The Digestive System - People Server at UNCW

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The Digestive System Part A

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Digestive System: Overview

ƒ The alimentary canal or gastrointestinal (GI) tract digests and absorbs food ƒ Alimentary canal – mouth, pharynx, esophagus, stomach, small intestine, and large intestine ƒ Accessory digestive organs – teeth, tongue, gallbladder, salivary glands, liver, and pancreas

Human Anatomy & Physiology, Sixth Edition Elaine N. Marieb Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Digestive System: Overview

Digestive Process

ƒ The GI tract is a “disassembly” line ƒ Nutrients become more available to the body in each step

ƒ There are six essential activities: ƒ Ingestion, propulsion, and mechanical digestion ƒ Chemical digestion, absorption, and defecation

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Figure 23.1

Digestive Process

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Gastrointestinal Tract Activities

ƒ Ingestion – taking food into the digestive tract ƒ Propulsion – swallowing and peristalsis ƒ Peristalsis – waves of contraction and relaxation of muscles in the organ walls

ƒ Mechanical digestion – chewing, mixing, and churning food

Figure 23.2 Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

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Peristalsis and Segmentation

Gastrointestinal Tract Activities

ƒ Chemical digestion – catabolic breakdown of food ƒ Absorption – movement of nutrients from the GI tract to the blood or lymph ƒ Defecation – elimination of indigestible solid wastes

Figure 23.3 Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

GI Tract

Receptors of the GI Tract

ƒ External environment for the digestive process

ƒ Mechano- and chemoreceptors respond to:

ƒ Regulation of digestion involves: ƒ Mechanical and chemical stimuli – stretch receptors, osmolarity, and presence of substrate in the lumen

ƒ Stretch, osmolarity, and pH ƒ Presence of substrate, and end products of digestion

ƒ They initiate reflexes that:

ƒ Extrinsic control by CNS centers

ƒ Activate or inhibit digestive glands

ƒ Intrinsic control by local centers

ƒ Mix lumen contents and move them along

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Nervous Control of the GI Tract

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Nervous Control of the GI Tract

ƒ Intrinsic controls ƒ Nerve plexuses near the GI tract initiate short reflexes ƒ Short reflexes are mediated by local enteric plexuses (gut brain)

ƒ Extrinsic controls ƒ Long reflexes arising within or outside the GI tract ƒ Involve CNS centers and extrinsic autonomic nerves Figure 23.4 Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

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Peritoneum and Peritoneal Cavity

Peritoneum and Peritoneal Cavity

ƒ Peritoneum – serous membrane of the abdominal cavity ƒ Visceral – covers external surface of most digestive organs ƒ Parietal – lines the body wall

ƒ Peritoneal cavity ƒ Lubricates digestive organs ƒ Allows them to slide across one another Figure 23.5a Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Peritoneum and Peritoneal Cavity

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Peritoneum and Peritoneal Cavity

ƒ Mesentery – double layer of peritoneum that provides: ƒ Vascular and nerve supplies to the viscera ƒ A means to hold digestive organs in place and store fat

ƒ Retroperitoneal organs – organs outside the peritoneum ƒ Peritoneal organs (intraperitoneal) – organs surrounded by peritoneum Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Blood Supply: Splanchnic Circulation

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Figure 23.5b

Histology of the Alimentary Canal

ƒ Arteries and the organs they serve include ƒ The hepatic, splenic, and left gastric: spleen, liver, and stomach ƒ Inferior and superior mesenteric: small and large intestines

ƒ Hepatic portal circulation: ƒ Collects nutrient-rich venous blood from the digestive viscera

ƒ From esophagus to the anal canal the walls of the GI tract have the same four tunics ƒ From the lumen outward they are the mucosa, submucosa, muscularis externa, and serosa

ƒ Each tunic has a predominant tissue type and a specific digestive function

ƒ Delivers this blood to the liver for metabolic processing and storage Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

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Histology of the Alimentary Canal

Mucosa

ƒ Moist epithelial layer that lines the lumen of the alimentary canal ƒ Its three major functions are: ƒ Secretion of mucus ƒ Absorption of the end products of digestion ƒ Protection against infectious disease

ƒ Consists of three layers: a lining epithelium, lamina propria, and muscularis mucosae Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Figure 23.6

Mucosa: Epithelial Lining

ƒ Consists of simple columnar epithelium and mucussecreting goblet cells ƒ The mucus secretions: ƒ Protect digestive organs from digesting themselves ƒ Ease food along the tract

ƒ Stomach and small intestine mucosa contain: ƒ Enzyme-secreting cells ƒ Hormone-secreting cells (making them endocrine and digestive organs) Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Mucosa: Other Sublayers

ƒ Submucosa – dense connective tissue containing elastic fibers, blood and lymphatic vessels, lymph nodes, and nerves ƒ Muscularis externa – responsible for segmentation and peristalsis ƒ Serosa – the protective visceral peritoneum

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Mucosa: Lamina Propria and Muscularis Mucosae

ƒ Lamina Propria ƒ Loose areolar and reticular connective tissue ƒ Nourishes the epithelium and absorbs nutrients ƒ Contains lymph nodes (part of MALT) important in defense against bacteria

ƒ Muscularis mucosae – smooth muscle cells that produce local movements of mucosa Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Enteric Nervous System

ƒ Composed of two major intrinsic nerve plexuses ƒ Submucosal nerve plexus – regulates glands and smooth muscle in the mucosa ƒ Myenteric nerve plexus – Major nerve supply that controls GI tract mobility

ƒ Replaced by the fibrous adventitia in the esophagus

ƒ Segmentation and peristalsis are largely automatic involving local reflex arcs

ƒ Retroperitoneal organs have both an adventitia and serosa

ƒ Linked to the CNS via long autonomic reflex arc

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

4

Mouth

Anatomy of the Oral Cavity: Mouth

ƒ Oral or buccal cavity: ƒ Is bounded by lips, cheeks, palate, and tongue ƒ Has the oral orifice as its anterior opening ƒ Is continuous with the oropharynx posteriorly

ƒ To withstand abrasions: ƒ The mouth is lined with stratified squamous epithelium ƒ The gums, hard palate, and dorsum of the tongue are slightly keratinized Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Lips and Cheeks

Figure 23.7a Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Oral Cavity and Pharynx: Anterior View

ƒ Have a core of skeletal muscles ƒ Lips: orbicularis oris ƒ Cheeks: buccinators

ƒ Vestibule – bounded by the lips and cheeks externally, and teeth and gums internally ƒ Oral cavity proper – area that lies within the teeth and gums ƒ Labial frenulum – median fold that joins the internal aspect of each lip to the gum Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Palate

ƒ Hard palate – underlain by palatine bones and palatine processes of the maxillae ƒ Assists the tongue in chewing ƒ Slightly corrugated on either side of the raphe (midline ridge)

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Figure 23.7b Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Palate

ƒ Soft palate – mobile fold formed mostly of skeletal muscle ƒ Closes off the nasopharynx during swallowing ƒ Uvula projects downward from its free edge

ƒ Palatoglossal and palatopharyngeal arches form the borders of the fauces

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5

Tongue

ƒ Occupies the floor of the mouth and fills the oral cavity when mouth is closed ƒ Functions include: ƒ Gripping and repositioning food during chewing ƒ Mixing food with saliva and forming the bolus

Tongue

ƒ Intrinsic muscles change the shape of the tongue ƒ Extrinsic muscles alter the tongue’s position ƒ Lingual frenulum secures the tongue to the floor of the mouth

ƒ Initiation of swallowing, and speech

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Tongue

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Tongue

ƒ Superior surface bears three types of papillae ƒ Filiform – give the tongue roughness and provide friction ƒ Fungiform – scattered widely over the tongue and give it a reddish hue ƒ Circumvallate – V-shaped row in back of tongue

ƒ Sulcus terminalis – groove that separates the tongue into two areas: ƒ Anterior 2/3 residing in the oral cavity ƒ Posterior third residing in the oropharynx Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Salivary Glands

ƒ Produce and secrete saliva that: ƒ Cleanses the mouth ƒ Moistens and dissolves food chemicals ƒ Aids in bolus formation ƒ Contains enzymes that break down starch

ƒ Three pairs of extrinsic glands – parotid, submandibular, and sublingual ƒ Intrinsic salivary glands (buccal glands) – scattered throughout the oral mucosa Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Figure 23.8 Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Salivary Glands

ƒ Parotid – lies anterior to the ear between the masseter muscle and skin ƒ Parotid duct – opens into the vestibule next to the second upper molar

ƒ Submandibular – lies along the medial aspect of the mandibular body ƒ Its ducts open at the base of the lingual frenulum

ƒ Sublingual – lies anterior to the submandibular gland under the tongue ƒ It opens via 10-12 ducts into the floor of the mouth Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

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Salivary Glands

Saliva: Source and Composition

ƒ Secreted from serous and mucous cells of salivary glands ƒ A 97-99.5% water, hypo-osmotic, slightly acidic solution containing ƒ Electrolytes – Na+, K+, Cl–, PO42–, HCO3– ƒ Digestive enzyme – salivary amylase ƒ Proteins – mucin, lysozyme, defensins, and IgA ƒ Metabolic wastes – urea and uric acid Figure 23.9a Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Control of Salivation

Teeth

ƒ Intrinsic glands keep the mouth moist

ƒ Primary and permanent dentitions have formed by age 21

ƒ Extrinsic salivary glands secrete serous, enzymerich saliva in response to:

ƒ Primary – 20 deciduous teeth that erupt at intervals between 6 and 24 months

ƒ Ingested food which stimulates chemoreceptors and pressoreceptors ƒ The thought of food

ƒ Strong sympathetic stimulation inhibits salivation and results in dry mouth Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

ƒ Permanent – enlarge and develop causing the root of deciduous teeth to be resorbed and fall out between the ages of 6 and 12 years ƒ All but the third molars have erupted by the end of adolescence ƒ There are usually 32 permanent teeth Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Deciduous Teeth

Permanent Teeth

Figure 23.10.1 Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Figure 23.10.2 Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

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Classification of Teeth

Dental Formula: Permanent Teeth

ƒ Teeth are classified according to their shape and function

ƒ A shorthand way of indicating the number and relative position of teeth

ƒ Incisors – chisel-shaped teeth adapted for cutting or nipping

ƒ Written as ratio of upper to lower teeth for the mouth

ƒ Canines – conical or fanglike teeth that tear or pierce

ƒ Primary: 2I (incisors), 1C (canine), 2M (molars)

ƒ Premolars (bicuspids) and molars – have broad crowns with rounded tips and are best suited for grinding or crushing

ƒ During chewing, upper and lower molars lock together generating crushing force Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Tooth Structure

ƒ Permanent: 2I, 1C, 2PM (premolars), 3M 2I

1C

2PM

3M

2I

1C

2PM

3M

X

2 (32 teeth)

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Tooth Structure

ƒ Two main regions – crown and the root ƒ Crown – exposed part of the tooth above the gingiva (gum)

ƒ Neck – constriction where the crown and root come together

ƒ Enamel – acellular, brittle material composed of calcium salts and hydroxyapatite crystals is the hardest substance in the body

ƒ Cementum – calcified connective tissue

ƒ Encapsules the crown of the tooth

ƒ Covers the root ƒ Attaches it to the periodontal ligament

ƒ Root – portion of the tooth embedded in the jawbone Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Tooth Structure

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Tooth Structure

ƒ Dentin – bonelike material deep to the enamel cap that forms the bulk of the tooth ƒ Periodontal ligament ƒ Anchors the tooth in the alveolus of the jaw ƒ Forms the fibrous joint called a gomaphosis

ƒ Gingival sulcus – depression where the gingiva borders the tooth

ƒ Pulp cavity – cavity surrounded by dentin that contains pulp ƒ Pulp – connective tissue, blood vessels, and nerves ƒ Root canal – portion of the pulp cavity that extends into the root ƒ Apical foramen – proximal opening to the root canal ƒ Odontoblasts – secrete and maintain dentin throughout life

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

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Tooth Structure

Tooth and Gum Disease

ƒ Dental caries – gradual demineralization of enamel and dentin by bacterial action ƒ Dental plaque, a film of sugar, bacteria, and mouth debris, adheres to teeth ƒ Acid produced by the bacteria in the plaque dissolves calcium salts ƒ Without these salts, organic matter is digested by proteolytic enzymes

Figure 23.11 Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

ƒ Daily flossing and brushing help prevent caries by removing forming plaque Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Tooth and Gum Disease: Periodontitis

ƒ Gingivitis – as plaque accumulates, it calcifies and forms calculus, or tartar ƒ Accumulation of calculus: ƒ Disrupts the seal between the gingivae and the teeth ƒ Puts the gums at risk for infection

ƒ Periodontitis – serious gum disease resulting from an immune response ƒ Immune system attacks intruders as well as body tissues, carving pockets around the teeth and dissolving bone Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

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