The Digestive System P A R T A. Digestive System: Overview The alimentary canal or gastrointestinal (GI) tract digests and absorbs food Alimentary canal

The Digestive System

P A R T A

Digestive System: Overview

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

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

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

Figure 23.1

Digestive Process

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

body in each step There are six essential activities:

Ingestion, propulsion, and mechanical digestion

Chemical digestion, absorption, and defecation

Figure 23.2

Gastrointestinal Tract Activities

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

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

Mechanical digestion – chewing, mixing, and churning food

Peristalsis and Segmentation

Figure 23.3

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

GI Tract

External environment for the digestive process Regulation of digestion involves:

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

Extrinsic control by CNS centersIntrinsic control by local centers

Receptors of the GI Tract

Mechano and chemoreceptors respond to:Stretch, osmolarity, and pHPresence of substrate, and end products of

digestion They initiate reflexes that:

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

Nervous Control of the GI Tract

Intrinsic controlsNerve plexuses near the GI tract initiate

short reflexesShort reflexes are mediated by local enteric

plexuses (gut brain) Extrinsic controls

Long reflexes arising within or outside the GI tract

CNS centers and extrinsic autonomic nerves

Nervous Control of the GI Tract

Figure 23.4

Peritoneum and Peritoneal Cavity Peritoneum – serous membrane of the

abdominal cavityVisceral – covers external surface of most

digestive organsParietal – lines the body wall

Peritoneal cavityLubricates digestive organs Allows them to slide across one another

Peritoneum and Peritoneal Cavity

Figure 23.5a

Peritoneum and Peritoneal Cavity Mesentery – double layer of peritoneum that

provides:Vascular and nerve supplies to the visceraHold digestive organs in place and store fat

Retroperitoneal organs – organs outside the peritoneum

Peritoneal organs (intraperitoneal) – organs surrounded by peritoneum

Peritoneum and Peritoneal Cavity

Figure 23.5b

Blood Supply: Splanchnic Circulation Arteries and the organs they serve include

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

Inferior and superior mesenteric: small and large intestines

Blood Supply: Splanchnic Circulation Hepatic portal circulation:

Collects nutrient-rich venous blood from the digestive viscera

Delivers this blood to the liver for metabolic processing and storage

Histology of the Alimentary Canal From esophagus to the anal canal the walls of

the GI tract have the same four tunicsFrom the lumen outward they are the

mucosa, submucosa, muscularis externa, and serosa

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

Histology of the Alimentary Canal

Figure 23.6

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 the tract

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

endocrine and digestive organs)

Mucosa: Lamina Propria and Muscularis Mucosae

Lamina PropriaLoose areolar and reticular connective tissueNourishes the epithelium and absorbs

nutrientsContains lymph nodes (part of MALT)

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

esophagus Retroperitoneal organs have both an

adventitia and serosa

Enteric Nervous System Composed of two major intrinsic nerve

plexuses:Submucosal nerve plexus – regulates

glands and smooth muscle in the mucosaMyenteric nerve plexus – Major nerve

supply that controls GI tract mobility Segmentation and peristalsis are largely

automatic involving local reflex arcs Linked to the CNS via long autonomic reflex

arc

Mouth

Oral or buccal cavity:Is bounded by lips, cheeks, palate, and

tongue Has the oral orifice as its anterior openingIs continuous with the oropharynx

Mouth

To withstand abrasions: The mouth is lined with stratified squamous

epithelium The gums, hard palate, and dorsum of the

tongue are slightly keratinized

Anatomy of the Oral Cavity: Mouth

Figure 23.7a

Lips and Cheeks

Have a core of skeletal musclesLips: orbicularis oris Cheeks: buccinators

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

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

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

Oral Cavity and Pharynx: Anterior View

Figure 23.7b

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

Palatoglossal and palatopharyngeal arches form the borders of the fauces

Tongue

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

Functions include:Gripping and repositioning food during

chewingMixing food with saliva and forming the

bolusInitiation of swallowing, and speech

Tongue

Intrinsic muscles change the shape of the tongue

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

floor of the mouth

Tongue

Superior surface bears three types of papillaeFiliform – give the tongue roughness and

provide friction Fungiform – scattered widely over the

tongue and give it a reddish hueCircumvallate – V-shaped row in back of

tongue

Tongue

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

Tongue

Figure 23.8

Salivary Glands

Produce and secrete saliva that: Cleanses the mouthMoistens and dissolves food chemicals Aids 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

Salivary Glands

Figure 23.9a

Saliva: Source and Composition

Secreted from serous and mucous cells of salivary glands

97-99.5% water, hypo-osmotic, slightly acidic solution containingElectrolytes – 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 and mechanoreceptors The thought of food

Strong sympathetic stimulation inhibits salivation and results in dry mouth

Teeth Primary and permanent dentitions have formed

by age 21 Primary – 20 deciduous teeth that erupt at

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

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

end of adolescenceUsually 32 permanent teeth

Deciduous Teeth

Figure 23.10.1

Permanent Teeth

Figure 23.10.2

Classification of Teeth

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

nippingCanines – 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

Dental Formula: Permanent Teeth

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

the mouth

2I 1C 2PM 3M

X 2 (32 teeth)2I 1C 2PM 3M

Dental Formula: Permanent Teeth

Deciduous: 2I (incisors), 1C (canine), 2M (molars) = 20

Permanent: 2I, 1C, 2PM (premolars), 3M = 32

Tooth Structure Two main regions – crown and the 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 bodyEncapsules 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 tissue Covers the rootAttaches it to the periodontal ligament

Tooth Structure

Periodontal ligamentAnchors the tooth in the alveolus of the jaw Forms the fibrous joint called a gomphosis

Gingival sulcus – depression where the gingiva borders the tooth

Tooth Structure

Dentin – bonelike 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

Tooth Structure

Figure 23.11

Tooth and Gum Disease

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

mouth debris, adheres to teethAcid produced by the bacteria in the plaque

dissolves calcium saltsWithout these salts, organic matter is

digested by proteolytic enzymes Daily flossing and brushing help prevent

caries by removing forming plaque

Tooth and Gum Disease: Periodontitis

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

Accumulation of calculus:Disrupts the seal between the gingivae and

the teeth Puts the gums at risk for infection

Periodontitis – serious gum disease resulting from an immune response

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

P A R T B


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 longitudinal Outer 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

Esophagus

Figure 23.12

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 (superior

third) to smooth muscle (inferior third)

Digestive Processes in the Mouth Food is ingested Mechanical digestion begins (chewing) Propulsion is initiated by swallowing Salivary amylase begins chemical breakdown

of starch The pharynx and esophagus serve as conduits

to pass food from the mouth to the stomach

Deglutition (Swallowing)

Coordinated activity of the tongue, soft palate, pharynx, esophagus, and 22 separate muscle groups

Buccal phase – bolus is forced into the oropharynx

Deglutition (Swallowing)

Pharyngeal-esophageal phase – controlled by the medulla and lower ponsAll routes except into the digestive tract are

sealed off Peristalsis moves food through the pharynx to

the esophagus

Figure 23.13

(d) (e)

Relaxedmuscles

Circular musclescontract,constrictingpassagewayand pushingbolus down

Bolus offoodLongitudinalmusclescontract,shorteningpassagewayahead of bolus

Gastroesophagealsphincter closed

Stomach

Relaxedmuscles

(a) (b) (c)

Tongue

Trachea

Pharynx

Epiglottis

Glottis

Upper esophagealsphincter contracted

Bolus of food

Upper esophagealsphincter relaxed

Epiglottis

Esophagus

Uvula

Bolus

Bolus

Upper esophagealsphincter contracted

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)

Figure 23.14a

Microscopic Anatomy of the Stomach Muscularis – has an additional oblique layer

that: Allows the stomach to churn, mix, and

pummel food physically Breaks down food into smaller fragments

Microscopic Anatomy of the Stomach Epithelial lining is composed of:

Goblet cells that produce a coat of alkaline mucusThe mucous surface layer traps a

bicarbonate-rich fluid beneath it Gastric pits contain goblet cells Gastric glands secrete gastric juice

Mucus and hormones

Microscopic Anatomy of the Stomach

Figure 23.15a


Figure 23.15b


Figure 23.15c

Glands of the Stomach Fundus and Body Gastric glands of the cardia and pylorus

Secrete mucus Gastric glands of the pyloric antrum

Secrete hormones 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 stomachPepsin itself via a positive feedback

mechanismEnteroendocrine cells – secrete gastrin,

histamine, endorphins, serotonin, cholecystokinin (CCK), and somatostatin into the lamina propria

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

Regulation of Gastric Secretion

Neural and hormonal mechanisms regulate the release of gastric juice

Stimulatory and inhibitory events occur in three phasesCephalic (reflex) phase: prior to food entryGastric phase: once food enters the

stomachIntestinal phase: as partially digested food

enters the duodenum

Cephalic Phase Excitatory events include:

Sight or thought of foodStimulation of taste or smell receptorsRelayed to hypothalamus, medulla

oblongata (vagus nucleus) Stimulates stomach glands

Inhibitory events include:Loss of appetite or depressionDecrease in stimulation of the

parasympathetic division

Gastric Phase

Excitatory events include:Stomach distension

Activation of stretch receptors (neural activation)

Activation of chemoreceptors by peptides, caffeine, and rising pH

Release of gastrin to the blood Increase release of HCl

Gastric Phase

Inhibitory events include:A pH lower than 2 Emotional upset that overrides the

parasympathetic division

Regulation and Mechanism of HCl Secretion HCl secretion is stimulated by ACh, histamine,

and gastrin through second-messenger systems

Release of hydrochloric acid:Is low if only one ligand binds to parietal

cellsIs high if all three ligands bind to parietal

cells Antihistamines block H2 receptors and

decrease HCl release

Regulation and Mechanism of HCl Secretion

Figure 23.17

Intestinal Phase – 2 components:

Excitatory phase – low pH; partially digested food enters the duodenum and encourages gastric gland activity through intestinal gastrin

Inhibitory phase (enterogastric reflex)– distension of duodenum, presence of fatty, acidic, or hypertonic chyme, and/or irritants in the duodenumInitiates inhibition of local reflexes and vagal

nuclei and stimulates sympathetic fibersCloses the pyloric sphincterReleases enterogastrones that inhibit gastric

secretion

Release of Gastric Juice: Stimulatory Events

Figure 23.16.1

Release of Gastric Juice: Inhibitory Events

Figure 23.16.2

Response of the Stomach to Filling Stomach pressure remains constant until

about 1L of food is ingested Relative unchanging pressure results from

reflex-mediated relaxation and plasticity

Response of the Stomach to Filling Reflex-mediated events include:

Receptive relaxation – as food travels in the esophagus, stomach muscles relax

Adaptive relaxation – the stomach dilates in response to gastric filling

Plasticity – intrinsic ability of smooth muscle to exhibit the stress-relaxation response

Gastric Contractile Activity

Peristaltic waves move toward the pylorus at the rate of 3 per minute

This basic electrical rhythm (BER) is set by pacemaker cells (cells of Cajal) on the longitudinal layerThey set the rate of contraction

Neural and hormonal stimulus Initiate the stimulusRegulate the strength of the stimulus


Most vigorous peristalsis and mixing occurs near the pylorus

Chyme is either:Delivered in small amounts to the

duodenum orForced backward into the stomach for

further mixing


Figure 23.18

Regulation of Gastric Emptying

Gastric emptying is regulated by:The neural enterogastric reflexHormonal (enterogastrone) mechanisms

These mechanisms inhibit gastric secretion and duodenal filling


Carbohydrate-rich chyme quickly moves through the duodenum

Fat-laden chyme is digested more slowly causing food to remain in the stomach longer


Figure 23.19

Small Intestine: Gross Anatomy

Runs from pyloric sphincter to the ileocecal valve

Has three subdivisions: duodenum, jejunum, and ileum

Mesentery proper attach the small intestine to the body wall

Small Intestine: Gross Anatomy

The bile duct and main pancreatic duct:Join the duodenum at the hepatopancreatic

ampulla Are controlled by the sphincter of Oddi

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 – fingerlike extensions of the mucosaMicrovilli – tiny projections of absorptive

mucosal cells’ plasma membranes

Duodenum and Related Organs

Figure 23.20

Small Intestine: Microscopic Anatomy

Figure 23.21

Small Intestine: Histology of the Wall The epithelium of the mucosa is made up of:

Absorptive cells and goblet cells Enteroendocrine cells Interspersed T cells called intraepithelial

lymphocytes (IELs) IELs immediately release cytokines upon

encountering Ag. Do not need “priming.”

Small Intestine: Histology of the Wall Cells of intestinal crypts secrete intestinal juice Peyer’s patches are found in the submucosa

of the ileum 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

The ______ in the small intestine have a similar structure and function to the ______ in the stomach.

a. gastric pits… intestinal glands b. intestinal glands… gastric pits c. goblet cells… enteroendocrine cells d. brush border… rugae

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

The ligamentum teres (round ligament):Is a remnant of the fetal umbilical veinRuns along the free edge of the falciform

ligament

Liver: Associated Structures

The lesser omentum anchors the liver to the stomach

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 ductThese two ducts form the bile duct

Gallbladder and Associated Ducts

Figure 23.20

Figure 23.24c

Liver: Microscopic Anatomy

Hexagonal-shaped liver lobules are the structural and functional units of the liverComposed of hepatocyte (liver cell) plates

radiating outward from a central veinPortal triads are found at each of the six

corners of each liver lobule

Figure 23.24d


Portal triads consist of a bile duct andHepatic artery – supplies oxygen-rich blood

to the liverHepatic portal vein – carries venous blood

with nutrients from digestive viscera


Liver sinusoids – enlarged, leaky capillaries located between hepatic plates

Kupffer cells – hepatic macrophages found in liver sinusoids


Hepatocytes’ functions include:Production of bileProcessing bloodborne nutrientsStorage of fat-soluble vitaminsDetoxification

Secreted bile flows between hepatocytes toward the bile ducts in the portal triads

Microscopic Anatomy of the Liver

Figure 23.24c, d

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

Enterohepatic circulation recycles bile salts The chief bile pigment is bilirubin, a waste

product of heme

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

Regulation of Bile Release

Acidic, fatty chyme causes the duodenum to release:Cholecystokinin (CCK) and secretin into the

bloodstream Bile salts and secretin transported in blood

stimulate the liver to produce bile Vagal stimulation causes weak contractions of

the gallbladder


Cholecystokinin causes:The gallbladder to contractThe hepatopancreatic sphincter to relaxSecretion of pancreatic juice

As a result, bile and pancreatic juice enters the duodenum


Figure 23.25

Acidic, fatty chyme entering duodenum causesrelease of cholecystokinin and secretin from duodenal wallenteroendocrine cells

Cholecystokinin and secretin enter the bloodstream

Cholecystokinin(via bloodstream)causes gallbladderto contract andhepatopancreaticsphincter to relax;bile entersduodenum

Bile saltsand secretintransported viabloodstreamstimulate liverto produce bilemore rapidly

Bile salts reabsorbed into blood

Vagal stimulation causesweak contractions of gallbladder

1

2

6

5

4

3


P A R T C

Pancreas

LocationLies deep to the greater curvature of the

stomachThe head is encircled by the duodenum and

the tail abuts the spleen

Pancreas

Exocrine functionSecretes pancreatic juice which breaks

down all categories of foodstuffAcini (clusters of secretory cells) contain

zymogen granules with digestive enzymes The pancreas also has an endocrine function –

release of insulin and glucagon

Acinus of the Pancreas

Figure 23.26a

Composition and Function of Pancreatic Juice Water solution of enzymes and electrolytes

(primarily HCO3–)

Neutralizes acid chymeProvides optimal environment for pancreatic

enzymes Pancreatic proteases are released in inactive

form and activated in the duodenumTrypsinogen is activated to trypsin

Composition and Function of Pancreatic Juice Trypsin activates:

Procarboxypeptidase to carboxypeptidaseChymotrypsinogen to chymotrypsin

Active enzymes secretedAmylase, lipases, and nucleases These enzymes require ions or bile for

optimal activity

Pancreatic Activation

Figure 23.27

Regulation of Pancreatic Secretion

Secretin and CCK are released when fatty or acidic chyme enters the duodenum

CCK and secretin enter the bloodstream Upon reaching the pancreas:

CCK induces the secretion of enzyme-rich pancreatic juice

Secretin causes secretion of bicarbonate-rich pancreatic juice

Vagal stimulation also causes release of pancreatic juice

Regulation of Pancreatic Secretion

Figure 23.28

Acidic chyme enteringduodenum causes theenteroendocrine cells ofthe duodenal wall to releasesecretin, whereas fatty,protein-rich chyme inducesrelease of cholecystokinin.

During cephalic and gastricphases, stimulation byvagal nerve fibers causesrelease of pancreatic juiceand weak contractions ofthe gallbladder.

Upon reaching thepancreas, cholecystokinininduces the secretion ofenzyme-rich pancreatic juice;secretin causes copioussecretion of bicarbonate-richpancreatic juice.

Cholecystokininand secretin enterbloodstream.

1

2

3

Digestion in the Small Intestine

As chyme enters the duodenum: Carbohydrates and proteins are only

partially digestedAlmost no fat digestion has taken place

Digestion in the Small Intestine

Digestion continues in the small intestineChyme is released slowly into the

duodenum Because it is hypertonic and has low pH,

mixing is required for proper digestionRequired substances needed are supplied

by the liver Virtually all nutrient absorption takes place

in the small intestine

Motility in the Small Intestine

The most common motion of the small intestine is segmentation and not peristalsis It is initiated by intrinsic pacemaker cells

(Cajal cells)Moves contents steadily toward the

ileocecal valve

Motility in the Small Intestine

After nutrients have been absorbed:Peristalsis begins in response to the motilin

hormone with each wave starting distal to the previous

Meal remnants, bacteria, mucosal cells, and debris are moved into the large intestine

Control of Motility

Local enteric neurons of the GI tract coordinate intestinal motility

Cholinergic neurons cause:Contraction and shortening of the circular

muscle layer proximallyShortening of longitudinal muscle distallyDistension of the intestine

Control of Motility

The gastroileal reflex initiated by the increased activity of the stomachIncreases segmentation in the ileum

Gastrin released by the stomachRelax the ileocecal sphincterIncreases the motility of the ileum

Allow chyme to pass into the large intestine

Large Intestine

Has three unique features:Teniae coli – three bands of longitudinal

smooth muscle in its muscularisHaustra – pocketlike sacs caused by the

tone of the teniae coliEpiploic appendages – fat-filled pouches of

visceral peritoneum

Large Intestine

Is 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

Large Intestine

Figure 23.29a

Colon

Has distinct regions: ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, and sigmoid colon

The transverse and sigmoid portions are anchored via mesenteries called mesocolons

The sigmoid colon joins the rectum The anal canal, the last segment of the large

intestine, opens to the exterior at the anus

Valves and Sphincters of the Rectum and Anus

Three valves of the rectum stop feces from being passed with gas

The anus has two sphincters:Internal anal sphincter composed of smooth

muscleExternal anal sphincter composed of skeletal

muscle These sphincters are closed except during

defecation

Mesenteries of Digestive Organs

Figure 23.30b


Figure 23.30c


Figure 23.30d

Large Intestine: Microscopic Anatomy Colon mucosa is simple columnar epithelium

except in the anal canal There are no villi, no plicae, and no cells that

secrete digestive enzymes Has numerous deep crypts lined with goblet

cells

Large Intestine: Microscopic Anatomy Anal canal mucosa is stratified squamous

epithelium Anal sinuses exude mucus and compress

feces Superficial venous plexuses are associated

with the anal canal Inflammation of these veins results in itchy

varicosities called hemorrhoids

Structure of the Anal Canal

Figure 23.29b

Bacterial Flora

The bacterial flora of the large intestine consist of:Bacteria surviving the small intestine that

enter the cecum and Those entering via the anus

These bacteria: Colonize the colonFerment indigestible carbohydratesRelease irritating acids and gases (flatus)Synthesize B complex vitamins and vitamin

K

Functions of the Large Intestine

Other than digestion of enteric bacteria, no further digestion takes place

Vitamins, water, and electrolytes are reclaimed Its major function is propulsion of fecal material

toward the anus Though essential for comfort, the colon is not

essential for life

Motility of the Large Intestine

Haustral contractionsSlow segmenting movements that move the

contents of the colonHaustra sequentially contract as they are

stimulated by distension Presence of food in the stomach:

Activates the gastrocolic reflex Initiates peristalsis ( mass movements)

that forces contents toward the rectum

Defecation

Distension of rectal walls caused by feces:Stimulates contraction of the rectal wallsRelaxes the internal anal sphincterContracts external anal sphincter

Voluntary signals stimulate relaxation of the external anal sphincter and defecation occurs

Defecation

Figure 23.32

Chemical Digestion: Carbohydrates Absorption: via cotransport with Na+, and

facilitated diffusionEnter the capillary bed in the villiTransported to the liver via the hepatic portal

vein Enzymes used: salivary amylase, pancreatic

amylase, and brush border enzymes

Chemical Digestion: Proteins

Absorption: similar to carbohydrates Enzymes used: pepsin in the stomach Enzymes acting in the small intestine

Pancreatic enzymes – trypsin, chymotrypsin, and carboxypeptidase

Brush border enzymes – aminopeptidases, carboxypeptidases, and dipeptidases

Figure 23.34

Chemical Digestion: Fats Happens mainly in the small intestine Two phases: Emulsification

By bile saltsEmulsion droplets

Chemical digestionPancreatic lipase

Free fatty acidsMonoglycerides

Chemical Digestion: Fats

Figure 23.35

Lipids Absorption

Glycerol and short chain fatty acids are:Absorbed into the capillary blood in villiTransported via the hepatic portal vein

Lipids Absorption

Micelles Fatty acids and monoglycerides enter intestinal

cells via diffusion They are resynthesized into triglycerides Triglycerides are combined with proteins within

the cells Resulting chylomicrons are extruded They enter lacteals and are transported to the

circulation via lymph

Figure 23.36

Lumen ofintestine

Absorptiveepithelial cellcytoplasm

ER

Golgiapparatus

ChylomicronLacteal

Fatty acids andmonoglyceridesassociated withmicelles inlumen of intestine

Fatty acids andmonoglyceridesresulting from fatdigestion leavemicelles and enterepithelial cell by diffusion.

Fatty acids are used to synthesize triglycerides in smooth endo-plasmic reticulum.

Fatty globules arecombined with proteins to form chylomicrons(within Golgi apparatus).

Vesicles containingchylomicrons migrate to the basal membrane,are extruded from the epithelial cell,and enter a lacteal (lymphatic capillary).

Lymph in the lacteal transports chylomicrons away from intestine.

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Fatty Acid Absorption

Chemical Digestion: Nucleic Acids Absorption: active transport via membrane

carriers Absorbed in villi and transported to liver via

hepatic portal vein Enzymes used: pancreatic ribonucleases and

deoxyribonuclease in the small intestines

Electrolyte Absorption

Most ions are actively absorbed along the length of small intestine Na+ is coupled with absorption of glucose

and amino acidsIonic iron is transported into mucosal cells

where it binds to ferritin Anions passively follow the electrical potential

established by Na+

Electrolyte Absorption

K+ diffuses across the intestinal mucosa in response to osmotic gradients

Ca2+ absorption:Is related to blood levels of ionic calciumIs regulated by vitamin D and parathyroid

hormone (PTH)

Water Absorption 95% of water is absorbed in the small

intestines by osmosis Water moves in both directions across

intestinal mucosa Net osmosis occurs whenever a concentration

gradient is established by active transport of solutes into the mucosal cells

Water uptake is coupled with solute uptake, and as water moves into mucosal cells, substances follow along their concentration gradients

Malabsorption of Nutrients

Results from anything that interferes with delivery of bile or pancreatic juice

Factors that damage the intestinal mucosa (e.g., bacterial infection)

Gluten enteropathy (adult celiac disease) – gluten damages the intestinal villi and reduces the length of microvilliTreated by eliminating gluten from the diet

(all grains but rice and corn)

Embryonic Development of the Digestive System

3rd week – endoderm has folded and foregut and hindgut have formed

The midgut is open and continuous with the yolk sac

Mouth and anal openings are nearly formed 8th week – accessory organs are budding from

endoderm

Embryonic Development of the Digestive System

Figure 23.37

Developmental Aspects

During fetal life, nutrition is via the placenta, but the GI tract is stimulated toward maturity by amniotic fluid swallowed in uterus

At birth, feeding is an infant’s most important function and is enhanced by Rooting reflex (helps infant find the nipple)

and sucking reflex (aids in swallowing)

Developmental Aspects

Digestive system has few problems until the onset of old age

During old age the GI tract activity declines, absorption is less efficient, and peristalsis is slowed

Cancer

Stomach and colon cancers rarely have early signs or symptoms

Metastasized colon cancers frequently cause secondary liver cancer

Prevention is by regular dental and medical examinations

Cancer

Colon cancer is the 2nd largest cause of cancer deaths in males (lung cancer is 1st)

Forms from benign mucosal tumors called polyps whose formation increases with age

Regular colon examination should be done for all those over 50

Documents

The Digestive System P A R T A. Digestive System: Overview The alimentary canal or gastrointestinal (GI) tract digests and absorbs food Alimentary canal