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