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CHAPTER 15: DIGESTION &
NUTRITION
Digestive System: Overview The alimentary canal or gastrointestinal
(GI) tract digests and absorbs food Alimentary canal – mouth, pharynx,
esophagus, stomach, small intestine, large intestine
Accessory digestive organs – teeth, tongue, gallbladder, salivary glands, liver, pancreas
Digestive Process
The GI tract is a “disassembly” lineNutrients become more available to the
body in each step There are 6 essential activities:
Ingestion, propulsion, and mechanical digestion
Chemical digestion, absorption, and defecation
Gastrointestinal Tract Activities
Ingestion – taking food into the digestive tract
Propulsion – swallowing and peristalsisPeristalsis – waves of contraction and
relaxation of muscles in the organ walls Mechanical digestion – chewing, mixing,
and churning food
Gastrointestinal Tract Activities
Chemical digestion – catabolic breakdown of food
Absorption – movement of nutrients from the GI tract to the blood or lymph
Defecation – elimination of indigestible solid wastes
Mucosa
Moist epithelial layer that lines the lumen of the alimentary canal
Three major functions:Secretion of mucusAbsorption of end products of digestionProtection against infectious disease
Consists of three layers: a lining epithelium, lamina propria, and muscularis mucosae
Mucosa: Epithelial Lining
Simple columnar epithelium and mucus-secreting goblet cells
Mucus secretions:Protect digestive organs from digesting themselvesEase food along tract
Stomach and small intestine mucosa contain:Enzyme-secreting cellsHormone-secreting cells (making them endocrine and
digestive organs)
Mucosa: Lamina Propria & Muscularis Mucosae Lamina Propria
Loose areolar and reticular connective tissue
Nourishes the epithelium and absorbs nutrients
Contains lymph nodes important in defense against bacteria
Muscularis mucosae – smooth muscle cells that produce local movements of mucosa
Mucosa: Other Sublayers
Submucosa – dense connective tissue containing elastic fibers, blood and lymphatic vessels, lymph nodes, and nerves
Muscularis externa – responsible for segmentation and peristalsis
Serosa – the protective visceral peritoneumReplaced by the fibrous adventitia in the esophagusRetroperitoneal organs have both adventitia and
serosa
Mouth
Oral or buccal cavity:Is bounded by lips, cheeks, palate, and
tongueHas the oral orifice as its anterior openingIs continuous with the oropharynx
posteriorly
Mouth
Able to withstand abrasions:The mouth is lined with stratified squamous
epitheliumThe gums, hard palate, and dorsum of the
tongue are slightly keratinized
Lips and Cheeks
Cheeks consist of:Outer layers of skinPads of subcutaneous fatMuscles associated with expression &
chewingInner linings of moist stratified squamous
epithelium
Lips & Cheeks
Lips – highly mobile structures that surround the mouth opening
Contain:Skeletal muscles Sensory receptors useful in judging the
temperature and texture of foods Normal reddish color comes from many
blood vessels near their surface
Tongue
Occupies the floor of the mouth and fills the oral cavity when mouth is closed
Functions include:Gripping and repositioning food during
chewing (papillae)Determining the taste of foods via taste
budsMixing food with saliva and forming the
bolusInitiation of swallowing & speech
Tongue
Posterior region of the tongue (root) is anchored to the hyoid bone
Covered with rounded masses of lymphatic tissue called lingual tonsils
Palate
Hard palate – underlain by palatine bones and palatine processes of the maxillaeAssists the tongue in chewingSlightly corrugated on either side of the
raphe (midline ridge)
Palate
Soft palate – mobile fold formed mostly of skeletal muscleCloses off the nasopharynx during
swallowingUvula projects downward from its free edge
Palatine tonsils lie beneath the epithelial lining of the mouth & help protect the body against infection
Teeth
Primary teeth – 20 deciduous teeth that erupt at intervals between 6 & 36 months
Permanent teeth – enlarge and develop causing the root of deciduous teeth to be resorbed and fall out between the ages of 6 & 12 yearsAll but the 3rd molars have erupted by the end of
adolescenceUsually 32 permanent teeth
Classification of Teeth
Teeth are classified according to their shape & functionIncisors – chisel-shaped teeth for cutting or rippingCanines – Fanglike teeth that tear or piercePremolars (bicuspids) and molars –have broad
crowns with rounded tips; best suited for grinding or crushing
During chewing, upper and lower molars lock together generating crushing force
Tooth Structure
Two main regions – crown & root Crown – exposed part of the tooth
above the gingiva Enamel – acellular, brittle material
composed of calcium salts and hydroxyapatite crystals; the hardest substance in the bodyEncases the crown of the tooth
Root – portion of the tooth embedded in the jawbone
Tooth Structure
Neck – constriction where the crown and root come together
Cementum – calcified connective tissueCovers the rootAttaches it to the periodontal ligament
Tooth Structure
Periodontal ligamentAnchors the tooth in the alveolus of the jawForms the fibrous joint called the
gomaphosis Gingival sulcus – depression where the
gingiva borders the tooth
Tooth Structure
Dentin – bone-like material deep to the enamel cap that forms the bulk of the tooth
Pulp cavity – cavity surrounded by dentin that contains pulp
Pulp – connective tissue, blood vessels and nerves
Tooth Structure
Root canal – portion of the pulp cavity that extends into the root
Apical foramen – proximal opening to the root canal
Odontoblasts – secrete and maintain dentin throughout life
Salivary Glands
Produce and secrete saliva that:Cleanses the mouthMoistens and dissolves food chemicalsAids in bolus formationContains enzymes that break down starch
Salivary Glands
Three pairs of extrinsic glands – parotid, submandibular, and sublingual
Intrinsic salivary glands (buccal glands) – scattered throughout the oral mucosa
Salivary Glands
Parotid – lies anterior to the ear between the masseter muscle and skinParotid duct opens into the vestibule next to
second upper molar Submandibular – lies along the medial aspect
of the mandibular bodyIts ducts open at the base of the lingual frenulum
Sublingual – lies anterior to the submandibular gland under the tongueIt opens via 10-12 ducts into the floor of the mouth
Saliva: Source and Composition
Secreted from serous and mucous cells of salivary glands
97-99.5% water, hypo-osmotic, slightly acidic solution containing:Electrolytes – Na+, K+, Cl-, PO4
2-, HCO3-
Digestive enzyme – salivary amylaseProteins – mucin, lysozyme, defensins, and IgAMetabolic wastes – urea and uric acid
Control of Salivation
Intrinsic glands keep the mouth moist Extrinsic salivary glands secrete serous,
enzyme-rich saliva in response to:Ingested food which stimulates
chemoreceptors & pressoreceptorsThe thought of food
Strong sympathetic stimulation inhibits salivation and results in dry mouth
Pharynx
From the mouth, the oro- and laryngopharynx allow passage of:Food and fluids to the esophagusAir to the trachea
Lined with stratified squamous epithelium and mucus glands
Has two skeletal muscle layersInner longitudinalOuter pharyngeal constrictors
Esophagus
Muscular tube going from the laryngopharynx to the stomach
Travels through the mediastinum and pierces the diaphragm
Joins the stomach at the cardiac orifice
Esophageal Characteristics Esophageal mucosa – nonkeratinized
stratified squamous epithelium The empty esophagus is folded
longitudinally and flattens when food is present
Glands secrete mucus as a bolus moves through the esophagus
Muscularis changes from skeletal (superiorly) to smooth muscle (inferiorly)
Stomach Chemical breakdown of proteins begins and
food is converted to chyme Cardiac region – surrounds the cardiac orifice Fundus – dome-shaped region beneath the
diaphragm Body – midportion of the stomach Pyloric region – made up of the antrum and
canal which terminates at the pylorus The pylorus is continuous with the duodenum
through the pyloric sphincter
Stomach
Greater curvature – entire extent of the convex lateral surface
Lesser curvature – concave medial surface
Lesser omentum – runs from the liver to the lesser curvature
Greater omentum – drapes inferiorly from the greater curvature to the small intestine
Stomach
Nerve supply – sympathetic and parasympathetic fibers of the autonomic nervous system
Blood supply – celiac trunk, and corresponding veins (part of the hepatic portal system)
Glands of the Stomach Fundus and Body Gastric glands of the fundus and body
have a variety of secretory cellsMucous neck cells – secrete acid mucusParietal cells – secrete HCl and intrinsic
factor
Glands of the Stomach Fundus and Body Chief cells – produce pepsinogen
Pepsinogen is activated to pepsin by:○ HCl in the stomach○ Pepsin itself via a positive feedback
mechanism
Stomach Lining The stomach is exposed to the harshest
conditions in the digestive tract To keep from digesting itself, the stomach has
a mucosal barrier with:A thick coat of bicarbonate-rich mucus on the
stomach wallEpithelial cells that are joined by tight junctionsGastric glands that have cells impermeable to HCl
Damaged epithelial cells are quickly replaced
Digestion in the Stomach The stomach:
Holds ingested foodDegrades this food both physically and
chemicallyDelivers chyme to the small intestineEnzymatically digests proteins with pepsinSecretes intrinsic factor required for
absorption of vitamin B12
Pancreas
Closely associated w/ duodenum of the small intestine
Secretes pancreatic juice into the duodenum at the same place where bile is secreted
Pancreatic juice:Contains enzymes that digest carbohydrates, fats,
nucleic acids, & proteinsHas a high concentration of bicarbonate ions
which neutralize the acidic chyme
Small Intestine: Gross Anatomy
Runs from the pyloric sphincter to the ileocecal valve
Has 3 subdivisions: duodenum, jejunum, and ileum
Small Intestine: Gross Anatomy
The bile duct and main pancreatic duct:Join the duodenum and the
hepatopancreatic ampulla The jejunum extends from the
duodenum to the ileum The ileum joins the large intestine at the
ileocecal valve
Small Intestine: Microscopic Anatomy Structural modifications of the small
intestine wall increase surface areaPlicae circulares: deep circular folds of the
mucosa and submucosaVilli: finger-like extensions of the mucosaMicrovilli: tiny projections of absorptive
mucosal cells’ plasma membrane
Small Intestine: Histology of the Wall The epithelium of the mucosa is made
up of:Absorptive cells and goblet cellsEnteroendocrine cellsInterspersed T cells called intraepithelial
lymphocytes
Small Intestine: Histology of the Wall Cells of intestinal crypts secrete
intestinal juice Peyer’s patches are found in the
submucosa Brunner’s glands in the duodenum
secrete alkaline mucus
Intestinal Juice
Secreted by intestinal glands in response to distension or irritation of the mucosa
Slightly alkaline and isotonic with blood plasma
Largely water, enzyme-poor but contains mucus
Liver
The largest gland in the body Superficially has four lobes – right, left,
caudate, and quadrate The falciform ligament:
Separates the right and left lobes anteriorlySuspends the liver from the diaphragm and
anterior abdominal wall
Liver: Associated Structures The lesser omentum anchors the liver to
the stomach The hepatic blood vessels enter the liver
at the porta hepatis The gallbladder rests in a recess on the
inferior surface of the right lobe
Liver: Associated Structures Bile leaves the liver via:
Bile ducts, which fuse into the common hepatic duct
The common hepatic duct, which fuses with the cystic duct○ These two ducts form the bile duct
Composition of Bile
A yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes
Bile salts are cholesterol derivatives that:Emulsify fatFacilitate fat and cholesterol absorptionHelp solubilize cholesterol
The Gallbladder
Thin-walled, green muscular sac on the ventral surface of the liver
Stores and concentrates bile by absorbing its water and ions
Releases bile via the cystic duct, which flows into the bile duct
Large Intestine
Has 3 unique features:Taenia coli – 3 bands of longitudinal smooth
muscle in its muscularisHaustra – pocket-like sacs caused by the
tone of the taenia coliEpiploic appendages – fat-filled pouches of
visceral peritoneum
Large Intestine
Subdivided into the cecum, appendix, colon, rectum and anal canal
The saclike cecum:Lies below the ileocecal valve in the right
iliac fossaContains a wormlike vermiform appendix