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C H A P T E R 23 The Digestive System

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Overview of the Digestive System • Organs are divided into two groups • The alimentary canal • Mouth, , and • , , and (colon) • Accessory digestive organs • Teeth and tongue • Gallbladder, salivary glands, liver, and pancreas

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The Alimentary Canal and Accessory Digestive Organs

Mouth (oral cavity) Parotid gland Tongue Sublingual gland Salivary glands Submandibular gland

Esophagus Pharynx

Stomach Pancreas (Spleen) Liver Gallbladder

Transverse colon Small intestine Large intestine Anus Vermiform

Figure 23.1

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1 Digestive Processes • Ingestion • Propulsion • Mechanical digestion • Chemical digestion • Absorption • Defecation

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Peristalsis

• Major means of propulsion • Adjacent segments of the alimentary canal relax and contract

Figure 23.3a 5

Segmentation

• Rhythmic local contractions of the intestine • Mixes food with digestive juices

Figure 23.3b 6

2 The Peritoneal Cavity and Peritoneum • Peritoneum – a • Visceral peritoneum – surrounds digestive organs • Parietal peritoneum – lines the body wall • Peritoneal cavity – a slit-like potential space

Falciform Anterior Visceral ligament peritoneum

Liver Peritoneal cavity (with serous fluid) Stomach Parietal peritoneum Kidney (retroperitoneal) Wall of Posterior body trunk

Figure 23.5 7

Mesenteries

• Lesser omentum attaches to lesser curvature of stomach

Liver

Gallbladder Lesser omentum

Stomach Duodenum

Transverse colon

Small intestine

Cecum

Urinary bladder (b) Figure 23.6b 8

Mesenteries

• Greater omentum— Greater omentum

a “fatty apron” of Transverse colon peritoneum Transverse • Greater omentum and mesocolon Descending colon transverse colon Jejunum reflected Mesentery Sigmoid mesocolon Sigmoid colon

Ileum

(c)

Figure 23.6c 9

3 Mesenteries

• Sagittal section through Liver the abdominopelvic Lesser omentum Pancreas cavity Stomach Duodenum Transverse • Mesenteries attach to mesocolon Transverse colon posterior abdominal wall Mesentery Greater omentum Jejunum Ileum Visceral peritoneum

Parietal peritoneum Urinary bladder Rectum

(d)

Figure 23.6d 10

Histology of the Alimentary Canal Wall • Same four layers from esophagus to anus • The mucosa – innermost layer • Consists of: • • • • The – external to the mucosa • Contains blood and lymphatic vessels, nerve fibers • The muscularis externa – external to the submucosa • Two layers • Circular muscularis • Longitudinal muscularis • The serosa – the outermost layer • The visceral peritoneum • Nerve plexuses • Myenteric nerve plexus • Submucosal nerve plexus 11

Histology of the Alimentary Canal

Intrinsic nerve plexuses Myenteric nerve plexus Submucosal nerve plexus

Glands in submucosa

Mucosa Epithelium Lamina propria Muscularis mucosae

Submucosa

Muscularis externa Longitudinal muscle Circular muscle Serosa Epithelium Connective tissue Nerve Gland in mucosa Lumen Duct of gland outside Mucosa-associated Mesentery Lymphatic vessel alimentary canal lymphoid tissue (a) Longitudinal and cross-sectional views through the small intestine Figure 23.7a 12

4 Histology of the Alimentary Canal

Mucosa

Submucosa

Muscularis externa

Serosa

(b) Light micrograph cross section through the small intestine (30×) Figure 23.7b 13

Smooth Muscle

Longitudinal layer of smooth muscle (shows smooth muscle fibers in cross section, 145×)

Small intestine Mucosa

(a) Location and plane of (b) Cross section of the intestine Circular layer of smooth section shown in (b) showing the smooth muscle muscle (shows longitudinal layers (one circular and the views of smooth muscle other longitudinal) running fibers, 145×) at right angles to each other

Figure 23.8a, b 14

The Mouth and Associated Organs • The mouth – oral cavity • Mucosal layer • Stratified squamous epithelium • Lamina propria • The lips and cheeks • Formed from orbicularis oris and buccinator muscles, respectively

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5 Anatomy of the Mouth Soft palate Palatoglossal arch Uvula

Hard palate Oral cavity Palatine tonsil Tongue Oropharynx Lingual tonsil Epiglottis Hyoid bone

Laryngopharynx

Esophagus

Trachea

(a) Sagittal section of the oral cavity and pharynx Figure 23.11a 16

Anatomy of the Mouth

• The labial frenulum • Connects lips to gum Upper lip • The palate Gingivae (gums) Superior labial frenulum • Forms the roof Palatine raphe Palatoglossal of the mouth Hard palate arch Soft palate Palatopharyngeal arch Uvula Palatine Posterior wall tonsil of oropharynx Tongue

Sublingual Lingual frenulum fold with Opening of openings of submandibular sublingual duct ducts Gingivae (gums) Vestibule Inferior labial Lower lip frenulum

(b) Anterior view Figure 23.11b 17

The Tongue • Interlacing fascicles of skeletal muscle • Grips food and repositions it • Helps form some consonants • Intrinsic muscles – within the tongue • Extrinsic muscles – external to the tongue

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6 The Superior Surface of the Tongue • Tongue papillae • Filiform papillae • Fungiform papillae • Vallate papillae

Figure 23.12 19

The Teeth • Dentition and dental formula • Deciduous teeth – 20 teeth • First appear at 6 months of age • Permanent teeth – 32 teeth • Most erupt by the end of adolescence • Dental formula – shorthand • Way to indicate number and position of teeth

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The Teeth

Incisors Central (7 yr) Lateral (8 yr) Canine (eyetooth) (11 yr) Premolars (bicuspids) First premolar (11 yr) Incisors Second premolar Central (6–8 mo) (12–13 yr) Lateral (8–10 mo) Molars Canine (eyetooth) First molar (16–20 mo) (6–7 yr) Molars Second molar First molar (12–13 yr) (10–15 mo) Deciduous Third molar Permanent Second molar (milk) teeth (wisdom tooth) teeth (about 2 yr) (17–25 yr) (a)

Figure 23.13a 21

7 The Teeth

Deciduous teeth Permanent teeth (b)

Figure 23.13b 22

Tooth Structure

Enamel Longitudinal Dentin • Crown Dentinal tubules

section of tooth in Pulp cavity (contains alveolus blood vessels Neck and nerves)

Gingiva (gum)

Cementum

Root canal Root Periodontal ligament

Apical foramen

Bone Figure 23.14 23

The Salivary Glands

Tongue

Teeth Parotid gland Ducts of sublingual Parotid duct gland Masseter muscle Frenulum of tongue Body of mandible (cut) Sublingual gland Posterior belly of digastric muscle Mylohyoid Submandibular muscle (cut) duct Anterior belly of Submandibular digastric muscle gland Mucous cells Serous cells forming (a) (b) demilunes

Figure 23.15 24

8 The Pharynx • Oropharynx and laryngopharynx – passages for air and food • Lined with stratified squamous epithelium • External muscle layer • Consists of superior, middle, and inferior pharyngeal constrictors

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The Esophagus • Gross anatomy – muscular tube • Begins as a continuation of the pharynx • Joins the stomach inferior to the diaphragm • Microscopic anatomy • Epithelium is stratified squamous epithelium • When empty – mucosa and submucosa in longitudinal folds • Mucous glands – primarily compound tubuloalveolar glands • Muscularis externa – skeletal muscle first third of length • Most external layer –

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Microscopic Structure of the Esophagus

Mucosa (contains a stratified squamous epithelium) Submucosa (areolar connective tissue) Lumen

Muscularis externa Circular layer Longitudinal layer

Adventitia (fibrous connective tissue) (a) Cross section through esophagus (5×) (b) Gastroesophageal junction, longitudinal section (85×) Figure 23.16 27

9 The Stomach • Site where food is churned into • Protein digestion begins • Secretes • Functions under acidic conditions

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The Stomach

Cardia Fundus

Esophagus

Muscularis externa Serosa Longitudinal layer Circular layer Oblique layer Body

Lumen Lesser curvature Rugae of mucosa

Greater curvature

Pyloric Pyloric Duodenum canal antrum Pyloric sphincter (a) (valve) at Figure 23.17a 29

The Stomach—Microscopic Anatomy

Gastric pits

Surface epithelium (mucous cells)

Surface Gastric epithelium pit Mucous neck cells

Mucosa Lamina Gastric propria gland Muscularis mucosae Submucosa (contains Chief cell submucosal plexus) Oblique layer

Muscularis externa Circular layer (contains myenteric Longitudinal plexus) layer Serosa Stomach wall (b) Enlarged view of and (a) Layers of the stomach wall, longitudinal section

Pepsinogen Pepsin -secreting HCl cells Gastric pits Surface mucous cell

Mitochondria Mucus neck cells

Parietal cell

HCl secreting parietal cells Gastric gland

Chief cell Enzyme secreting chief cells

Enteroendocrine cell Muscularis mucosae (c) Location of the HCl-producing parietal cells and (d) Micrograph of the stomach mucosa, view similar to part (b) (115×) pepsin-secreting chief cells in a gastric gland Figure 23.18 30

10 The Small Intestine – Gross Anatomy • Longest portion of the alimentary canal • Site of most enzymatic digestion and absorption • Three subdivisions • Duodenum • Brunner’s glands • Jejunum • Ileum • Peyer’s patches

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The Small Intestine—Structural Features

Vein carrying blood to hepatic portal vessel

Muscle layers Lumen Circular Microvilli folds (brush Absorptive border) Villi cells Lacteal Blood Absorptive cells capillaries Vilus (a) Mucosa Goblet associated cells lymphoid Villi tissue Intestinal Enteroendocrine crypt cells Muscularis Venule mucosae Lymphatic vessel Duodenal (b) gland Submucosa

(c) Intestinal crypt Figure 23.20 32

The Duodenum

• Receives digestive enzymes and bile • Main pancreatic duct and common bile duct enter duodenum • Sphincters control entry of bile and pancreatic juices

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11 The Duodenum and Related Organs

Right and left hepatic ducts of liver Common hepatic duct Cystic duct Bile duct and sphincter Accessory pancreatic duct

Mucosa with folds Tail of pancreas Pancreas Gallbladder Jejunum Main pancreatic duct and Hepatopancreatic sphincter ampulla and sphincter Duodenum Head of pancreas

Figure 23.19 34

The Large Intestine • Digested residue contains few nutrients • Small amount of digestion by bacteria • Main functions – absorb water and electrolytes • Mass peristaltic movements force feces toward the rectum

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Gross Anatomy of Large Intestine • Subdivided into: • Cecum, vermiform appendix, colon, rectum, anal canal • Special features of large intestine • Teniae coli, haustra, and epiploic appendages • Cecum – a blind pouch • Vermiform appendix – contains lymphoid tissue • Neutralizes pathogens • Colon – divided into distinct segments • Ascending, transverse, descending, and sigmoid colon • Rectum – descends along the inferior half of the sacrum • Anal Canal – the last subdivision of the large intestine

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12 Gross Anatomy of Large Intestine

Left colic (splenic) flexure Right colic Transverse (hepatic) flexure mesocolon Epiploic Transverse colon appendages

Superior mesenteric artery Descending colon Ascending colon IIeum Cut edge of mesentery IIeocecal valve Teniae coli

Sigmoid colon Cecum

Vermiform appendix Rectum

Anal canal (a)

Figure 23.21a 37

Gross Anatomy of Large Intestine

Rectal valve Rectum Hemorrhoidal Levator ani muscle

Anal canal

External anal sphincter

Internal anal sphincter

Pectinate line Anus (b) Figure 23.21b 38

Microscopic Anatomy of Large Intestine • Villi are absent • Contains numerous goblet cells • Intestinal crypts – simple tubular glands • Lined with simple columnar epithelial tissue • Epithelium changes at anal canal • Becomes stratified squamous epithelium

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13 The Liver • Largest gland in the body • Performs over 500 functions • Digestive function – bile production • Performs many metabolic functions

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Visceral Surface of the Liver

Hepatic portal vein Hepatic veins Inferior vena Caudate cava lobe

Bare area Left lobe

Ligamentum venosum in fissure Right hepatic Porta hepatis artery Right hepatic Left hepatic artery duct

Left hepatic duct Cystic duct Common hepatic duct Gallbladder Fissure Falciform ligament Right lobe Ligamentum teres Quadrate lobe (b)

Figure 23.25b 41

Microscopic Anatomy of Liver

(a)Lobule (b) Central vein Connective tissue septum Interlobular veins (to hepatic vein) Central vein

Sinusoids Bile canaliculi Plates of hepatocytes

Bile duct (receives bile from bile canaliculi) Fenestrated lining (endothelial cells) of sinusoids

Bile duct Portal venule Portal triad Hepatic Portal arteriole macrophages in sinusoid walls Portal vein

(c) Figure 23.26 42

14 The Gallbladder • Stores and concentrates bile • Expels bile into duodenum • Bile emulsifies fats • Cholecystokinin—released from enteroendocrine cells in response to fatty chyme

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The Gallbladder

Right and left hepatic ducts of liver Common hepatic duct Cystic duct Bile duct and sphincter Accessory pancreatic duct

Mucosa with folds Tail of pancreas Pancreas Gallbladder Jejunum Major duodenal papilla Main pancreatic duct and Hepatopancreatic sphincter ampulla and sphincter Duodenum Head of pancreas

Figure 23.19 44

The Pancreas • Pancreatic duct penetrates duodenal wall • Endocrine functions • Insulin (beta cells) and glucagons (alpha cells) • Exocrine functions – Acinar cells make, store, and secrete pancreatic enzymes • Majority of pancreatic secretions • Pancreatic juice secreted into small intestine • Carbohydrases • Lipases • Nucleases • Proteolytic enzymes

Figure 23.27 45

15 The Pancreas

Figure 23.27 46

Disorders of the Digestive System • Intestinal obstruction • Mechanical obstructions • Adhesions, tumors, or foreign objects • Nonmechanical obstruction • Halt in • Trauma • Intestines touched during surgery

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Disorders of the Digestive System • Inflammatory bowel disease • Inflammation of intestinal wall • Crohn’s disease • • Viral hepatitis – jaundice and flu-like symptoms • Major types – A, B, C, and G • Cystic Fibrosis and the Pancreas

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16 The Digestive System in Later Life • Middle age – gallstones and ulcers • Old age – activity of digestive organs decline • Fewer digestive juices and enzymes produced • Absorption is less efficient • Dehydration of fecal mass leads to constipation • Diverticulosis and cancer of digestive organs

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