Digestive System: OverviewDigestive System: Overview

The alimentary canal or gastrointestinal (GI) The alimentary canal or gastrointestinal (GI) tract digests and absorbs foodtract digests and absorbs food

Alimentary canal – mouth, pharynx, Alimentary canal – mouth, pharynx, esophagus, stomach, small intestine, and large esophagus, stomach, small intestine, and large intestineintestine

Accessory digestive organs – teeth, tongue, Accessory digestive organs – teeth, tongue, gallbladder, salivary glands, liver, and gallbladder, salivary glands, liver, and pancreaspancreas

Digestive ProcessDigestive Process

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

each stepeach step There are six essential activities: There are six essential activities:

Ingestion, propulsion, and mechanical digestion Ingestion, propulsion, and mechanical digestion Chemical digestion, absorption, and defecationChemical digestion, absorption, and defecation

Gastrointestinal Tract ActivitiesGastrointestinal Tract Activities

Ingestion – taking food into the digestive tract Ingestion – taking food into the digestive tract Propulsion – swallowing and peristalsisPropulsion – swallowing and peristalsis

Peristalsis – waves of contraction and relaxation of Peristalsis – waves of contraction and relaxation of muscles in the organ wallsmuscles in the organ walls

Mechanical digestion – chewing, mixing, and Mechanical digestion – chewing, mixing, and churning food churning food

Peristalsis and SegmentationPeristalsis and Segmentation

Figure 23.3

Gastrointestinal Tract ActivitiesGastrointestinal Tract Activities

Chemical digestion – catabolic breakdown of Chemical digestion – catabolic breakdown of foodfood

Absorption – movement of nutrients from the Absorption – movement of nutrients from the GI tract to the blood or lymphGI tract to the blood or lymph

Defecation – elimination of indigestible solid Defecation – elimination of indigestible solid wasteswastes

GI TractGI Tract

External environment for the digestive processExternal environment for the digestive process Regulation of digestion involves:Regulation of digestion involves:

Mechanical and chemical stimuli – stretch Mechanical and chemical stimuli – stretch receptors, osmolarity, and presence of substrate in receptors, osmolarity, and presence of substrate in the lumenthe lumen

Extrinsic control by CNS centersExtrinsic control by CNS centers Intrinsic control by local centersIntrinsic control by local centers

Receptors of the GI TractReceptors of the GI Tract

Mechano- and chemoreceptors respond to:Mechano- and chemoreceptors respond to: Stretch, osmolarity, and pHStretch, osmolarity, and pH Presence of substrate, and end products of Presence of substrate, and end products of

digestiondigestion They initiate reflexes that:They initiate reflexes that:

Activate or inhibit digestive glands Activate or inhibit digestive glands Mix lumen contents and move them alongMix lumen contents and move them along

Nervous Control of the GI TractNervous Control of the GI Tract

Intrinsic controlsIntrinsic controls Nerve plexuses near the GI tract initiate short Nerve plexuses near the GI tract initiate short

reflexesreflexes Short reflexes are mediated by local enteric Short reflexes are mediated by local enteric

plexuses (gut brain)plexuses (gut brain) Extrinsic controlsExtrinsic controls

Long reflexes arising within or outside the GI tract Long reflexes arising within or outside the GI tract CNS centers and extrinsic autonomic nervesCNS centers and extrinsic autonomic nerves

Peritoneum and Peritoneal Peritoneum and Peritoneal CavityCavity

Peritoneum – serous membrane of the Peritoneum – serous membrane of the abdominal cavityabdominal cavity Visceral – covers external surface of most Visceral – covers external surface of most

digestive organsdigestive organs Parietal – lines the body wallParietal – lines the body wall

Peritoneal cavityPeritoneal cavity Lubricates digestive organs Lubricates digestive organs Allows them to slide across one anotherAllows them to slide across one another

Peritoneum and Peritoneal Peritoneum and Peritoneal CavityCavity

Mesentery – double layer of peritoneum that Mesentery – double layer of peritoneum that provides:provides: Vascular and nerve supplies to the visceraVascular and nerve supplies to the viscera Hold digestive organs in place and store fatHold digestive organs in place and store fat

Retroperitoneal organs – organs outside the Retroperitoneal organs – organs outside the peritoneumperitoneum

Peritoneal organs (intraperitoneal) – organs Peritoneal organs (intraperitoneal) – organs surrounded by peritoneumsurrounded by peritoneum

Blood Supply: Splanchnic Blood Supply: Splanchnic CirculationCirculation

Splanchnic- pertaining to the digestive visceraSplanchnic- pertaining to the digestive viscera Arteries and the organs they serve includeArteries and the organs they serve include

The hepatic, splenic, and left gastric: spleen, liver, The hepatic, splenic, and left gastric: spleen, liver, and stomachand stomach

Inferior and superior mesenteric: small and large Inferior and superior mesenteric: small and large intestines intestines

Blood Supply: Splanchnic Blood Supply: Splanchnic CirculationCirculation

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

digestive visceradigestive viscera Delivers this blood to the liver for metabolic Delivers this blood to the liver for metabolic

processing and storageprocessing and storage

Histology of the Alimentary Histology of the Alimentary CanalCanal

From esophagus to the anal canal the walls of From esophagus to the anal canal the walls of the GI tract have the same four tunicsthe GI tract have the same four tunics From the lumen outward they are the mucosa, From the lumen outward they are the mucosa,

submucosa, muscularis externa, and serosasubmucosa, muscularis externa, and serosa Each tunic has a predominant tissue type and a Each tunic has a predominant tissue type and a

specific digestive functionspecific digestive function

MucosaMucosa

Moist epithelial layer that lines the lumen of Moist epithelial layer that lines the lumen of the alimentary canalthe alimentary canal

Three major functions:Three major functions: Secretion of mucusSecretion of mucus Absorption of end products of digestionAbsorption of end products of digestion Protection against infectious diseaseProtection against infectious disease

Consists of three layers: a lining epithelium, Consists of three layers: a lining epithelium, lamina propria, and muscularis mucosaelamina propria, and muscularis mucosae

Mucosa: Epithelial LiningMucosa: Epithelial Lining

Simple columnar epithelium and mucus-Simple columnar epithelium and mucus-secreting goblet cellssecreting goblet cells

Mucus secretions:Mucus secretions: Protect digestive organs from digesting themselvesProtect digestive organs from digesting themselves Ease food along the tractEase food along the tract

Stomach and small intestine mucosa contain:Stomach and small intestine mucosa contain: Enzyme-secreting cells Enzyme-secreting cells Hormone-secreting cells (making them endocrine Hormone-secreting cells (making them endocrine

and digestive organs)and digestive organs)

Mucosa: Lamina Propria and Mucosa: Lamina Propria and Muscularis MucosaeMuscularis Mucosae

Lamina PropriaLamina Propria Loose areolar and reticular connective tissueLoose areolar and reticular connective tissue Nourishes the epithelium and absorbs nutrientsNourishes the epithelium and absorbs nutrients Contains lymph nodes (part of MALT) important Contains lymph nodes (part of MALT) important

in defense against bacteriain defense against bacteria Muscularis mucosae – smooth muscle cells Muscularis mucosae – smooth muscle cells

that produce local movements of mucosathat produce local movements of mucosa

Mucosa: Other SublayersMucosa: Other Sublayers

Submucosa – dense connective tissue Submucosa – dense connective tissue containing elastic fibers, blood and lymphatic containing elastic fibers, blood and lymphatic vessels, lymph nodes, and nervesvessels, lymph nodes, and nerves

Muscularis externa – responsible for Muscularis externa – responsible for segmentation and peristalsissegmentation and peristalsis

Serosa – the protective visceral peritoneumSerosa – the protective visceral peritoneum Replaced by the fibrous adventitia in the Replaced by the fibrous adventitia in the

esophagus esophagus Retroperitoneal organs have both an adventitia and Retroperitoneal organs have both an adventitia and

serosaserosa

Enteric Nervous SystemEnteric Nervous System

Enteric- pertaining to the intestinesEnteric- pertaining to the intestines Composed of two major intrinsic nerve plexuses:Composed of two major intrinsic nerve plexuses:

Submucosal nerve plexus – regulates glands and smooth Submucosal nerve plexus – regulates glands and smooth muscle in the mucosamuscle in the mucosa

Myenteric nerve plexus – Major nerve supply that controls Myenteric nerve plexus – Major nerve supply that controls GI tract mobilityGI tract mobility

Segmentation and peristalsis are largely automatic Segmentation and peristalsis are largely automatic involving local reflex arcsinvolving local reflex arcs

Linked to the CNS via long autonomic reflex arcLinked to the CNS via long autonomic reflex arc

MouthMouth

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

MouthMouth

To withstand abrasions: To withstand abrasions: The mouth is lined with stratified squamous The mouth is lined with stratified squamous

epithelium epithelium The gums, hard palate, and dorsum of the tongue The gums, hard palate, and dorsum of the tongue

are slightly keratinizedare slightly keratinized

Lips and CheeksLips and Cheeks Have a core of skeletal musclesHave a core of skeletal muscles

Lips: orbicularis oris Lips: orbicularis oris Cheeks: buccinatorsCheeks: buccinators

Vestibule – bounded by the lips and cheeks Vestibule – bounded by the lips and cheeks externally, and teeth and gums internallyexternally, and teeth and gums internally

Oral cavity proper – area that lies within the Oral cavity proper – area that lies within the teeth and gumsteeth and gums

Labial frenulum – median fold that joins the Labial frenulum – median fold that joins the internal aspect of each lip to the guminternal aspect of each lip to the gum

PalatePalate

Hard palate – underlain by palatine bones and Hard palate – underlain by palatine bones and palatine processes of the maxillaepalatine processes of the maxillae Assists the tongue in chewingAssists the tongue in chewing Slightly corrugated on either side of the raphe Slightly corrugated on either side of the raphe

(midline ridge)(midline ridge)

PalatePalate

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

Palatoglossal and palatopharyngeal arches Palatoglossal and palatopharyngeal arches form the bordersform the borders

TongueTongue

Occupies the floor of the mouth and fills the Occupies the floor of the mouth and fills the oral cavity when mouth is closedoral cavity when mouth is closed

Functions include:Functions include: Gripping and repositioning food during chewingGripping and repositioning food during chewing Mixing food with saliva and forming the bolusMixing food with saliva and forming the bolus Initiation of swallowing, and speechInitiation of swallowing, and speech

TongueTongue

Intrinsic muscles change the shape of the Intrinsic muscles change the shape of the tonguetongue

Extrinsic muscles alter the tongue’s positionExtrinsic muscles alter the tongue’s position Lingual frenulum secures the tongue to the Lingual frenulum secures the tongue to the

floor of the mouthfloor of the mouth

TongueTongue

Superior surface bears three types of papillaeSuperior surface bears three types of papillae Filiform – give the tongue roughness and provide Filiform – give the tongue roughness and provide

friction friction Fungiform – scattered widely over the tongue and Fungiform – scattered widely over the tongue and

give it a reddish huegive it a reddish hue Circumvallate – V-shaped row in back of tongueCircumvallate – V-shaped row in back of tongue

TongueTongue

Sulcus terminalis – groove that separates the Sulcus terminalis – groove that separates the tongue into two areas:tongue into two areas: Anterior 2/3 residing in the oral cavityAnterior 2/3 residing in the oral cavity Posterior third residing in the oropharynxPosterior third residing in the oropharynx

TongueTongue

Figure 23.8

Salivary GlandsSalivary Glands

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

Salivary GlandsSalivary Glands

Three pairs of extrinsic glands – parotid, Three pairs of extrinsic glands – parotid, submandibular, and sublingualsubmandibular, and sublingual

Intrinsic salivary glands (buccal glands) – Intrinsic salivary glands (buccal glands) – scattered throughout the oral mucosascattered throughout the oral mucosa

Salivary GlandsSalivary Glands

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

upper molarupper molar Submandibular – lies along the medial aspect Submandibular – lies along the medial aspect

of the mandibular bodyof the mandibular body Its ducts open at the base of the lingual frenulumIts ducts open at the base of the lingual frenulum

Sublingual – lies anterior to the submandibular Sublingual – lies anterior to the submandibular gland under the tonguegland under the tongue It opens via 10-12 ducts into the floor of the mouthIt opens via 10-12 ducts into the floor of the mouth

Salivary GlandsSalivary Glands

Figure 23.9a

Saliva: Source and CompositionSaliva: Source and Composition

Secreted from serous and mucous cells of Secreted from serous and mucous cells of salivary glandssalivary glands

97-99.5% water, hypo-osmotic, slightly acidic 97-99.5% water, hypo-osmotic, slightly acidic solution containingsolution containing Electrolytes – NaElectrolytes – Na++, K, K++, Cl, Cl––, PO, PO44

2–2–, HCO, HCO33––

Digestive enzyme – salivary amylaseDigestive enzyme – salivary amylase Proteins – mucin, lysozyme, defensins, and IgAProteins – mucin, lysozyme, defensins, and IgA Metabolic wastes – urea and uric acidMetabolic wastes – urea and uric acid

Control of SalivationControl of Salivation

Intrinsic glands keep the mouth moistIntrinsic glands keep the mouth moist Extrinsic salivary glands secrete serous, Extrinsic salivary glands secrete serous,

enzyme-rich saliva in response to: enzyme-rich saliva in response to: Ingested food which stimulates chemoreceptors Ingested food which stimulates chemoreceptors

and pressoreceptors and pressoreceptors The thought of food The thought of food

Strong sympathetic stimulation inhibits Strong sympathetic stimulation inhibits salivation and results in dry mouthsalivation and results in dry mouth

TeethTeeth Primary and permanent dentitions have formed Primary and permanent dentitions have formed

by age 21by age 21 Primary – 20 deciduous teeth that erupt at Primary – 20 deciduous teeth that erupt at

intervals between 6 and 24 monthsintervals between 6 and 24 months Permanent – enlarge and develop causing the Permanent – enlarge and develop causing the

root of deciduous teeth to be resorbed and fall root of deciduous teeth to be resorbed and fall out between the ages of 6 and 12 yearsout between the ages of 6 and 12 years All but the third molars have erupted by the end of All but the third molars have erupted by the end of

adolescenceadolescence Usually 32 permanent teethUsually 32 permanent teeth

Deciduous TeethDeciduous Teeth

Figure 23.10.1

Permanent TeethPermanent Teeth

Figure 23.10.2

Classification of TeethClassification of Teeth

Teeth are classified according to their shape Teeth are classified according to their shape and functionand function Incisors – chisel-shaped teeth for cutting or Incisors – chisel-shaped teeth for cutting or

nippingnipping Canines – fanglike teeth that tear or pierceCanines – fanglike teeth that tear or pierce Premolars (bicuspids) and molars – have broad Premolars (bicuspids) and molars – have broad

crowns with rounded tips; best suited for grinding crowns with rounded tips; best suited for grinding or crushingor crushing

During chewing, upper and lower molars lock During chewing, upper and lower molars lock together generating crushing forcetogether generating crushing force

2I2I 1C1C 2PM2PM 3M3M

XX 2 (32 teeth)2 (32 teeth)2I2I 1C1C 2PM2PM 3M3M

Dental Formula: Permanent Dental Formula: Permanent TeethTeeth

A shorthand way of indicating the number and A shorthand way of indicating the number and relative position of teethrelative position of teeth Written as ratio of upper to lower teeth for the Written as ratio of upper to lower teeth for the

mouthmouth Primary: 2I (incisors), 1C (canine), 2M (molars)Primary: 2I (incisors), 1C (canine), 2M (molars) Permanent: 2I, 1C, 2PM (premolars), 3M Permanent: 2I, 1C, 2PM (premolars), 3M

Tooth StructureTooth Structure

Two main regions – crown and the rootTwo main regions – crown and the root Crown – exposed part of the tooth above the Crown – exposed part of the tooth above the

gingivagingiva Enamel – acellular, brittle material composed Enamel – acellular, brittle material composed

of calcium salts and hydroxyapatite crystals; of calcium salts and hydroxyapatite crystals; the hardest substance in the bodythe hardest substance in the body Encapsules the crown of the toothEncapsules the crown of the tooth

Root – portion of the tooth embedded in the Root – portion of the tooth embedded in the jawbonejawbone

Tooth StructureTooth Structure

Neck – constriction where the crown and root Neck – constriction where the crown and root come togethercome together

Cementum – calcified connective tissue Cementum – calcified connective tissue Covers the rootCovers the root Attaches it to the periodontal ligamentAttaches it to the periodontal ligament

Tooth StructureTooth Structure

Periodontal ligamentPeriodontal ligament Anchors the tooth in the alveolus of the jaw Anchors the tooth in the alveolus of the jaw Forms the fibrous joint called a gomaphosisForms the fibrous joint called a gomaphosis

Gingival sulcus – depression where the Gingival sulcus – depression where the gingiva borders the toothgingiva borders the tooth

Tooth StructureTooth Structure

Dentin – bonelike material deep to the enamel Dentin – bonelike material deep to the enamel cap that forms the bulk of the toothcap that forms the bulk of the tooth

Pulp cavity – cavity surrounded by dentin that Pulp cavity – cavity surrounded by dentin that contains pulp contains pulp

Pulp – connective tissue, blood vessels, and Pulp – connective tissue, blood vessels, and nervesnerves

Tooth StructureTooth Structure

Root canal – portion of the pulp cavity that Root canal – portion of the pulp cavity that extends into the rootextends into the root

Apical foramen – proximal opening to the root Apical foramen – proximal opening to the root canalcanal

Odontoblasts – secrete and maintain dentin Odontoblasts – secrete and maintain dentin throughout lifethroughout life

Tooth and Gum DiseaseTooth and Gum Disease

Dental caries – gradual demineralization of Dental caries – gradual demineralization of enamel and dentin by bacterial actionenamel and dentin by bacterial action Dental plaque, a film of sugar, bacteria, and mouth Dental plaque, a film of sugar, bacteria, and mouth

debris, adheres to teethdebris, adheres to teeth Acid produced by the bacteria in the plaque Acid produced by the bacteria in the plaque

dissolves calcium saltsdissolves calcium salts Without these salts, organic matter is digested by Without these salts, organic matter is digested by

proteolytic enzymes proteolytic enzymes Daily flossing and brushing help prevent caries by Daily flossing and brushing help prevent caries by

removing forming plaqueremoving forming plaque

Tooth and Gum Disease: Tooth and Gum Disease: PeriodontitisPeriodontitis

Gingivitis – as plaque accumulates, it calcifies Gingivitis – as plaque accumulates, it calcifies and forms calculus, or tartarand forms calculus, or tartar

Accumulation of calculus:Accumulation of calculus: Disrupts the seal between the gingivae and the Disrupts the seal between the gingivae and the

teeth teeth Puts the gums at risk for infection Puts the gums at risk for infection

Periodontitis – serious gum disease resulting Periodontitis – serious gum disease resulting from an immune responsefrom an immune response

Immune system attacks intruders as well as Immune system attacks intruders as well as body tissues, carving pockets around the teeth body tissues, carving pockets around the teeth and dissolving boneand dissolving bone