23 Overview of the Digestive System - Mission...

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© 2014 Pearson Education, Inc.

PowerPoint® Lecture Presentations prepared by Leslie Hendon University of Alabama, Birmingham

23 PART 1 The Digestive System


Overview of the Digestive System

•  Organs are divided into two groups •  Alimentary canal • Mouth, pharynx, and esophagus •  Stomach, small intestine, and large intestine

•  Accessory digestive organs •  Teeth and tongue • Gallbladder, salivary glands, liver, and pancreas

•  Accessory organs are connected to the alimentary canal by ducts •  Secretions contribute to breakdown of foodstuffs


Figure 23.1 The alimentary canal and the accessory digestive organs.

Mouth (oral cavity)

Tongue

Esophagus

Liver

Gallbladder

Small intestine

Anus

Duodenum Jejunum Ileum

Parotid gland Sublingual gland Submandibular gland

Salivary glands

Pharynx

Stomach

Pancreas

(Spleen)

Transverse colon

Descending colon

Ascending colon

Cecum

Sigmoid colon

Rectum

Appendix

Anal canal

Large intestine


Abdominal Regions

•  Four lines divide abdominal wall into nine regions •  Midclavicular lines—vertical lines of grid

•  Subcostal plane—superior horizontal line • Connects inferior points of costal margin

•  Transtubercular plane—inferior horizontal line • Connects tubercles of iliac crests


Figure 23.2a Divisions of the anterior abdominal wall.

Right hypochondriac

region

Epigastric region

Left hypochondriac

region

Right lumbar region

Umbilical region

Left lumbar region

Right iliac (inguinal)

region

Hypogastric (pubic) region

Left iliac (inguinal)

region

Nine regions delineated by four planes


Figure 23.2b Divisions of the anterior abdominal wall.

Liver

Gallbladder

Ascending colon of large intestine

Small intestine

Cecum

Appendix

Diaphragm

Stomach

Transverse colon of large intestine

Descending colon of large intestine

Initial part of sigmoid colon

Urinary bladder

Anterior view of the nine regions showing the superficial organs

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Abdominal Quadrants

•  A simpler method of sectioning the anterior abdominal wall •  Right upper quadrant

•  Left upper quadrant

•  Right lower quadrant

•  Left lower quadrant


Figure 23.3 Peritoneum and the Digestive System Organs (1 of 2)


Figure 23.3 Peritoneum and the Digestive System Organs (2 of 2)


The Peritoneal Cavity and Peritoneum

•  Peritoneum—a serous membrane •  Visceral peritoneum—surrounds digestive

organs

•  Parietal peritoneum—lines the body wall

•  Peritoneal cavity—a slitlike potential space



•  Mesentery—a double layer of peritoneum •  Holds organs in place

•  Sites of fat storage

•  Provides a route for circulatory vessels and nerves



•  Ventral mesenteries •  Falciform ligament •  Binds anterior part of liver to anterior abdominal

wall

•  Lesser omentum •  Attaches liver to the lesser curvature of stomach

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•  Dorsal mesenteries •  Greater omentum • Connects greater curvature of stomach to

posterior abdominal wall

•  A “fatty apron”

•  Transverse mesocolon • Holds transverse colon in place

•  Sigmoid mesocolon • Connects the sigmoid colon to posterior pelvic wall


Figure 23.4a The mesenteries.

Falciform ligament

Greater omentum

Liver

Gallbladder

Spleen

Stomach

Ligamentum teres

Small intestine

Cecum


Figure 23.4b The mesenteries.

Lesser omentum

Liver

Gallbladder

Stomach Duodenum

Transverse colon

Small intestine

Cecum

Urinary bladder


Figure 23.4c The mesenteries.

Greater omentum

Transverse mesocolon

Mesentery

Sigmoid mesocolon

Transverse colon

Descending colon

Jejunum Sigmoid colon Ileum


Figure 23.4d The mesenteries.

Gallbladder

Transverse mesocolon

Ascending colon

Sigmoid mesocolon

Duodenum, superior part

Duodenum, horizontal part

Liver

Ileum Cecum

Pancreas

Descending colon

Rectum

Mesentery (cut)

Greater omentum

Duodenojejunal flexure

Transverse colon Stomach, pyloric part

Gastrosplenic ligament

Spleen

Lesser omentum

Ligamentum teres



•  Retroperitoneal organs •  Behind the peritoneum

•  Peritoneal organs •  Digestive organs that keep their mesentery

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Secondarily Retroperitoneal Organs

•  Initially formed within peritoneum

•  Become retroperitoneal •  Fuse to posterior abdominal wall


Table 23.1 Summary of Intraperitoneal and Secondarily Retroperitoneal Digestive Organs in the Abdomen and Pelvis


Digestive Processes

•  Ingestion—occurs in the mouth

•  Propulsion—movement of food •  Peristalsis—major means of propulsion

•  Mechanical breakdown—prepares food for chemical digestion •  Chewing, churning food in stomach,

segmentation •  Segmentation is rhythmic local constrictions of

intestine


Digestive Processes

•  Chemical digestion—complex molecules broken down to chemical components •  Mouth

•  Stomach

•  Small intestine

•  Absorption—transport of digested nutrients

•  Defecation—elimination of indigestible substances as feces


Figure 23.5 Activities of the gastrointestinal tract.

Large intestine

Small intestine

Anus

Feces

Mainly H2O

Blood vessel

Lymph vessel

Stomach

Esophagus

Pharynx

Food

• Swallowing (oropharynx) • Peristalsis

(esophagus, stomach, small intestine, large intestine)

• Chewing (mouth) • Churning (stomach) • Segmentation

(small intestine)

Defecation

Digestion

Mechanical breakdown

Ingestion

Propulsion

Absorption


Peristalsis

•  Major means of propulsion

•  Adjacent segments of the alimentary canal relax and contract

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Figure 23.6a Peristalsis and segmentation. From mouth

Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, moving food along the tract distally.


Segmentation

•  Rhythmic local contractions of the intestine

•  Mixes food with digestive juices


Figure 23.6b Peristalsis and segmentation.

Segmentation: Nonadjacent segments of alimentary tract organs alternately contract and relax, moving the food forward then backward. Food is mixed and slowly propelled.


Histology of the Alimentary Canal Wall

•  Same four layers from esophagus to anus •  The mucosa—innermost layer • Consists of

• Epithelium

• Lamina propria

• Muscularis mucosae

•  The submucosa—external to the mucosa •  Contains blood and lymphatic vessels, nerve

fibers


Histology of the Alimentary Canal Wall

•  The muscularis externa—external to the submucosa •  Two layers • Circular muscularis—inner layer •  Longitudinal muscularis—outer layer

•  The serosa—the outermost layer •  Is the visceral peritoneum


Figure 23.7a Histological layers of the alimentary canal.

Myenteric nerve plexus

Mucosa

Submucosa

Muscularis externa

Serosa

Longitudinal and cross-sectional views through the small intestine

Glands in submucosa

Epithelium Lamina propria Muscularis mucosae

Circular layer Longitudinal layer

Epithelium Connective tissue

Lumen

Mucosa associated lymphoid tissue

Duct of gland outside alimentary canal

Gland in mucosa

Lymphatic vessel Vein

Artery Nerve

Mesentery

Intrinsic nerve plexuses

Submucosal nerve plexus

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Figure 23.7b Histological layers of the alimentary canal.

Light micrograph cross section through the small intestine (85×)

Serosa

Muscularis externa

Submucosa

Mucosa


Smooth Muscle

•  Primarily found in walls of viscera

•  Fibers elongated

•  Have one centrally located nucleus

•  Grouped into sheets •  Longitudinal layer—parallel to long axis of

organ

•  Circular layer—deeper layer, fibers run around circumference of organ


Figure 23.8 Arrangement of smooth muscle in the walls of hollow organs.

Location and plane of section shown in (b)

Cross section of the intestine showing the smooth muscle layers (one circular and the other longitudinal) running at right angles to each other

Circular layer of smooth muscle (shows longitudinal views of smooth muscle fibers, 215×)

Small intestine

Longitudinal layer of smooth muscle (shows smooth muscle fibers in cross section, 215×)

Mucosa


Smooth Muscle Contraction

•  Mechanism of contraction •  Myofilaments operate by interaction with

cytoskeleton

•  Dense bodies—correspond to Z-discs of skeletal muscle •  Sliding myofilaments shorten the muscle cell by

pulling on cytoskeleton

•  Entry of Ca2+ into sarcoplasm stimulates contraction


Smooth Muscle Contraction

•  Contraction is slow and sustained •  Takes 30x longer to contract and relax

•  Resistant to fatigue

•  Smooth muscle of arteries and visceral organs must sustain contraction over long periods

•  Energy requirements are low • Mitochondria are not abundant


Figure 23.9 Cytoskeletal elements involved in the contraction of smooth muscle.

Intermediate filament

Caveolae

Nucleus

Nucleus

Gap junctions

Dense bodies

Dense bodies

Relaxed smooth muscle fiber (note that gap junctions connect adjacent fibers)

Contracted smooth muscle fiber

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Innervation of Smooth Muscle

•  Innervated by ANS

•  Few fibers per sheet innervated •  Sheet of smooth muscle contracts as a unit • Called single-unit innervation

•  Multiunit innervation •  Each smooth muscle cell innervated

•  Iris of eye and arrector pili muscles


Figure 23.10 Innervation of smooth muscle.

Autonomic nerve fibers innervate most smooth muscle fibers.

Varicosities release their neurotransmitters into a wide synaptic cleft (a diffuse junction).

Varicosities

Smooth muscle cell

Synaptic vesicles

Mitochondrion


Nerve Plexuses

•  Myenteric nerve plexus •  Lies between circular and longitudinal muscularis •  Controls peristalsis and segmentation

•  Submucosal nerve plexus •  Lies in submucosa •  Signals glands to secrete

•  Innervation •  Sympathetic and parasympathetic motor fibers •  Visceral sensory fibers


The Mouth and Associated Organs

•  Mouth (oral cavity) •  Mucosa-lined cavity •  Boundaries are

•  Lips anteriorly

•  Cheeks laterally

•  Palate superiorly

•  Tongue inferiorly

•  Fauces of oropharynx posteriorly


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


Figure 23.11a Anatomy of the mouth.

Sagittal section of the oral cavity and pharynx

Trachea

Esophagus

Laryngopharynx

Hyoid bone

Epiglottis

Lingual tonsil

Oropharynx Tongue

Palatine tonsil

Oral cavity

Hard palate

Palatoglossal arch Soft palate

Uvula

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Anatomy of the Mouth

•  The labial frenulum •  Connects lips to gum

•  The palate •  Forms the roof of the mouth

•  Boundaries of the fauces •  Palatoglossal arches

•  Palatopharyngeal arches


Figure 23.11b Anatomy of the mouth.

Anterior view

Lower lip

Oral vestibule

Sublingual fold with openings of sublingual ducts

Palatine tonsil

Uvula

Soft palate

Hard palate

Palatine raphe

Gingivae (gums)

Inferior labial frenulum

Gingivae (gums)

Opening of submandicular duct

Lingual frenulum

Tongue

Posterior wall of oropharynx

Palatopharyngeal arch

Palatoglossal arch

Superior labial frenulum

Upper lip


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

•  Lingual frenulum •  Secures tongue to floor of mouth


The Superior Surface of the Tongue

•  Tongue papillae •  Filiform papillae—no taste buds

•  Fungiform papillae

•  Vallate papillae

•  Sulcus terminalis •  Marks border between mouth and pharynx

•  Posterior one-third of tongue lies in oropharynx •  Lined with lingual tonsil


Figure 23.12 The dorsal surface of the tongue. Epiglottis

Palatopharyngeal arch Palatine tonsil

Lingual tonsil

Palatoglossal arch

Terminal sulcus

Foliate papillae

Vallate papilla

Medial sulcus of the tongue

Dorsum of tongue

Fungiform papilla

Filiform papilla © 2014 Pearson Education, Inc.

The Teeth

•  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 •  Formula for adult dentition indicates number and

position of teeth •  2I, 1C, 2P, 3M

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Figure 23.13a Human dentition.

Third molar (wisdom tooth) (17–25 yr)

Permanent teeth

Deciduous (milk) teeth

Second molar (12–13 yr)

First molar (6–7 yr)

Second premolar (12–13 yr)

First premolar (11 yr)

Lateral (8 yr) Central (7 yr)

Second molar (about 2 yr)

First molar (10–15 mo)

Lateral (8–10 mo) Central (6–8 mo)

Canine (eyetooth) (16–20 mo)

Canine (eyetooth) (11 yr)

Incisors

Molars

Incisors

Premolars (bicuspids)

Molars


Figure 23.13b Human dentition.

Permanent teeth Deciduous teeth


Vessels and Nerves to the Teeth

•  Upper teeth •  Innervation is superior alveolar nerves

branching from maxillary division of CN V

•  Lower teeth •  Innervation is inferior alveolar nerves

branching from mandibular branch of CN V

•  Arterial supply to teeth •  Superior and inferior alveolar arteries (branching

from maxillary arteries)


Tooth Structure

•  Crown—exposed surface

•  Root—in tooth socket

•  Outer layer is enamel •  Dentin—underlies enamel

•  Pulp cavity—center of tooth


Figure 23.14 Longitudinal section of a canine tooth within its bony tooth socket.

Enamel

Dentin

Dentinal tubules

Pulp cavity (contains blood vessels and nerves)

Gingival sulcus

Gingiva (gum)

Cement

Root canal

Periodontal ligament

Apical foramen

Bone

Root

Neck

Crown


The Salivary Glands

•  Produce saliva •  Compound tubuloalveolar glands •  Parotid glands •  Parotid duct—parallel to zygomatic arch • Contain only serous cells

•  Submandibular glands •  Lie along medial surface of mandible

•  Sublingual glands •  Lie in floor of oral cavity • Contain primarily mucous cells

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Figure 23.15 The major salivary glands.

Serous cells forming demilunes

Mucous cells

Posterior belly of digastric muscle

Body of mandible (cut)

Masseter muscle

Anterior belly of digastric muscle

Mylohyoid muscle (cut)

Frenulum of tongue

Teeth

Tongue

Ducts of sublingual gland

Sublingual gland

Parotid gland

Parotid duct

Submandibular duct

Submandibular gland


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


The Esophagus

•  Gross anatomy—muscular tube •  Begins as a continuation of the pharynx

•  Joins the stomach inferior to the diaphragm • Cardiac sphincter—closes lumen to prevent

stomach acid from entering esophagus


The Esophagus

•  Microscopic anatomy •  Epithelium is stratified squamous epithelium

•  When empty, mucosa and submucosa are in longitudinal folds

•  Mucous glands—primarily compound tubuloalveolar glands


The Esophagus

•  Muscularis externa •  Skeletal muscle •  First one-third of length

•  Mixture of skeletal and smooth muscle • Middle one-third of length

•  Smooth muscle •  Inferior one-third of length

•  Adventitia •  Most external layer of esophagus


Figure 23.16 Microscopic structure of the esophagus.

Esophagus- stomach junction

Simple columnar epithelium of stomach

Lumen


Mucosa (stratified squamous epithelium)

Muscularis externa

Adventitia (fibrous connective tissue)

Submucosa (areolar connective tissue)

Cross section through esophagus (3×)

Esophagus-stomach junction, longitudinal section (85×)

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The Stomach

•  Site where food is churned into chyme •  Secretion of pepsin begins protein digestion •  Functions under acidic conditions

•  Food remains in stomach approximately 4 hours

•  Regions of the stomach •  Cardial part •  Fundus •  Body •  Pyloric part


Figure 23.17a Gross anatomy of the stomach.

Esophagus

Muscularis externa Longitudinal layer Circular layer Oblique layer

Lesser curvature

Cardia Fundus

Serosa

Body

Lumen

Rugae of mucosa

Greater curvature

Pyloric antrum

Pyloric canal

Pyloric sphincter (valve) at pylorus Duodenum


Figure 23.17b Gross anatomy of the stomach.

Liver (cut)

Lesser curvature

Fundus

Body

Spleen

Greater curvature


Microscopic Anatomy of the Stomach

•  Muscularis has three layers •  Circular and longitudinal layers and oblique layer

•  Epithelium is simple columnar epithelium

•  Mucosa dotted with gastric pits •  Gastric glands—deep to gastric pits


Microscopic Anatomy of the Stomach

•  Gastric glands of fundus and body •  Mucous neck cells •  Secrete a special mucus

•  Parietal (oxyntic) cells •  Secrete hydrochloric acid and gastric intrinsic

factor

•  Chief (zymogenic) cells •  Secrete pepsinogen

•  Pepsinogen is activated to pepsin when it encounters acid in the gastric glands


Figure 23.18 Microscopic anatomy of the stomach.

Surface epithelium

Lamina propria

Muscularis mucosae

Oblique layer


Stomach wall

Submucosa (contains submucosal plexus)

Muscularis externa (contains myenteric plexus)

Mucosa

Layers of the stomach wall, longitudinal section

Enlarged view of gastric pits and gastric glands

Enteroendocrine cell

Gastric gland

Gastric pit

Chief cell

Parietal cell

Mucous neck cells

Surface epithelium (mucous cells)

Gastric pits

Location of the HCl-producing parietal cells and pepsin-secreting chief cells in a gastric gland

Pepsinogen Pepsin HCl

Mitochondria

Parietal cell

Chief cell

Enteroendocrine cell

Micrograph of the stomach mucosa, view similar to part (b) (110×)

Muscularis mucosae

Enzyme-secreting chief cells

HCl-secreting parietal cells

Mucous neck cells

Surface mucous cell

Mucus-secreting cells

Gastric gland

Gastric pits

Serosa

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The Small Intestine—Gross Anatomy

•  Longest portion of the alimentary canal •  Site of most enzymatic digestion and

absorption •  Three subdivisions

•  Duodenum •  Jejunum •  Ileum

•  Innervation

•  Parasympathetic fibers from vagus nerve •  Sympathetic from thoracic splanchnic nerves


The Duodenum

•  Receives digestive enzymes and bile

•  Main pancreatic duct and common bile duct enter duodenum •  Sphincters control entry of bile and pancreatic

juices


Figure 23.19 The duodenum of the small intestine, and related organs.

Mucosa with folds

Gallbladder

Duodenum Head of pancreas

Main pancreatic duct and sphincter

Jejunum Pancreas

Tail of pancreas

Accessory pancreatic duct

Bile duct and sphincter

Cystic duct

Common hepatic duct

Right and left hepatic ducts of liver

Major duodenal papilla

Hepatopancreatic ampulla and sphincter


The Small Intestine—Microscopic Anatomy

•  Modifications for absorption •  Circular folds •  Transverse ridges of mucosa and submucosa

•  Villi •  Fingerlike projections of the mucosa

• Covered with simple columnar epithelium

•  Microvilli •  Further increase surface area for absorption


Histology of the Intestinal Wall

•  Absorptive enterocytes •  Uptake digested nutrients

•  Goblet cells •  Secrete mucus that lubricates chyme

•  Enteroendocrine cells •  Secrete hormones

•  Intestinal crypts •  Epithelial cells secrete intestinal juice


Figure 23.20 Structural modifications of the small intestine that increase the surface area for digestion and absorption.

Muscle layers

Circular folds

Villi

Lumen

Vein carrying blood to hepatic portal vessel

Microvilli (brush border)

Vilus

Absorptive enterocytes

Lacteal

Goblet cell

Blood capillaries

Mucosa associated lymphoid tissue Intestinal crypt

Muscularis mucosae

Duodenal gland

Submucosa

Lymphatic vessel

Venule

Enteroendocrine cells

Intestinal crypt

Absorptive enterocytes

Villi

Goblet cells

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