The digestive system
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The Digestive System. Chapter 22. Introduction. A healthy digestive system is essential to the maintenance of life – homeostasis

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Introduction l.jpg
Introduction

  • A healthy digestive system is essential to the maintenance of life – homeostasis

  • Includes the structures of the body that take in food, break it into nutrient molecules, absorb these molecules into the circulatory system, and then eliminate the indigestible wastes


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Overview of the Digestive System

  • Various organs are divided into 2 main groups: alimentary canal and accessory digestive organs

  • Alimentary (aliment = nourishment) canal or gastrointestinal (GI) tract – the muscular digestive tube that winds through the body

    - mouth, pharynx, and esophagus

    - stomach, small intestine, and large intestine (end leads to the terminal opening or anus)

    - in a cadaver its about 9m (30 feet)

    - food material in the alimentary canal is outside the body (canal is open to the external environment at both ends)


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Accessory Digestive Organs

  • Include the teeth and tongue, the gallbladder and various large digestive glands – salivary glands, liver, and pancreas

    - lie external to and are connected to the alimentary canal by ducts

    - accessory digestive glands secrete saliva, bile, and digestive enzymes (all help breakdown foodstuffs)



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Digestive Processes

Perform 6 essential food-processing activities:

  • Ingestion – taking of food into the mouth

    2. Propulsion – movement of food through the alimentary canal

    - voluntary swallowing, involuntary peristalsis (‘around contraction’) involves alternate waves of contraction and relaxation of musculature in the organ walls

  • Mechanical digestion – prepares food for chemical digestion

    - chewing, churning of food in stomach and segmentation:

    - rhythmic local constrictions of the intestine mix food with digestive juices, increase efficiency of nutrient absorption


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  • Chemical digestion – complex molecules are broken down to chemical components

    - carried out by enzymes secreted by digestive glands into the lumen of the alimentary canal

    - mouth, stomach, small intestine

  • Absorption – transport of digested end products from the lumen into the blood and lymphatic capillaries

  • Defecation – elimination of indigestible substances as feces


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  • Schematic summary of digestive processes chemical components

  • Sites at which the 6 steps occur are indicated

  • The mucosa of almost the entire alimentry canal secretes mucus

    - protects the canal and lubricates its contents

Figure 22.2


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Peristalsis chemical components

  • Major means of propulsion

  • Adjacent segments of the alimentary canal alternately contract and relax, moving food distally along the canal

Figure 22.3a


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Segmentation chemical components

  • Non adjacent areas of the intestine alternately contract and relax

  • The active regions are separated by inactive regions, segmentation moves food onward and then backward to mix the food with digestiv juices

Figure 22.3b


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Abdominal Regions chemical components

4 grid lines divide abdominal wall into 9 regions:

  • Midclavicular lines – vertical lines of grid

  • Transtubercular plane – inferior horizontal line from the clavicle midpoint

    - connects tubercles of iliac crests

  • Subcostal plane - superior line that connects inferior points of the costal margins


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Figure 22.4a chemical components


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Figure 22.4b


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Figure 22.4c


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Histology of the chemical componentsAlimentary Canal Wall

Same 4 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

  • The muscularis externa – external to submucosa

    - 2 layers: circular muscularis (inner) and longitudinal muscularis (outer)

    - in some areas circular layer thickens to form sphinctrers

  • The serosa – outermost layer

    - is the visceral peritoneum


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Figure 22.5 chemical components


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Smooth Muscle chemical components

  • Primarily found in walls of viscera

  • Fibers elongated – have one centrally located nucleus

  • Grouped into sheets:

    - Longitudinal layer is parallel to the long axis of the organ

    - Circular layer, deeper layer, with fibers that run around the circumference of the organ


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Smooth Muscle chemical components

Figure 22.6a, b


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Smooth Muscle Contraction chemical components

  • Myofilaments operate by interaction with cytoskeleton

  • Dense bodies – correspond to Z-discs of skeletal muscle

  • Entry of CA2+ ions into the sacroplasm initiates contraction - caveolae, tiny infoldings

  • Contraction, slow, sustained, and resistant to fatigue (30X longer to contract and relax)

Figure 22.7a–c


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Innervation of Smooth Muscle chemical components

  • Innervated by ANS – only a few smooth muscle fibers in each sheet

    - impulse spreads thorugh gap junctions between adjacent fibers

    - contraction stimulated also by stretching of muscle fibers and hormones

    - single-unit innervation, whole sheet contracts as a single unit

    - multiunit innervation exception: iris of the eye and arrector pili muscles of the skin


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Innervation of Smooth Muscle chemical components

Figure 22.8

  • ANS fibers release their neurotransmitters from varicosities (row of knots on the motor axon) into a wide synaptic cleft


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Nerve Plexuses chemical components

  • Myenteric nerve plexus – lies between circular and longtiudinal muscularis

    - controls peristalsis and segmentation

  • Submucosal nerve plexus – lies in submucosa

    - signals glands to secrete

  • Innervation – botyh plexuses contain sympathetic and parasympathetic motor fibers

    - visceral sensory fibers


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The Peritoneal Cavity chemical componentsand Peritoneum

  • Peritoneum – a serous membrane

    - visceral peritoneum surrounds the digestive organs

    - parietal peritoneium lines the body wall

  • Peritoneal cavity – a slit-like potential space


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The Peritoneal Cavity and Peritoneum chemical components

  • Mesentery – double layer of peritoneum

    • Holds organs in place

    • Sites of fat storage

    • Provides a route for circulatory vessels and nerves

Figure 22.9a


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Peritoneum chemical components

  • Retroperitoneal organs

    • Behind the peritoneum

  • Peritoneal organs

    • Digestive organs that keep their mesentery

Figure 22.9b


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Mesenteries chemical components

Figure 22.10a


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  • Lesser omentum chemical components attaches to lesser curvature of stomach

Figure 22.10b


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Figure 22.10c


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Figure 22.10d


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Secondarily Retroperitoneal Organs chemical components

  • Initially formed within peritoneum

  • Become retroperitoneal - Fuse to posterior abdominal wall

Figure 22.11


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Table 22.1 chemical components


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The Mouth and Associated Organs chemical components

  • The mouth – oral cavity

    - mucosal layer of stratified squamous epithelium and a lamina propria

  • Lips and cheeks – formed from orbicularis oris and buccinator muscles (respectively)


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Oral cavity and pharynx chemical components

Figure 22.12a


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  • The chemical componentslabial frenulum - connects lips to gum

  • The palate - forms the roof of the mouth

Figure 22.12b


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The Tongue chemical components

  • Interlacing fascicles of skeletal muscle

  • Grips food and repositions it

  • Helps form some consonants

  • Intrinsic muscles – within the tongue

  • Extrinsic muscle – external to the tongue

  • Lingual frenulum – secures tongue to floor of month


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  • Tongue papillae chemical components

    - filiform papillae has no taste buds

    - fungiform papillae – taste buds on the tops

    - circumvallate papillae – marks border between mouth and pharynx; posterior /3 of tongue lies in oropharynx is lined with lingual tonsil


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Figure 22.13 chemical components


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The Teeth chemical components

  • 20 Deciduous teeth – first appear at 5 months of age

  • 32 Permanent teeth – most erupt by the end of adolescence

  • Dental formula – shorthand to indicate numbers and positions of the different classes of teeth

    2I, 1C, 2P, 3M X 2

    2I, 1C, 2P, 3M


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Figure 22.14 chemical components


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Tooth Structure chemical components

  • Longitudinal section of tooth in alveolus

Figure 22.15


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The Salivary Glands chemical components

  • Produce saliva – complex mixture of water, ions, mucus, and enzymes

    - moistens the mouth, dissolves food chemicals, wets food, binds food into a bolus, enzymes begin the digestion of carbohydrates

  • Compound tubuloalveolar glands

    - parotid, largest, lies anterior to the ear with a parotid duct that runs parallel to the zygomatic arch

    - contains only serous cells


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The Salivary Glands mandible

Figure 22.16


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The Pharynx glands with mostly mucous cells, a few serous cells

  • Oropharynx and laryngopharynx

    - passageways for food, fluids, and inhaled air

    - lined with stratified squamous epithelium

    - external muscle layer consists of superior, middle, and inferior pharyngeal constrictors


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The Esophagus glands with mostly mucous cells, a few serous cells

  • Gross anatomy – muscular tube that propels swallowed food to the stomach

    - 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


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  • Microscopic anatomy glands with mostly mucous cells, a few serous cells

    - epithelium is stratified squamous

    - when empty mucosa and submucosa are thrown into longitundinal folds

    - mucous glands are primarily compound tubuloalveolar glands

    - muscularis externa, skeletal muscle (first 1/3 of length)

    - adventitia, most external layer


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Esophagus glands with mostly mucous cells, a few serous cells

Figure 22.17a, b


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The Stomach glands with mostly mucous cells, a few serous cells

  • J-shaped stomach is the widest part of the alimentary canal

    - temporary storage tank where food is churned into chyme

    - breakdown of proteins by secreting pepsin, a protein-digesting enzyme, functions only under acidic condition and hydrochloric acid (destroys harmful bacteria)

    - while most nutrients are absorbed in the small intestine, some are absorbed through the stomach (water, electrolytes, aspirin and alcohol)

    - food remains for roughly 4 hours


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Figure 22.18a glands with mostly mucous cells, a few serous cells


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Figure 22.18b glands with mostly mucous cells, a few serous cells


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Microscopic Anatomy glands with mostly mucous cells, a few serous cellsof the Stomach

  • Muscularis has 3 layers – circular and longitudinal layers and an oblique layer

  • Epithelium – simple columnar

  • Mucosa dotted with millions of cup-shaped gastric pits

    - open into tubular gastric glands


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  • Gastric glands of fundus and body glands with mostly mucous cells, a few serous cells

    - mucous neck clls occur in the upper ends or neck of the gastric glands

    - secrete a special mucus

  • Parietal (oxyntic) cells – occur mainly in the middle regions of the glands

    - secrete hydrochloric acid and gastric intrinsic factor (for absorption of B12 by the small intestine)

  • Chief (zymogenic) cells occur mainly in the basal parts of the glands

    - secretes pepsinogen, which is activated to pepsin when it encounters acid


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Figure 22.19a–d glands with mostly mucous cells, a few serous cells


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The Small Intestine glands with mostly mucous cells, a few serous cells– Gross Anatomy

  • Longest portion of the alimentary canal

  • Site of most enzymatic digestion and absorption

  • 3 subdivisions

    - Duodenum

    - Jejunum

    - Ileum


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The Duodeum glands with mostly mucous cells, a few serous cells

  • Receives digestive enzymes and bile

  • Main pancreatic duct and common bile duct enters the duodenum


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The Duodenum and Related Organs glands with mostly mucous cells, a few serous cells

Figure 22.20


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The Small Intestine glands with mostly mucous cells, a few serous cells

  • Modifications for absorption

    - circular folds (plicae circulares)

    - transverse ridges of mucos and submucosa

    - villi, finger like projections of the mucosa, covered with simple columnar epithelium

    - microvilli, further increases surface area for absorption


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Histology of the Intestinal Wall glands with mostly mucous cells, a few serous cells

  • Absorptive cells – uptake digested nutrients

  • Goblet cells – secrete mucus that lubricates chyme

  • Enteroendocrine cells – secrete hormones

  • Intestinal crypts – epithelial cells that secrete intestinal juice


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Figure 22.21a–d glands with mostly mucous cells, a few serous cells


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Gross Anatomy - Large Intestine glands with mostly mucous cells, a few serous cells

Figure 22.22a


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Figure 22.22b glands with mostly mucous cells, a few serous cells


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Defecation Reflex glands with mostly mucous cells, a few serous cells

Figure 22.23


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Mucosa of the Large Intestine glands with mostly mucous cells, a few serous cells

Figure 22.24


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Visceral Surface of the Liver glands with mostly mucous cells, a few serous cells

Figure 22.26


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Microscopic Anatomy of Liver glands with mostly mucous cells, a few serous cells

Figure 22.27a, c, d


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The Exocrine Pancreas: Histology glands with mostly mucous cells, a few serous cells

Figure 22.29a, b


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Embryonic Development glands with mostly mucous cells, a few serous cellsof the Digestive System

Figure 22.30a, b


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The Gallbladder glands with mostly mucous cells, a few serous cells

Figure 22.20


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