14. digestive system

The Digestive System
Histology

Introduction
• The digestive system consists of a muscular
digestive (tube) tract and accessory organs
- digestive tract: oral cavity (mouth), pharynx, esophagus,
stomach, small intestine, and large intestine
- accessory organs: teeth, tongue, and glandular organs
(salivary glands, lever, pancreas) which secrete into ducts

• Food enters the digestive tract, along the way
secretions of the glandular organs assist in
preparing organic and inorganic nutrients for
absorption

Functions of the
Digestive System
• Ingestion – occurs when foods and liquids enter
the digestive tract via the mouth
• Mechanical processing – squashing with the
tongue, tearing and crushing with the teeth
- swirling, mixing, churning, and propulsive motions
provide mechanical processing after swallowing

• Digestion – chemical and enzymatic breakdown
of carbohydrates, lipids, and proteins
- small organic molecules can be absorbed by the
digestive epithelium

• Secretion – digestion involves the action of acids,
enzymes, and buffers
- produced by the lining but mostly by the accessory organs
(pancreas)

• Absorption – movement of organic molecules,
electrolytes, vitamins, and water across the
digestive epithelium
- into the interstitial fluid of the digestive tract

• Excretion – into the digestive tract, primarily by the
accessory glands (especially the liver)
• Compaction – progressive dehydration of
indigestible materials and organic wastes (feces)
- defecation is the elimination of feces from the body

Lining of the Digestive Tract
• Plays a defensive role by protecting surrounding
tissues against:
1. the corrosive effects of digestive acids and
enzymes
2. mechanical stresses, such as abrasion
3. pathogens that are swallowed with food or
that reside within the digestive tract

Components of the Digestive System

Fig 25.1

Histological Organization
of the Digestive Tract
Major layers of the digestive tract include the:
• Mucosa
• Submucosa
• Muscularis externa
• Serosa

The Mucosa
• A mucous membrane consisting of a layer of loose
CT covered by an epithelium moistened by
glandular secretions
- plicae (folds) increase surface area for absorption
- lamina propria, underlying layer of areolar tissue (BVs,
sensory nerve endings, lymphatic vessels, smooth muscle
fibers
- muscularis mucosae, smooth muscle fibers arranged in 2
thin concentric layers:
- the circular layer, inner layer encircles the lumen and
- the longitudinal layer, outer layer whose muscle fibers lie
parallel to the long axis of the tract

Histology of the Mucosa
Organ
Mouth

Epithelium
Nonkeratinized Stratified
Squamous

Pharynx

Squamous

Esophagus

Squamous

Stomach
Small Intestine
Large Intestine
Anus

Simple Columnar
Simple Columnar
Simple Columnar

Histology of the Mucosa
Organ

Folds of the epithelium

Esophagus

none

Stomach

L: Rugae, S: gastric pits

Small Intestine L: Plicae circulares, Villi S:
Crypts of Lieberkuhn, microvilli
Large Intestine L: Haustra S: Intestinal glands

Figure 25.2a
Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Submucosa
• A layer of dense, irregular CT – surrounds the
muscularis mucosae
- contains large BVs and lymphatics
- some regions contain exocrine glands that secrete
buffers and enzymes into the lumen
- in the outer margin the submucosal plexus, a network
of nerve fibers and scattered neuron cell bodies
innervates the mucosa
- include the sensory neurons, parasympathetic ganglia,
and sympathetic postganglionic fibers

Histology of the Submucosa
Organ

Specialized structures

Esophagus

Submucosal mucous glands

Stomach

None

Duodenum

Brunner’s glands

Ileum

Peyer’s Patches

Large Intestine None

The Muscularis Externa
• Dominated by smooth muscle fibers – surrounds
the submucosa
- fibers are arranged in circular (inner) and longitudinal
(outer) layers
- essential in mechanical processing and in propulsion of
materials along the digestive tract
- movements coordinated by the myenteric (Auerbach)
plexus, a network of parasympathetic ganglia and
sympathetic postganglionic fibers (sandwiched between
the muscle layers)
- muscularis externa forms sphincters (valves) that help
prevent materials from moving along the tract at the
wrong time or direction

Histology of the Muscularis
Organ

Smooth muscle layers

Esophagus

2, circular and longitudinal

Stomach

3, oblique, circular, and
longitudinal

Small Intestine 2, circular and longitudinal
Large Intestine 2, circular and longitudinal

The Serosa
• A serous membrane – covers the muscularis
externa along most regions of the digestive tract
within the peritoneal cavity
- no serosa surrounds the muscularis externa of the oral
cavity, pharynx, esophagus, and rectum
- instead adventitia, a fibrous sheath of collagen fibers
attaches the digestive tract to adjacent structures

• Serosa or visceral peritoneum is continuous with
the parietal peritoneum that lines the inner
surfaces of the body wall

Histology of the Serosa
Organ

Serosa

Esophagus

Adventitia due to the fact that
the esophagus is not in a cavity

Stomach

Visceral Peritoneum

Small Intestine Visceral Peritoneum
Large Intestine Visceral Peritoneum
Anus

Adventitia

Histology of the Digestive System
Basic Histological Layers
1. Mucosa
a. Epithelium
b. Lamina Propria
c. Muscularis Mucosae
2. Submucosa
a. Submucosal plexus
“Plexus of Meissner”
3. Muscularis
a. Myenteric plexus
“Plexus of Auerbach
4. Serosa

Histological Structure of the Digestive Tract

Figure 25.2b


Muscularis Layers and the
Movement of Digestive Materials
• Visceral smooth muscle tissue – digestive tract
– a single smooth muscle cell: 5-10um in diameter and 30200um in length
- surrounded by CT (no tendons or aponeuroses)
- contractile proteins not organized into sacromeres
- muscle cells are nonstriated, involuntary muscle
- contractions are as strong as skeletal or cardiac muscle
- muscle cells are arranged in sheets
- adjacent cells are electrically connected by gap junction
- 1 cell contracts spreads like a wave throughout the tissue
stimulus: activation of a motor neuron, local response to
chemicals, hormones, O2 & CO2 levels, stretching, irritation

• Contractile filaments of smooth muscle cells are
not rigidly organized
- plasticity, tolerate extreme stretching, a stretched cell
adapts to its new length and retains ability to contract on
demand
- pacemaker cells in the muscularis mucosae and
muscularis externa undergo spontaneous depolarization
- which trigger contractions leading to 2 types of
movement: peristalsis and segmentation

Peristalsis
• Muscularis externa via
peristalsis propels materials
from 1 region of the tract to
another
• Waves of contractions
move a bolus of food –
1. circular muscles contract
behind the digestive contents
2. followed by longitudinal
muscle contraction that
shorten adjacent segments
3. wave of contraction in
circular muscle layer forces
bolus forward

Fig 25.3a

Segmentation
• Contractions in most areas
of the small intestine and
some regions of the large
intestine churn and fragment
the digestive materials
• The contents are mixed with
intestinal secretions
• No net movement is
produced

Figure 25.3

Peristalsis and Segmentation
• May be triggered by pacesetter cells, hormones,
chemicals, and physical stimulation
• Peristaltic waves can also be initiated by afferent
and efferent fibers
- glossopharyngeal, vagus or pelvic nerves

• Afferent fibers synapse within the myenteric
plexus – produce localized myenteric reflexes
- short reflexes do not involve the CNS
- enteric nervous system, neural network that coordinates
short reflexes (as many neurons and NTs as the SC)
- long reflexes, involve interneurons and motor neurons
in the CNS control large-scale peristaltic waves

Mesenteries
• Most regions of the digestive tract are suspended
by sheets of serous membrane
- connect the parietal with the visceral peritoneum

• Mesenteries - fused, double sheets of peritoneal
membrane
- stabilize positions of attached organs
- prevent entanglement of intestines
- areolar CT between the mesothelial surfaces provide a
route for BVs, nerves, and lymphatics, to and from the
digestive tract
- during development, the digestive tract and accessory
organs are suspended by dorsal and ventral mesenteries

• Lesser omentum - remnant of the embryonic
ventral mesentery
- persists only on ventral surface of the stomach, between
the stomach and liver
- and the falciform ligament, that lies between the liver
and the anterior abdominal wall and diaphragm

• Greater omentum - dorsal mesentry becomes
enlarged to form a pouch
- loose CT within the mesentery contains a thick layer of
adipose tissue, the lipids are an important energy reserve
- provides insulation that reduces heat loss
- contains numerous lymph nodes to help protect the body
from pathogens that evaded the defenses of the digestive
tract


Figure 25.4c

• Mesentery proper – all but the first 25cm of the
small intestine is suspended by this thick
mesenterial sheet
- provides stability but permits some independent
movement

• Mesocolon – attached to the large intestine
• Transverse mesocolon – suspends the middle
portion of the large intestine (transverse colon)
• Sigmoid mesocolon – suspends the sigmoid
colon that leads to the rectum and anus


Figure 25.4d

The Oral Cavity
• Digestive tract – path of food from the mouth to the
anus

• The oral cavity includes:
- the tongue, salivary glands, teeth, mastication

• Functions include:
1) analysis of material before swallowing
2) mechanical processing through the actions of the
teeth, tongue, and palatal surfaces
3) lubrication by mixing with mucous and salivary
secretions
4) limited digestion of carbohydrates by a salivary enzyme

The Oral Cavity
• Oral (buccal) cavity – lined by the oral mucosa
- stratified squamous epithelium protects from abrasion
- mucosa of the cheeks (lateral walls) are formed by buccal
fat pads and the buccinator muscles
- mucosa of the cheeks are continuous with the lips (labia)
- the vestibule, space between the cheeks, lips, and teeth
- gingivae, ridge of oral mucosa or the gums surrounds the
base of each tooth
- roof is formed by the hard and soft palates
- hard separates oral from the nasal cavity; soft separates
the oral from the nasopharynx (closes it off for swallowing)
- uvula helps prevent food from entering the pharynx
prematurely

Figure 25.5


• Mylohyoid muscle – gives additional support
• Posterior soft palate supports the uvula and the 2
pairs of muscular pharyngeal arches
- palatoglossal arches extend between the soft palate

and the base of the tone and consist of a mucous
membrane and an underlying palatoglossus muscle
- palatopharyngeal arches extend from the soft palate to
the side of the pharynx; consist of a mucous membrane
and the underlying palatopharyngeus muscle

• Palatine tonsils – lie between the palatoglossal
and palatopharyngeal arches
• Fauces – entrance to the oropharynx

Lips and Cheeks
• Both structures important in mastication and
speech
• Lips (labia): orbicularis oris muscle within
– Keratinized stratified squamous exterior is thin and
color of blood in dermis gives a red/pink color.
– Labial frenula (mucous folds) extend from alveolar
processes of maxilla and mandible to the upper
and lower lips, respectively.
– Many facial muscles act to move lips

• Cheeks: lateral walls of oral cavity
– Buccinator muscle
– Buccal fat pad

Palate and Palatine Tonsils
• Palate
– Hard palate: anterior, supported by palatine
process of maxilla and palatine bone
• Slightly corrugated on either side of midline raphe

– Soft palate: posterior, consists of skeletal muscle
and connective tissue
• Closes off nasopharynx during swallowing
• Uvula: projects from posterior of soft palate

• Palatine tonsils: lateral walls of fauces
– Housed within palatoglossal and palatopharyngeal
arches

The Tongue
• Primary functions include:
1) mechanical processing by compression,
abrasion, and distortion
2) manipulation to assist in chewing and
preparing the material for swallowing
3) sensory analysis by touch, temperature, and
taste receptors
4) secretion of mucins and an enzyme that aids
in fat digestion

• Tongue divisions include:
- an anterior body or oral portion
- posterior root or pharyngeal portion
- superior surface, dorsum of the body, contains
numerous papillae

• Thickened epithelium covering each papilla
provides additional friction
- many have taste buds along the edges

• Secretions of small glands of the tongue’s
epithelium extend into the lamina propria
- contain water mucins and lingual lipase (triglycerides)

• Lingual frenulum (‘small bridle’) connects the
body of the tongue to the mucosa of the oral floor

Tongue
• Functions to moves food in mouth, sensory analysis by
taste buds, participates in speech and swallowing
• Muscular organ with free anterior surface and attached
posterior surface.
– Covered with moist stratified squamous epithelium

– Intrinsic muscles: change shape of the tongue
– Extrinsic muscles: protrude or retract tongue, move
side to side
• Innervated by the hypoglossal nerve

• Lingual frenulum attaches tongue inferiorly to floor of oral
cavity

Tongue

• Terminal sulcus: groove that divides tongue into anterior 2/3 in oral cavity
posterior 1/3 in oropharynx
• Anterior region: in oral cavity; has papillae, some of which have taste buds
• Posterior region: in oropharynx; no papillae; abundant lymphoid tissue
(lingual tonsils)

Salivary Glands
• 3 pairs of salivary glands secrete into the oral
cavity – parotid, sublingual, submandibular
– each covered by a fibrous capsule
- saliva is transported through a network of fine ducts to a
single large drainage duct
- the main duct penetrates the capsule and opens onto the
surface of the oral mucosa

• Salivary glands produce 1.0 – 1.5 L saliva/day
- 70% from the submandibular; 25% from the parotid; 5%
from the sublingual
- 99.4% water + ions, buffers, metabolites, enzymes
- mucins, glycoproteins produce lubricating effects

• Parotid salivary glands - the largest ~20g
- secretions are drained by a parotid (Stensen’s) duct
- salivary amylase breaks down complex carbohydrates

• Sublingual glands - covered by the mucous
membrane of the floor of the mouth
- numerous sublingual ducts (of Rivinus) open along
either side of the lingual frenulum

• Submandibular gands – found in the floor of the
mouth along the medial surfaces of the mandible
inferior to the mylohyoid line
- submandibular (Wharton’s) ducts open into the mouth
on either side of the lingual frenulum, posterior to the teeth

Regulation of the Salivary Glands
• Controlled by the ANS – receives both
parasympathetic and sympathetic innervation
- an object placed within the mouth can stimulate receptors
triggering a salivary reflex
- innervation by CN VII, IX or X

• Parasympathetic stimulation accelerates secretion
by all of the salivary glands
- produce large amounts of watery saliva

• Sympathetic activation results in secretion of a
small volume of viscous saliva
- high enzyme concentration reduced volume produces the
sensation of dry mouth


Figure 25.6a


Figure 25.6b

The Teeth
• Perform chewing, or mastication of food
- breaks down tough CTs and plant fibers
- saturates material with salivary secretions and enzymes

• Dentine – mineralized matrix similar to bone
- contains no living cells; is the bulk of each tooth
- cytoplasmic processes extend into the dentine from cells in
the central pulp cavity (spongy and highly vascularized)
- root canal, receives the BVs and nerves for the pulp cavity:
dental artery, vein, & nerve enter through the apical foramen

• Root of each tooth is anchored into the alveolus by
collagen fibers of the periodontal ligament
- extends from the dentine of the root to the alveolar bone
- creates a strong articulation, the gomphosis

•

Enamel – covers the
dentine of the crown
- contains densely packed
calcium phosphate crystal

•

Epithelial cells of the
gingival sulcus form tight
attachments to the tooth

•

Cementum covers the
dentine of the root,
providing protection and
anchoring the periodontal
ligament
- similar to bone, less
resistant to erosion than
dentine

Figure 25.7a


Figure 25.7d

•

Incisors – blade-shaped
teeth useful for clipping or
cutting

•

Cuspids or canines – conical
with a sharp ridgeline and
pointed tip; useful for tearing
or slashing

•

Bicuspids or premolars –
contain 1 or 2 roots, have
flattened crowns with
prominent ridges; used for
crushing, mashing, and
grinding

•

Molars – very large flattened
crowns with prominent ridges,
typical have 3 or more roots;
used for crushing and grinding


Fig 25.7c

Figure 25.7b

Figure 25.7e

Mastication
• Muscles of mastication close the jaws and slide or rock
the lower jaw from side to side
- food is forced back and forth between the vestibule
and the rest of the oral cavity
- movement results in part from the masticatory muscles
but also the buccal, labial, and lingual muscles
- material is shredded and moistened with salivary
secretions
- the tongue compacts the debris into a small oval mass
or bolus, that can be swallowed

The Pharynx
• Serves as a common passageway for food, liquids,
and air – divisions include:
nasopharynx, oropharynx, laryngopharynx

• Deep to the lamina propria of the mucosa is a
dense layer of elastic fibers, bound to underlying
skeletal muscles involved in swallowing
- the pharyngeal constrictors (superior, middle, and inferior)
push the bolus toward the esophagus
- the palatopharyngeus and stylopharyngeus muscles
elevate the larynx
- the palatal muscles raise the soft palate and adjacent
portions of the pharyngeal wall

The Swallowing Process
• Pharyngeal muscles cooperate with muscles of
the oral cavity and esophagus to initiate the
swallowing process or deglutition
- complex process initiates voluntarily but proceeds
involuntarily once initiated
- divided into buccal, pharyngeal, and esophageal
phases

Figure 25.8a, b


Begins with compression of the bolus against the hard palate
Subsequent retraction of the tongue forces the bolus into the
pharynx and in elevation of the soft palate by the palatal
muscles, thereby isolating the nasopharynx
Buccal phase – strictly voluntary; however in the oropharynx,
involuntary reflexes are initiated and the bolus is moved toward
the stomach

Figure 25.8c, d


Begins when the bolus comes in contact with the palatal arches, the posterior
pharyngeal wall, or both
Elevation of the larynx by the palatopharyngeus and stylopharyngeus
muscles, and folding of the epiglottis direct the bolus past the closed glottis
In less than a second, the pharyngeal constrictor muscles propel the bolus
into the esophagus
During the time it takes for the bolus to travel through the pharynx and into the
esophagus, the respiratory centers are inhibited and breathing ceases

Starts at opening of
upper esophageal
sphincter
After passing
through - bolus is
pushed along the
esophagus by
peristaltic waves

Fig 25.8

Approach of the
bolus triggers the
opening of the weak
lower esophageal
sphincter (cardiac
sphincter) and the
bolus continues into
the stomach

The Esophagus
• A hollow muscular tube – transports foods and
liquids to the stomach
- located posterior to the trachea
- enters the peritoneal cavity through the esophageal
hiatus, before emptying into the stomach
- about 25 cm long and 2 cm in diameter
- begins at the level of the cricoid cartilage anterior to
vertebra C6 and ends anterior to vertebra T7

• Receives blood from the esophageal arteries and
branches of the
1) thyrocervical trunk & external carotid arteries of the neck,
2) bronchial arteries and esophageal arteries of the
mediastinum, and the
3) inferior phrenic artery and left gastric artery of the
abdomen

• Venous blood from the esophageal capillaries
collect
- into the esophageal, inferior thyroid, azygos, & gastric veins

• Innervation is by the vagus and sympathetic trunks
via the esophageal plexus
• Sphincter muscles are not well-defined, the
- upper and lower esophageal sphincters (cardiac sphincter),
are similar in function to other sphincters

Microscopic View of
the Esophagus

Endoscopic View of
the Esophagus

The Stomach
• Performs 3 major functions:
1) bulk storage of ingested food
2) mechanical breakdown of ingested food, and
3) chemical digestion of ingested food through
disruption of chemical bonds by acids and enzymes
• Mixing of ingested substances with acids and
enzymes secreted by stomach glands produces a
viscous, strongly acidic, soupy mixture or chyme

Figure 25.12

The stomach – intraperitoneal, occupies the left hypochondriac,
epigastric, and portions of the umbilical and left lumbar regions

Anatomy of the Stomach
• J-shaped stomach:
- short lesser curvature (medial surface), long greater
curvature (lateral surface)
- cardia, where esophagus contacts the medial surface
- cardiac orifice, esophageal lumen opens into this orifice
- fundus, contacts inferior & posterior surface of diaphargm
- body, area between the fundus and the curve of the J
- pylorus, the curve of the J, divided into the pyloric antrum
and pyloric canal ; as mixing movements occur during
digestion, the pylorus changes shape
- pyloric sphincter regulates release of chyme from the
pyloric orifice into the duodenum

Figure 25.10a

b) radiograph of the
stomach & duodenum,
after swallowing a
barium solution

Figure 25.10b, c

c) the pyloric region,
pyloric valve, & duodenum



•
•

Figure 25.11a

Stomach volume increases at mealtimes; decreases as chyme enters
the small intestine
Rugae – longitudinal folds permit expansion of the gastric lumen; during
expansion, the epithelial lining, flattens out, and the rugae become less
prominent; in a full stomach rugae almost disappear

Figure 25.11b

•
•

Greater omentum forms a large pouch (hangs like an apron from the
greater curvature – adipose tissue provides padding and insulation,
protects anterior and lateral surfaces, an important energy reserve
Lesser omentum a smaller pouch in the ventral mesentery between the
lesser curvature and liver – provides an access route for BVs and other
structures entering or leaving the liver

Blood Supply to the Stomach
Celiac artery–3 branches supply blood to the stomach
• Left gastric artery supplies blood to the lesser
curvature and cardia
• Splenic artery supplies the fundus and the greater
curvature through the left gastroepiploic artery
• Common hepatic artery supplies blood to the
lesser and greater curvatures of the pylorus
- through the right gastric artery, the right gastroepiploic
artery and the gastroduodenal artery
- gastric and gastroepiploic veins drain blood from the
stomach into the hepatic portal vein

Musculature of the Stomach
• Muscularis mucosae and muscularis externa
contain extra layers of smooth muscle
- extra layers strength the stomach wall
- perform the mixing and churning activites essential to
chyme formation

• Muscularis mucosae contains an additional outer,
circular layer of muscle fibers
• The muscularis externa has an extra inner,
oblique layer of smooth muscle

Histology of the Stomach
• Simple columnar epithelium lines all regions of the
stomach – a secretory sheet
- produces a carpet of mucus that covers the luminal
surfaces and protects the epithelium against the acids and
enzymes in the gastric lumen

• Gastric pits (shallow depressions) open onto the
gastric surface
- mucous cells at the base, or neck, actively divide to
replace superficial cells shed continuously into the chyme
- continual replacement of epithelial cells provides an
additional defense against the gastric contents ( when
stomach acid and enzymes penetrate the mucous layers)

Gastric Secretory Cells
• In the stomach fundus and body, each gastric pit
communicates with several gastric glands
- simple branched tubular glands dominated by 3 types
of secretory cells: parietal cells, chief cells, and
enteroendocrine cells (scattered between the parietal
and chief cells)
- parietal and chief cells work together to secrete about
1500 ml of gastric juice per day


Fig 25.13a, c, d

Parietal Cells
• Or oxyntic cells - secrete intrinsic factor and
hydrochloric acid (HCl)
- common along proximal portions of each gastric gland
- intrinsic factor facilitates absorption of vitamin B12
(necessary for normal erythropoiesis) across the
intestinal lining
- HCl lowers pH of the gastric juice, kills microorganisms,
breaks down cell walls and CTs in food, and activates
secretions of the chief cells

Chief Cells
• Or zymogen cells – secretes pepsinogen
- most abundant near the base of a gastric gland
- pepsinogen converted by HCl to pepsin, an active
proteolytic enzyme
- in newborn infants, also produce rennin and gastric
lipase (enzymes important for digestion of milk)
- rennin coagulates milk proteins, and gastric lipase
initiates digestion of milk fats

Enteroendocrine Cells
• Produce at least 7 different secretions
- G cells, enteroendocrine cells most abundant in gastric
pits of the pyloric region secrete the hormone gastrin
- gastrin, released when food enters the stomach,
stimulates the secretory activity of both parietal and chief
cells; also promotes smooth muscle activity in the
stomach wall (enhances mixing and churning activity)

e) parietal and mucous neck cells
– outer portion of a gastric
gland
f) chief and parietal cells –
deepest portions of a gastric
gland

Figure 25.13b, e, f

Low and High power view of
the Stomach Mucosa

Regulation of the Stomach
• Production of acid and enzymes by the gastric
mucosa can be directly controlled by the CNS
- and indirectly regulated by local hormones

• Parasympathetic innervation via the vagus nerve
- sight or thought of food triggers motor output in the vagus
nerve  postganglionic fiber stimulation of parietal cells,
chief cells, and mucous cells causes an increase in acids,
enzymes, and mucus production  arrival of food stimulates
stretch receptors in the stomach wall and chemoreceptors in
the mucosa  reflexive contractions occur in the muscularis
layers of the stomach wall and gastrin is released by
enteroendocrine cells

• Sympathetic innervation via branches of the
celiac plexus
- activation leads to the inhibition of gastric activity
- in addition, the small intestine release two hormones
that inhibit gastric secretion
- secretin and cholecystokinin stimulate secretion by
both the pancreas and liver; the depression of gastric
activity is a secondary, but complementary effect

Chemical Digestion in the Stomach
• 1. pepsinogen

HCl

pepsin (pH 1 - 3)
proteins

• 2. gastric lipase
• 3. rennin (infant only)

peptides

Mode of Hydrochloric Acid
Secretion
• Pepsid AC
• Zantac

Three Phases of Stomach Control
•
•
•

Cephalic Phase
Gastric Phase
Intestinal Phase

Stomach Regulation-First Phase
•

1. cephalic phase
a. psychic stimuli
b. vagus nerve
c. increased motility and secretion

Stomach Regulation-Cephalic Phase
Cephalic phase
PSYCHIC STIMULI

thought and anticipation of food

sight, taste, smell of food

sound of food preparation

parasympathetic output via the vagus nerve (X)

stimulation of stomach’s enteric nervous system

increased gastric secretion + increased gastric motility

Stomach Regulation-Second Phase
•

2. gastric phase
a. stretch receptors and
chemoreceptors
b. local parasympathetic response
c. gastrin

Stomach Regulation-Gastric Phase
food enters the stomach

increased stretch of stomach
wall

increased pH stimulates chemoreceptors

input to brainstem

direct stimulation of stomach’s
enteric nervous system

parasympathetic output via
the vagus nerve (X)

increased gastrin secretion

increased gastric secretion + increased gastric motility

Positive Feedback Control of
Gastric Secretion

Negative Feedback of the Gastric Phase
CONTROLLED CONDITION
Food entering stomach disrupts
homeostasis by causing an increase in
gastric juice pH AND stretch (distention) of
stomach wall

RETURN TO HOMEOSTASIS

RECEPTOR

In response, there is increased acidity in
stomach chyme and the mixing waves
begin emptying the stomach. An empty
stomach is a return to homeostasis.

Chemoreceptors and stretch receptors
increased pH and stretch of stomach wall,
and generate nerve impulses that pass to
the control centers

EFFECTORS

CONTROL CENTER
Enteric nervous system and medullary
neurons generate parasympathetic
impulses that pass to the effectors

Parietal cells of the gastric mucosa
secrete HCl and the muscularis contracts
more vigorously (increased frequency and
strength of mixing waves)

Stomach Regulation-Third Phase
•

3. intestinal phase
a. stretch receptors and
chemoreceptors
b. enterogastric reflex
c. hormones
(1) gastrin (+)
(2) cholecystokinin (CCK) (-)
(3) secretin (-)
(4) gastric inhibitory peptide
(GIP) (-)

Stomach Regulation-Intestinal Phase
chyme enters the duodenum

increased stretch of duodenal wall

increased enteric endocrine cell activity

enterogastric
reflex

direct stimulation of
duodenum’s enteric
nervous system

secretion of
cholecystokinin
secretin

input to brainstem

decreased parasympathetic
output from the vagus nerve (X)
to stomach

inhibits

inhibits

enteric gastrin

increased sympathetic
output to stomach
inhibits

decreased stomach activity

increased stomach activity

NET EFFECT
gastric inhibition

Gastric Emptying
STIMULATION OF GASTRIC EMPTYING

INHIBITION OF GASTRIC EMPTYING

distention of stomach

partially digested
proteins
alcohol
caffeine

distention of duodenum


increased vagal activity

enterogastric reflex

partially digested
proteins, fatty acids,
glucose in duodenum

secretion of
cholecystokinin and
secretin

contraction of gastroesophageal sphincter
relaxation of pyloric sphincter
increased rate of mixing waves
increased gastric secretion

contraction of pyloric sphincter
decreased rate of mixing waves
decreased gastric secretion

increased rate of emptying

decreased rate of emptying

Stomach Absorption
•
•
•
•
•

Accomplishments of digestion to this point in the GI tract
starch  maltose by salivary amylase (action stops in stomach)
proteins  partially digested proteins (action of pepsin)
lipids  partially digested fats (action of lingual and gastric lipase)
creation of chyme from food, drink, saliva, and gastric juice

•

Stomach Absorption

•
•
•
•

1.
2.
3.
4.

water
electrolytes
certain drugs (aspirin)
alcohol

The Small Intestine
• Primary role in the digestion and absorption of
nutrients – about 90%
- averages 6 m in length (range 5 -8.3m) with a diameter
ranging from 4 cm at the stomach to 2.5 cm at the junction
with the large intestine
- stabilized by mesenteries attached to the dorsal body wall
- movement during digestion is restricted by the stomach,
large intestine, abdominal wall, and pelvic girdle
- plicae circulares, transverse folds in the intestinal lining
are permanent and do not disappear as intestine fills;
roughly 800 plicae increases surface area for absorption

• 3 subdivisions – the duodenum, jejunum, ileum

The Small Intestine

Fig 25.14

The Duodenum
• Shortest and widest segment - about 25 cm long
– a mixing bowl that receives chyme from the stomach and
digestive enzymes from the pancreas and liver
- almost all digestive enzymes enter from the pancreas
- connected to the pylorus of the stomach
- interconnection guarded by the pyloric sphincter
- from its start at the pyloric sphincter, curves in a C that
encloses the pancreas
- contains numerous mucous glands and the duodenal
submucosal (Brunner’s) glands
- bile duct and pancreatic duct come together at a muscular
chamber, the duodenal ampulla or hepatopancreatic
ampulla

The Jejunum
• About 2.5 m long – where the bulk of chemical
digestion and nutrient absorption occurs
- duodenojejunal flexure, marks the boundary between
the duodenum and the jejunum (the small intestine reenters the peritoneal cavity to become intraperitoneal)
- supported by a sheet of mesentery
- plicae and villi are prominent over the proximal half
- small, isolated, individual lymphoid nodules are present
in the lamina propria

The Ileum
• Last segment and the longest, averages 3.5 m
- ends at the ileocecal valve, a sphincter which controls
flow of materials from the ileum into the cecum of the
large intestine
- plicae and villi diminish in size and number
- lymphoid nodules become more numerous and fuse
together to form large masses called aggregated
lymphoid nodules, or Peyer’s patches
- Peyer’s patches are most abundant near the entrance to
the large intestine, which normally contains large numbers
of potentially harmful bacteria

Support of the Small Intestine
• Duodenum has no supporting mesentery
• Jejunum and ileum – supported by an extensive,
fan-shaped mesentery, the mesentery proper
- BVs, lymphatics, nerves pass through the mesentery CT
- BVs involved: intestinal arteries, branches of the superior
mesenteric artery and superior mesenteric vein
- parasympathetic innervation provided by the vagus nerve
- sympathetic innervation involves postganglionic fibers
from the superior mesenteric ganglion

Histology of the Small Intestine
• Intestinal villi – fingerlike projections of the mucosa
- each villus is covered by a simple columnar epithelium
- the apical epithelial surfaces are carpeted with microvilli
(‘brush border’)
- epithelium also contain plicae circularis, each plica
supports a forest of villi; each villus is covered by epithelial
cells whose exposed surfaces contain microvilli
- arrangement increases total area for absorption to more
than 200 m2
- at the base of the villi are the entrances to the intestinal
crypts or crypts of Lieberkuhn where stem cell division
continually renew epithelial cells
- crypts also contain enteroendocrine cells that produce
several horomones, including cholecystokinin and secretin

Fig 25.15

• Lamina propria of each villus contains an
extensive network of capillaries - absorb
and carry nutrients to the hepatic portal
circulation
• Also contain a terminal lymphatic called a
lacteal – transports materials that cannot
enter the capillaries such as large lipidprotein complexes

Fig 25.15d,e

Histology of the Intestinal Wall


Regions of the Small Intestine

Figure 25.16


Scanning view of the Small Intestine
demonstrating Plicae Circularis

High power view of
the duodenal Mucosa

X-sectional view of the
duodenum

Gross view and low-power
view
of the ileum

High-power view of the Ileum
demonstrating Peyer’s patches

Regulation of the Small Intestine
• As absorption occurs, weak peristaltic contractions
slowly move materials along the small intestine
- movements are controlled primarily by neural reflexes
involving the submucosal and myenteric plexuses
- parasympathetic (vagal) stimulation increases sensitivity of
these reflexes, accelerates peristaltic contractions and
segmentation movements
- the ileocecal valve allows passage of material into the large
intestine
- hormonal and CNS controls regulate the secretory output
of intestinal juice
- sympathetic stimulation inhibits secretion

Intestinal Juice and Brush Border
Enzymes
•
•
•
•
•
•

Maltase
Lactase
Peptidases
Dextrinases
Nucleosidases
Phosphatases

Small Intestine-Mechanical
Digestion
• 1. segmentation
• 2. peristalsis

Review of Chemical Digestion of Carbohydrates
STARCH
mouth

SUCROSE

LACTOSE

SUCROSE

LACTOSE

salivary
amylase

stomach

small intestine

pancreatic
amylase

MALTOSE
brush border
maltase

brush border
sucrase

brush border
lactase

glucose + glucose

glucose + fructose

glucose + galactose

(absorbed into blood of villus)



Review the Chemical Digestion of Proteins

Review the Chemical Digestion of Lipids

Regulation of small intestinal
secretion and motility
• 1. local reflexes
• 2. parasympathetic reflexes
(vagus nerve)
• 3. gastrin

Regulation of the Small Intestine
GASTRIC PHASE
psychic stimuli

stretch of stomach

chemoreceptors in stomach

stretch of small intestine

gastroileal reflex
increased parasympathetic
impulses via vagus nerve


increased small intestinal
motility secretion
+
relaxation of ileocecal sphincter

increased enteric nervous
system activity

Small Intestine Absorption
•
•

1. monosaccharides
2. amino acids
hepatic portal blood 
liver  inferior vena cava 
general circulation

monosaccharides
amino acids

thoracic
duct
inferior
vena cava

lacteal
with
chylomicrons

hepatic
portal vein
superior
mesenteric vein

•

3. fats
triglycerides
chylomicrons
lymph  lacteals 
intestinal trunk  thoracic duct
 general circulation

•

4. water

blood
capillary

lymphatic vessel

Water absorption
• GI tract fluids/24 hours
Ingested or secreted
into GI tract

•
•
•
•
•
•
•

saliva = 1 L
ingested liquids = 2L
gastric juice = 2 L
bile = 1L
pancreatic juice = 2 L
intestinal juice = 1L
total = 9 L

Absorbed into blood

small intestine =
8L
large intestine =
0.9 L
Excreted
in feces

0.1 L

The Large Intestine
• Aka the large bowel, shaped like a horse-shoe begins at the end of the ileum, ends at the anus
- average length of ~1.5 m and a width of ~7.5 cm

• Divided into 3 parts:
1) Cecum – first portion, appears as a pouch
2) Colon - largest portion
3) Rectum – last 15 cm and the end of the
digestive tract

• Major functions of the large intestine:
1) reabsorption of water and electrolytes, and
compaction of intestinal contents into feces
2) absorption of important vitamins produced by
bacterial action
3) storage of fecal material before defecation
• Blood supply:
- receives blood from tributaries of the superior
mesenteric and inferior mesenteric arteries
- venous blood is collected by the superior
mesenteric and inferior mesenteric veins

Figure 25.17

The Cecum
• The ileum attaches to the medial surface of the
cecum – opens into the cecum at the ileal papilla
• Muscles encircling the opening form the ileocecal
valve – regulates passage of materials
• Cecum collects and stores arriving materials –
begins the process of compaction
• Vermiform appendix – ~9 cm is attached to the
posteromedial surface of the cecum
- the mesoappendix (band of mesentery) connects the
appendix to the ileum and cecum
- mucosa and submucosa dominated by lymphoid nodules,
primarily functions as a lymphoid system organ (tonsils)

The Colon
• Has a larger diameter and a thinner wall than the
small intestine
1) Wall of the colon forms a series of pouches or haustra
(sing., haustrum) – permits distension and elongation;
creases between the haustra extend into the mucosal
lining
2) Taeniae coli – 3 longitudinal ribbons of the smooth
muscles
3) Omental (fatty) appendices – teardrop-shaped sacs
of fat of the serosa

Regions of the Colon
• Colon is subdivided into 4 regions: the ascending
colon, transverse colon, descending colon, and
sigmoid colon
• The ascending colon – begins at the superior
border of the cecum
- ascends to the inferior surface of the liver
- at the right colic flexure, or hepatic flexure
colon turns to the left; marks the end of the
ascending colon and beginning of the transverse
colon

• The transverse colon – curves anteriorly at the
hepatic flexure
- crosses the abdomen from right to left
- initial segment is intraperitoneal, supported by the
transverse mesocolon
- the left side passes inferior to the greater curvature of
the stomach and becomes secondarily retroperitoneal; the
gastrocolic ligament attaches it to the stomach
- near the spleen, at the left colic flexure or splenic
flexure makes a right-angle bend and proceeds caudally

• The descending colon proceeds inferiorly along
the left side of the abdomen
- it is secondarily retroperitoneal and so it is firmly attached
to the abdominal wall
- at the iliac fossa, descending colon enters sigmoid colon

• The sigmoid colon – S-shaped segment
- only ~15 cm begins at the sigmoid flexure and ends at
the rectum
- it curves posterior to the urinary bladder and is suspended
by the sigmoid mesocolon

• The sigmoid colon discharges fecal waste into the
rectum
- the last portion, the anal canal contains small longitudinal
folds called the anal columns
- anal canal ends at the anus, or anal orifice
- epidermis close to the anus becomes keratinized and
identical to the surface of the skin
- veins in the lamina propria and submucosa of the anal
canal can become distended, producing hemorrhoids
- circular muscle layer of the muscularis externa forms the
internal anal sphincter – not under voluntary control
- external anal sphincter, a ring of skeletal muscle
encircles the distal portion of the anal canal – under
voluntary control


Figure 25.18

Histology of the Large Intestine
• Characteristics that distinguish the large intestine
from the small intestine:
- wall is relatively thin, diameter of the colon is ~3 times
larger than the small intestine
- lacks villi
- goblet cells are much more abundant
- distinctive intestinal crypts with deeper glands dominated
by goblet cells; secretion occurs as local stimuli trigger
reflexes involving the local nerve plexuses
- large lymphoid nodules are scattered throughout the
lamina propria and extend into the submucosa
- longitudinal layer of the muscularis externa reduced to the
muscular bands of the taeniae coli

Figure 25.19

High power view of the colon
demonstrating intestinal glands

Regulation of the Large Intestine
• Movement of ingested materials:
- from cecum to transverse colon occurs slowly thorugh
peristaltic activity and haustral churning,
- allows fecal material to be converted into a sludgy paste
- powerful peristaltic contractions (mass movements) from
the transverse colon occurs a few times per day
- stimulus: distension of the stomach and duodenum
- commands are relayed over the intestinal nerve plexuses
- contractions force fecal materials into the rectum causes
conscious urge to defecate leads to internal sphincter
relaxation (defecation reflex)
- fecal material moves into the anal canal
- defecation occurs by voluntary relaxation of external anal
sphincter

Chemical digestion in the
large intestine
• 1. bacteria fermentation
• 2. bacteria secrete vitamin K and some
B complex vitamins

Large Intestine Absorption
• 1. simple molecules and vitamins
• 2. most remaining water (~900 ml/day)
• Feces consists of:
1. water (about 100 ml/day)
2. undigested foodstuffs (plant fibers = cellulose)
3. bacteria
4. products of bacterial decomposition
5. sloughed epithelial cells

Defecation Reflex in the Adult
•
•

•

•

1. distention of the rectum
stimulates stretch receptors
2. sacral parasympathetic area output,
causing:
a. contraction of the descending colon,
sigmoid colon, and rectum; and
b. reflex relaxation of the
internal anal sphincter
3. voluntary relaxation of the
external anal sphincter
(in the infant, this is also reflexive)
4. expulsion of feces

Mechanical Digestion in the Large
Intestine
•
•
•

1. haustral churning
2. mass peristalsis
(gastrocolic reflex)
3. peristalsis

Fig 25.24

Inflammation of the Colon – IBD is of unknown origin
- thought to be an autoimmune disorder as an individual develops an
immune reaction to their own intestinal tract (affects ~1 million in the US)
- 2 major forms: ulcerative colitis (develop ulcers of the colon) and Crohn’s
disease (distal segment of the ileum)

Accessory Glandular Digestive Organs
• Include the salivary glands, the liver, the
gallbladder, and the pancreas
- glandular organs produce and store enzymes and
buffers essential to normal digestive function
- in addition salivary glands, liver, and pancreas have
exocrine functions

The Liver
• The largest visceral organ and one of the most
versatile – weighs ~ 1.5 kg
- lies within the right hypochondriac and epigastric
regions

• Provides essential metabolic and synthetic
actions that fall into 3 basic categories:
metabolic regulation, hematological regulation, bile
production

• Metabolic regulation - all blood leaving the
digestive tract enters the hepatic portal system
- circulating levels of carbohydrates, lipids, and amino
acids are regulated by the liver
- hepatocytes extract absorbed nutrients or toxins from
the blood before it enters the hepatic veins
- hepatocytes monitor circulating levels of metabolites and
adjust them as necessary
- excess nutrients are removed and stored, deficiencies
are corrected by mobilizing stored reserves or performing
appropriate synthetic activities
- circulating toxins and metabolic waste are removed for
subsequent inactivation, storage, or excretion
- fat-soluble vitamins (A,D,K,E) are absorbed and stored

• Hematological regulation – the liver is the largest
blood reservoir (receives ~25% of the cardiac output)
- as blood passes through the liver sinusoids:
1) phagocytic cells in the liver remove old or damaged
RBCs, cellular debris, and pathogens from circulation
2) hepatocytes synthesize plasma proteins that contribute
to the osmotic concentration of the blood, transports
nutrients, establishes clotting and complement systems

• Synthesis and secretion of bile - by the liver
- bile is stored in the gallbladder and excreted into the
lumen of the duodenum
- consists mostly of water + minor amounts of ions, bilirubin
(pigment derived from hemoglobin), and bile salts
- water and ions assist in dilution and buffering of acids in
chyme; bile salts associate with lipids in order to facilitate
their breakdown into fatty acids suitable for absorption

Anatomy of the Liver
• The largest intraperitoneal organ
- anterior surface, a ventral mesentery, the falciform
ligament, marks division between left and right lobes
- thickening in the inferior margin is the round ligament, or
ligamentum teres, a fibrous band that marks the path of
the degenerated fetal umbilical vein
- coronary ligament suspends the liver from the inferior
surface of the diaphragm
- the superior, anterior, and posterior surfaces are referred
to as the diaphragmatic surfaces
- inferior surface is referred to as the visceral surface
- inferior to the small caudate lobe is the quadrate lobe

Blood Supply to the Liver
• Afferent BVs travel within the CT of the lesser
omentum and converge at the porta hepatis
(‘doorway to the liver’)
• Hepatic artery proper and the hepatic portal
vein deliver blood to the liver
- blood returns to the systemic circuit through the hepatic
veins that open into the inferior vena cava
- the arterial supply provides oxygenated blood and the
hepatic portal vein supplies nutrients and other chemicals
absorbed from the liver

Figure 25.20

Fig 25.20

The classical description of the 4 lobes was based on the
superficial topography of the liver
New terminology subdivides the lobes into segments based on
the major subdivisions of the hepatic artery, portal vein, and
hepatic ducts

Liver Lobules
• Each lobe is divided by CT into ~100,000 liver
lobules – basic functional units of the liver
- hepatocytes form a series of plates (like spokes around
a wheel), each plate is only 1 cell thick
- exposed heptocyte surfaces are covered with short
microvilli
- sinusoids between adjacent plates empty into the central
vein
- sinusoidal lining includes a large number of Kupffer
(stellate reticuloendothelial) cells, part of the monocytemacrophage system
- Kupffer cells engulf pathogens, cell debris, damaged
blood cells and also any heavy metals (tin and mercury)

• Blood enters liver sinusoids from small branches
- 6 portal areas, or hepatic triads, one at each of the 6
corners of the lobule; contains 3 structures:
1) a branch of the hepatic portal vein
2) a branch of the hepatic artery proper
3) a small branch of the bile duct
- as blood flows through the sinusoids, hepatocytes absorb
and secrete materials into the bloodstream
- blood then leaves the sinusoids and enters the central
vein of the lobule
- central veins merge to form the hepatic veins that empty
into the inferior vena cava

CELL TYPES & ARRANGEMENT OF LIVER
Fenestrated Endothelial cells

with gut endotoxins
& other bad stuff

SINUSOID
Reticular
fiber

Hepatocytes in plates

Tight junctions
Microvilli

Kupffer cell deals

Bile canaliculus

STELLATE CELL
makes collagen
fibrils & ECM
materials

SPACE OF DISSE
has ECM materials,
but no distinct basal
lamina

Liver Histology

Figure 25.21


Bile Secretion and Transport
• Bile is secreted into a network of narrow channels
called bile canaliculi
- canaliculi eventually connect with fine bile ductules that
carry bile to a bile duct in the nearest portal area

• Right and left hepatic ducts collect bile from all
of the bile ducts of the liver lobes
- ducts unite to form the common hepatic duct that leaves
the liver
- bile within the common hepatic duct may either 1) flow
into the common bile duct that empties into the duodenum
2) enter the cystic duct that leads to the gallbladder

Bile
• 1. is a detergent
• 2. emulsification of fats
• Produced continuously at slow rate
• Secretion increased in response to:
vagus nerve – psychic and gastric phases
secretin – from the duodenum during intestinal
phase

Physiology of the Liver
1. carbohydrate metabolism
a. glycogenesis
b. glycogenolysis
c. gluconeogenesis
2. lipid metabolism

4. detoxification
5. synthesis and
excretion of bile
6. storage
7. phagocytosis of
RBCs
8. activation of vitamin D

3. protein metabolism
a. deamination (-NH2)
b. urea formation
c. plasma protein production

Gallbladder
•
•
•
•
•
•

1.
2.
3.
4.
5.
6.

anatomy
rugae
cystic duct
stores/concentrates bile
sphincter of Oddi
CCK

Biliary Tract
•
•
•
•
•
•

common hepatic duct +
cystic duct =
common bile duct +
main pancreatic duct =
ampulla of Vater
sphincter of Oddi

The Gallbladder
• Hollow pear-shaped muscular sac - stores and
concentrates bile before
- its excretion into the small intestine
- located in a fossa in the visceral surface of the right lobe

• Divided into 3 regions: fundus, body, and neck
- the cystic duct leads from the gallbladder toward the porta
hepatis, where the common hepatic duct and cystic duct
unite to create the common bile duct
- at the duodenum, a muscular hepatopancreatic sphincter
(sphincter of Oddi) surrounds the common bile duct,
contraction seals off the passageway, prevents bile from
entering the small intestine

The Gallbladder and
Associated Bile Ducts

Figure 25.22


• The gallbladder has 2 major functions – bile
storage and bile modification
- when the hepatopancreatic sphincter is closed, bile
enters the cystic duct
- in the interim bile enters the cystic duct for storage
- at capacity contains 40-70 ml of bile
- composition gradually changes: water is absorbed, bile
salts and other components become more concentrated
- bile ejection occurs under stimulation of the hormone
cholecystokinin (CCK)
- CCK released into the bloodstream at the duodenum,
when chyme arrives with large amounts of lipid and
partially digested proteins
- CCK causes relaxation of the hepatopancreatic sphincter
and contraction of the gallbladder

Regulation of Bile Secretion
REGULATION OF BILE
SECRETION
acid chyme in
duodenum

stimulated

cholecystokinin
secretion

gallbladder
contraction

relaxation of
sphincter of Oddi

release of bile into
duodenum

The Pancreas
• Lies posterior to the stomach, extends laterally
from the duodenum toward the spleen
- about 15 cm long, about 80 g (3 oz) and divided into 3
regions: broad head, slender body, short-blunted tail
- thin transparent CT capsule wraps the pancreas

• Primarily an exocrine organ – produces digestive
enzymes and buffers
- large pancreatic duct (duct of Wirsung) delivers
secretions to the duodenal ampulla
- small accessory pancreatic duct (duct of Santorini)
empties into the duodenum at the lesser duodenal papilla

Pancreas
•
•
•
•
•
•

1.
2.
3.
4.
5.
6.

gross anatomy
main pancreatic duct
hepatopancreatic ampulla
accessory pancreatic duct
99% exocrine
1% endocrine

Pancreatic Juice
•
•
•
•
•

1.
2.
3.
4.
5.

sodium bicarbonate (NaHCO3)
pancreatic amylase
pancreatic lipase and cholesterol esterase
nucleases -- DNAse and RNAse
protein-digesting enzymes
a. trypsinogen (inactive)
b. chymotrypsinogen (inactive)
c. procarboxypeptidase (inactive)

Pancreatic Regulation-Neural
Control and Endocrine Control
• 1. vagus nerve
• 2. CCK = enzymes
• 3. secretin = NaHCO3

Pancreatic Regulation
NEURAL CONTROL
psychic stimuli

stretch of stomach

increased parasympathetic
impulses via vagus nerve

increased pancreatic secretion

ENDOCRINE CONTROL
acid chyme in duodenum

stimulated

increased secretin

increased cholecystokinin

increased secretion
of bicarbonate ions

increased secretion
of enzymes

Figure 25.23

• Arterial blood reaches the
pancreas through branches of the
splenic, superior mesenteric
and common hepatic arteries.

• Major branches include the pancreatic arteries and the
pancreaticoduodenal arteries ( superior and inferior)
• Splenic vein and its branches drain the pancreas

Histology of the Pancreas
• Partitions of CT divide pancreatic tissue into lobules
- BVs and tributaries of the pancreatic ducts are found within
these CT septa

• Pancreas is a compound tubuloacinar gland
- within each lobule, ducts branch repeatedly before ending
in pancreatic acini
- a pancreatic acinus is lined by simple cuboidal epithelium,
secretes a mixture of water, ions, and digestive enzymes
(pancreatic juice) into the duodenum
- pancreatic ducts secrete buffers (1° sodium bicarbonate)
important in neutralizing the acid in chyme and stabilizing pH
- pancreatic islets are scattered between the acini, account
for only ~1% of the cellular population

Pancreatic enzymes classified
according to their intended
targets
• Lipases digest lipids
• Carbodhydrases such as
pancreatic amylase digest
sugars and starches
• Nucleases attack nucleic
acids
• Proteolytic enzymes break
proteins apart such as
proteinases (large protein
complexes) and peptidases
(small peptide chains)

Figure 25.23

Regulation of Pancreatic Secretion
• Occurs primarily in response to hormonal
instructions from the duodenum
- the hormone secretin is released when acidic chyme
arrives in the small intestine
- secretin triggers the production of watery pancreatic
juice containing buffers especially sodium bicarbonate
- a duodenal hormone, cholecystokinin, stimulates the
production and secretion of pancreatic enzymes

Aging and the Digestive System
• Rate of epithelial stem cell division declines
• Smooth muscle tone decreases
• The effects of cumulative damage become
apparent
• Cancer rates increase
• Changes in other systems have direct of indirect
effects on the digestive system

14. digestive system

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