia Skeletal muscle fibers (superior third) to Smooth muscle fibers (inferior third) Adventitia CT outside muscularis externa (no serosa) Anchors esophagus against dorsal body wall

Esophageal Sphincters Circular smooth muscles that control entrance/exit to esophagus Upper esophageal sphincter In superior 1 in. Prevents air from entering Lower esophageal sphincter At inferior end Prevents backflow from stomach

Begin our discussion of the 4 accessory digestive organs Today in class we will: Complete our more detailed discussion of the gastrointestinal tract components Stomach Small intestine Large intestine Anus Begin our discussion of the 4 accessory digestive organs Teeth Tongue Glandular organs Salivary glands

Stomach Muscular tube with extra layers of smooth muscle cells Strengthen stomach wall Assist in mixing/churning activities required to form chyme Muscularis mucosae/interna – has extra outer, circular layer Muscularis externa – has oblique layer

Figure 24-13

Stomach Has expanded J shape Lesser curvature = shorter, inner, medial surface Greater curvature = longer, outer, lateral surface

Figure 24-12b

Stomach Has 4 regions Cardia Fundus Body (corpus) Pylorus Smallest region where esophagus enters (at gastroesophageal/cardiac sphincter) Fundus Superior hump to left of cardia that contacts the diaphragm Body (corpus) Main (middle) region of the stomach between fundus and curve Pylorus Last part of stomach that enters the duodenum through pyloric sphincter

Stomach Rugae Folds of gastric mucosa Temporary features Let gastric lumen expand (almost disappear when stomach full)

Stomach Gastric pits Gastric glands Depressions that open onto gastric surface Each communicates with several gastric glands Mucous cells line neck Gastric glands In fundus and body of stomach 2 types of secretory cells Parietal cells Chief cells

Figure 24-13b

Small Intestine 3 Regions Duodenum (10 in.) Jejunum (8 feet) First part; connects to pylorus “Mixing bowl” Receives chyme from stomach and secretions from pancreas and liver Curves in a C that encloses head of the pancreas Jejunum (8 feet) Middle part Site for most of chemical digestion and nutrient absorption Ileum (12 feet) Last part Connects to large intestine via ileocecal valve

Small Intestine Plicae circulares Wall inner folds in intestinal lining Permanent feature -- do not disappear when small intestine fills Wall Mucosa Intestinal epithelium Simple columnar epithelium; covers villi (pl; singular = villus) Villus capillaries/villus capillary network Lacteals Intestinal glands (goblet cells, enteroendocrine cells) Muscularis mucosae/interna Submucosa, muscularis externa, and serosa as described previously

Figure 24-16

Figure 24-17

Large Intestine (Bowel) 3 Main sections Cecum Colon (4.5 ft) Rectum (6 in.)

Large Intestine (Bowel) Cecum Pouch-like structure that connects to ileum (at ileocecal valve) Vermiform appendix = small, worm-like appendage of lymphoid tissue attached to cecum

Large Intestine (Bowel) Colon Ascending colon – along right side; takes right turn at superior end = right colic/hepatic flexure Transverse colon – crosses from R to L; takes right turn downward at left end = left colic/splenic flexure Descending colon – along left side; curves medially at sigmoid flexure Sigmoid colon – S-shaped; empties into rectum Haustra Pouches caused by longitudinal bands of muscularis externa (taeniae coli) Permit expansion and elongation of colon

Large Intestine (Bowel) Rectum Anal canal = last portion Contain small longitudinal folds (anal columns) Anus/anal orifice = exit of anal canal Contains 2 sphincters Internal anal sphincter – smooth muscle, involuntary control External anal sphincter – skeletal muscle, voluntary control

Figure 24-24a

Teeth Located in alveoli (bony sockets) on alveolar processes of maxillary bones and mandible Number Primary (deciduous) 5/side/jaw = 20 Secondary +3 molars/side/jaw = 32 Replace primary teeth

Figure 24-9a, b

Tooth Type No./ Jaw Location Shape Function No. Roots Incisors 4 Center front Blade Clip/cut 1 Cuspids/ Canines 2 Lateral to incisors Conical w/sharp ridge, pointed tip Tear/ slash/ shear Bicuspids/ Premolars Posterior to cuspids Flattened crown, prominent ridges Crush/ mash/ grind 1 or 2 Molars (wisdom teeth) 6 Posterior to bicuspids Large, flattened crown, prominent ridges Crush/ grind 3 or more

Teeth Parts Crown – above gum line Neck – boundary between crown and root Root – below gum line; sits in alveolus

Teeth Layers/Regions Outer Inner Enamel Cementum Dentin Pulp cavity Thin, white layer that covers crown Hardest biologically manufactured substance Cementum Thin layer that covers root; protects and helps anchor tooth Inner Dentin Makes up bulk of tooth Mineralized matrix (CaPO4 crystals) similar to bone, except acellular Pulp cavity Central region Receives blood vessels and nerves from root canal Root canal Narrow tunnel at root/base of tooth Blood vessels and nerves enter through apical foramen

Figure 24-8a, b

Salivary Glands Outside oral cavity, but secrete into oral cavity 3 pairs Parotid glands Extend from mastoid process of temporal bone across outer surface of masseter muscle Parotid (Stenson’s) duct – from parotid through buccinator to oral cavity Sublingual glands Under the floor of the mouth Many small sublingual (Rivinus’) ducts open along side of lingual frenulum Submandibular glands Along inner surfaces of mandible Submandibular (Wharton’s) ducts – open into mouth on either side of lingual frenulum immediately posterior to teeth

Figure 24-7a, b

Today in class we will: Complete our discussion of the 4 accessory digestive organs Glandular organs Pancreas Liver Histology Bile duct system Gall bladder Trace the path of food through the digestive system

Pancreas Lies posterior to stomach, extends laterally from duodenum toward spleen Covered by thin, CT capsule 3 regions Head – burrowed in loop of duodenum Tail – tapered end against spleen Body – slender middle region Ducts – deliver digestive enzymes and buffers to duodenum Main pancreatic duct (of Wirsung) – large, main duct  major duodenal papilla with common bile duct  duodenal ampulla (chamber) Accessory pancreatic duct (of Santorini) – may branch from pancreatic duct  minor duodenal papilla  duodenal ampulla

Fig. 24-18, p. 888

Gallbladder Hollow, pear-shaped, muscular sac Located in recess on posterior surface of liver’s right lobe 3 regions Fundus – bottom of pouch Body – main region Neck – narrow end where cystic duct exits

Figure 24-19

Liver Largest visceral organ Covered by tough, fibrous capsule Divided into 4 lobes Right lobe – largest, in right hypochondriac region Left lobe – narrow part extending into left hypochondriac region Caudate lobe – on inferior side; superior, near IVC Quadrate lobe – inferior to caudate, near gallbladder Falciform ligament – on anterior surface between R and L lobes

Liver Histology Hepatocytes = liver cells; simple cuboidal cells Each liver lobe divided into lobules Liver lobule = basic functional unit of liver Has hexagonal shape (6 corners) Hepatic triad at each corner contains Branch of hepatic portal vein Branch of hepatic artery Branch of bile duct Separated by interlobular septa Each has a central vein Hepatocytes Form 1-cell wide plates arranged like spokes around a wheel around central vein Plates are separated by sinusoids = modified blood vessels for large solutes can pass out of/into blood stream

Liver Histology Branches of hepatic portal vein (from intestine to liver) and hepatic artery (to liver from systemic circulation)  sinusoids Hepatocytes absorb solutes and secrete materials (plasma proteins) into sinusoids Blood leaves sinusoids and enters central vein of lobule Central veins   hepatic veins  IVC  heart Kupffer cells Phagocytic cells (lymphatic cells) Functions Engulf pathogens, cell debris and damaged blood vessels Store iron, lipids, heavy metals absorbed by digestive tract

Figure 24-20

Bile Duct System Hepatocytes  bile  bile canaliculi  bile ductules  R and L hepatic ducts  common hepatic duct Common hepatic duct + cystic duct (to/from gallbladder)  common bile duct Common bile duct meets pancreatic duct at duodenal ampulla  duodenal papilla Hepatopancreatic sphincter - encircles Common bile duct, pancreatic duct, and duodenal papilla

Figure 24-21a, b