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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
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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
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Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
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

Physically prepares food for chemical digestion.
Includes chewing, mixing of food w/ saliva by the
tongue, churning food in the stomach, and
segmentation of the intestine (mixing food
w/digestive enzymes)
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Peristalsis and Segmentation
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Figure 23.3
Gastrointestinal Tract Activities

Chemical digestion



Series of catabolic steps in which food molecules
are broken down to amino acids, simple sugars,
free fatty acids by enzymes (mouth & s. intestine)
Absorption – movement of nutrients from the GI
tract to the blood or lymph (mainly s. intestine)
Defecation – elimination of indigestible solid
wastes
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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 centers

Intrinsic control by local centers
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Receptors of the GI Tract


Mechano- and chemoreceptors respond to:

Stretch, osmolarity, and pH

Presence of substrate, and end products of digestion
They initiate reflexes that:

Activate or inhibit digestive glands that secrete digestive
enzymes into the lumen or hormones into the blood

Mix lumen contents and move them along by stimulating
smooth muscles of the GI tract walls
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Nervous Control of the GI Tract


Intrinsic controls (short reflexes)

Nerve plexuses near the GI tract initiate short
reflexes

Short reflexes are mediated by local enteric
plexuses (gut brain)
Extrinsic controls (long reflexes)

Long reflexes arising within or outside the GI tract

Involve CNS centers and extrinsic autonomic
nerves
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Nervous Control of the GI Tract
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Figure 23.4
Peritoneum and Peritoneal Cavity

Peritoneum – serous membrane of the abdominal cavity



Visceral – covers external surface of most digestive organs
& is continuous w/ the parietal peritoneum
Parietal – lines the body wall
Peritoneal cavity

Lies between the visceral & parietal peritonea

Contains serous fluid

Allows the visceral & parietal peritonea to slide across one
another
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Peritoneum and Peritoneal Cavity
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Figure 23.5a
Peritoneum and Peritoneal Cavity



Mesentery – double layer of peritoneum that extends to the
digestive organs from the body wall and provides:

Vascular, lymphatic and nerve supplies to the digestive
viscera

Hold digestive organs in place and store fat
Retroperitoneal organs – organs outside the peritoneum

Adhere to the dorsal abdominal wall and lack mesentary

E.g. pancreas and parts of the l. intestine
Peritoneal organs (intraperitoneal) – organs surrounded by
peritoneum & remain in the peritoneal cavity

E.g. stomach
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Peritoneum and Peritoneal Cavity
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Figure 23.5b
Blood Supply: Splanchnic Circulation

Involves those arteries that branch off the abdominal aorta
to serve the digestive organs & the hepatic portal
circulation

Hepatic, splenic and left gastric branches of the celiac
trunk serving the spleen, liver and stomach


Hepatic portal circulation:

Collects nutrient-rich venous blood from the
digestive viscera

Delivers this blood to the liver for metabolic
processing and storage
Mesenteric arteries serving the s. & l. intestine
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Histology of the Alimentary Canal

From esophagus to the anal canal the walls of the
GI tract have the same four tunics (layers)

From the lumen outward they are the:


mucosa, submucosa, muscularis externa, and
serosa
Each tunic has a predominant tissue type and a
specific digestive function
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Histology of the Alimentary Canal
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Figure 23.6
Mucosa (Mucous Membrane)

Moist epithelial layer that lines the lumen of the
alimentary canal

Three major functions:


Secretion of mucus, enzymes & hormones

Absorption of end products of digestion into the
blood

Protection against infectious disease
Consists of three sublayers: a lining epithelium,
lamina propria, and muscularis mucosae
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Mucosa: Epithelial Lining

Simple columnar epithelium and mucus-secreting
goblet cells
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Mucus secretions:


Protect digestive organs from digesting themselves

Ease food along the tract
Stomach and small intestine mucosa contain:

Enzyme-secreting cells

Hormone-secreting cells (making them endocrine
and digestive organs)
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Mucosa: Lamina Propria and Muscularis
Mucosae
 Lamina Propria


Loose areolar and reticular connective tissue

Nourishes the epithelium and absorbs nutrients

Contains lymph nodes (part of MALT) important in
defense against bacteria
Muscularis mucosae – smooth muscle cells that
produce local movements of mucosa
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Mucosa: Other Sublayers
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

Submucosa – dense connective tissue containing elastic fibers,
blood and lymphatic vessels, lymph nodes, and nerves
Muscularis externa

responsible for segmentation and peristalsis

Inner circular layer that may thicken to form sphincters

Outer longitudinal layer
Serosa – the protective visceral peritoneum

Replaced by the fibrous adventitia in the esophagus

Retroperitoneal organs have both an adventitia and serosa
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Enteric Nervous System

Composed of two major intrinsic nerve plexuses:


Submucosal nerve plexus – regulates glands and
smooth muscle in the mucosa
Myenteric nerve plexus – Located between the
circular & longitudinal muscle layers.


Major nerve supply that controls GI tract
mobility
Segmentation and peristalsis are largely automatic
involving local reflex arcs
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Enteric Nervous System

Linked to the CNS via long autonomic reflex arc

Parasympathetic inputs enhance secretory activity
and motility
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Sympathetic impulses inhibit digestive activities
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Saliva: Source and Composition

Secreted from serous and mucous cells of salivary glands

97-99.5% water, hyposmotic, slightly acidic solution
containing

Electrolytes – Na+, K+, Cl–, PO42–, HCO3–

Digestive enzyme – salivary amylase

Proteins – mucin, lysozyme, defensins, and IgA

Metabolic wastes – urea and uric acid

Lingual lipase (enzyme) digests fat

Protection agaisnt microrganisms: e.g. IgA & lysozyme
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Control of Salivation
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Intrinsic salivary glands secrete saliva continuously to keep mouth moist

Extrinsic salivary glands: submandibular (submax.), sublingual &
parotid are activated when food enters the mouth

Extrinsic salivary glands secrete serous, enzyme-rich saliva in response
to:

Ingested food which stimulates chemoreceptors and mechanoreceptors

The thought of food

Controlled by parasympathetic nervous system

Sympathetic N.S. releases mucin rich saliva

Strong activation of the sympathetic N.S. inhibits saliva release (dry
mouth)
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Control of Salivation

Chemo- & mechanoreceptors send signals to the salivary
nuclei in the brain stem (pons & medulla)


Chemoreceptors: activated strongly by acidic substances
Mechanoreceptors: “
stimulus
“ any mechanical

Efferent impulses are sent via motor fibers in the facial &
glossopharyngeal nerves

Strong sympathetic stimulation inhibits salivation and
results in dry mouth
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Pharynx

From the mouth, the oro- and laryngopharynx
allow passage of:

Food and fluids to the esophagus

Air to the trachea

Lined with stratified squamous epithelium and
mucus glands

Has two skeletal muscle layers

Inner longitudinal

Outer pharyngeal constrictors
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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
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Digestive Processes in the Mouth

Mouth & salivary glands are involved in the digestive
process

Food is ingested

Mechanical digestion begins (chewing)

Propulsion is initiated by swallowing

Salivary amylase begins chemical breakdown of
polysaccharides (starch & glycogen)

Lingual lipase secreted in the mouth acts in the acidic
condition of the stomach

The pharynx and esophagus serve only as conduits to
pass food from the mouth to the stomach
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Mastication (Chewing)

Partly voluntary, partly reflexive

The pattern & rhythm of continued jaw movements
are controlled mainly by stretch reflexes and in
response to pressure inputs from receptors in the
cheeks, gums, tongue
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Deglutition (Swallowing)

Food is first compacted by the tongue

Deglutination involves the coordinated activity of
the tongue, soft palate, pharynx, esophagus, and 22
separate muscle groups
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Deglutition (Swallowing)

Deglutination involves 2 main phases:

1) Buccal phase (mouth)


Voluntary

Bolus is forced into the oropharynx

Bolus stimulates mechanoreceptors in the pharynx generating reflex
activity
2) Pharyngeal-esophageal phase

Involuntary

Motor impulses from medulla & pons via the vagus nerve cause
contraction of muscles of the pharynx & esophagus

All routes except into the digestive tract are sealed off

Peristalsis moves food through the pharynx to the esophagus

The gastroesophogeal sphincter relaxes as the peristaltic wave reaches
the end of the esophagus allowing food to enter the stomach
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Bolus of food
Tongue
Uvula
Pharynx
Epiglottis
Bolus
Epiglottis
Glottis
Esophagus
Trachea
(a) Upper esophageal
sphincter contracted
Bolus
(b) Upper esophageal
sphincter relaxed
Relaxed
muscles
Bolus of
food
Longitudinal
muscles
contract,
shortening
passageway
ahead of bolus
Gastroesophageal
sphincter closed
(d)
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(c) Upper esophageal
sphincter contracted
Circular muscles
contract,
constricting
passageway
and pushing
bolus down
Relaxed
muscles
Gastroesophageal
sphincter open
Stomach
(e)
Figure 23.13
Stomach

Temporary storage area

Chemical breakdown of proteins begins and food
is converted to chyme


Nerve supply – sympathetic and parasympathetic
fibers of the autonomic nervous system
Blood supply – celiac trunk, and corresponding
veins (part of the hepatic portal system)
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Figure 23.14a
Digestion in the Stomach

The stomach:

Holds ingested food

Degrades this food both physically and chemically

Delivers chyme to the small intestine

Enzymatically digests proteins with pepsin


Protein digestion is the only significant type of
enzymatic digestion that occurs in the stomach
Secretes intrinsic factor required for absorption of vitamin
B12
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Digestion in the Stomach

Proteins are denatured by HCl produced by stomach glands
to prepare proteins for enzymatic cleavage

Major enzymes in stomach:

Pepsinogen (inactive) is released by chief cells and is
cleaved by HCl forming pepsin (active) in the stomach
lumen (protective mechanism)

Rennin: cleaves casein & produced by infants

Lingual lipase (intrinsic salivary gland): triglyceride
digestion

Intrinsic factor: required for intestinal absorption of vitamin
B12 needed to produce mature RBCs
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Regulation of Gastric Secretion

Neural and hormonal mechanisms regulate the
release of gastric juice

Nervous control is provided by long (vagus) and
short (enteric) nerve reflexes

When the vagus nerves stimulate the stomach,
secretory activity in all its glands increases

The hormone gastrin stimulates secretion of
enzymes and HCl
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Regulation of Gastric Secretion

Stimulatory and inhibitory events occur in three
phases

Cephalic (reflex) phase: prior to food entry

Gastric phase: once food enters the stomach

Intestinal phase: as partially digested food enters
the duodenum

The effector site in all 3 phases is the stomach
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Phase 1: Cephalic Phase

Reflex phase that occurs before food enters stomach

Several minutes in duration

Excitatory events include:


Sight, smell, taste or thought of food

Stimulation of taste or smell receptors relays signals to the
hypothalamus which stimulates the vagal nuclei of the
medulla oblongata causing motor impulses to be
transmitted via the vagus nerve to parasympathetic enteric
ganglion which stimulate stomach glands
Inhibitory events include:

Loss of appetite or depression

Decrease in stimulation of the parasympathetic division
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Phase 2: Gastric Phase

Once food reaches the stomach

Several hours in duration

Excitatory events include:

Stomach distension

Activation of stretch receptors (neural activation)

Activation of chemoreceptors by peptides, caffeine, and
rising pH


All of these initiate both local reflexes and long
vagal reflexes resulting in ACh release causing…
Release of gastrin to the blood and increase in HCl
production in the stomach
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Phase 2: Gastric Phase (cont.)

Inhibitory events include:

A pH lower than 2

Emotional upset that overrides the parasympathetic
division
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Phase 3: Intestinal Phase


Excitatory phase – low pH; partially digested food enters
the duodenum of the s. intestine stimulating intestinal
mucosal cells to release hormones that mimic gastrin
(intestinal gastrin)
Inhibitory phase – distension of duodenum, presence of
fatty, acidic, or hypertonic chyme, and/or irritants in the
duodenum initiates the enterogastric reflex:

1) Initiates inhibition of local reflexes

2) Inhibition of vagal nuclei in the medulla

3) Activate sympathetic fibers constricting the pyloric
sphincter preventing entry into the s. intestine
Thus, gastric secretory activity stops protecting the s.
intestine from excess acid
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Release of Gastric Juice: Stimulatory Events
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Figure 23.16.1
Release of Gastric Juice: Inhibitory Events
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Figure 23.16.2
The Hormone Gastrin

Chemical stimuli provided by partially digested
proteins, caffeine, and rising pH activate gastrin
secreting cells (G cells) in the stomach stimulating
them to produce more HCl

As pH falls, G cells are inhibited

G cells are also activated by neural reflexes (fear,
anxiety, etc…) and the sympathetic N.S. overrides
the parasympathetic N.S.
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Regulation and Mechanism of HCl Secretion

HCl secretion is stimulated by ACh, histamine, and
gastrin through second-messenger systems (e.g
Ca++ for ACh & gastrin; cAMP for histamine)

Release of HCl:


Is low if only one ligand binds to parietal cells

Is high if all three ligands bind to parietal cells
Antihistamines block H2 receptors and decrease
HCl release
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Regulation and Mechanism of HCl Secretion
Bicarbonate
Alkaline Tide: blood draining
from the stomach is more
alkaline than the blood serving
it
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Figure 23.17
Response of the Stomach to Filling

Stomach pressure remains constant until about 1L
of food is ingested

Relative unchanging pressure results from reflexmediated muscle relaxation
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Gastric Contractile Activity

Peristaltic waves begin at the gastroesophogeal sphincter and
move toward the pylorus at the rate of 3 per minute

Contractions become more powerful approaching the pylorus
region (not much going on at the fundus or body)

Pyloric valve acts as a filter letting only liquid and small
particles to pass.
Retropulsion: back and forth mixing
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Gastric Contractile Activity

Basic electrical rhythm (BER) is initiated by pacemaker
cells in the longitudinal smooth muscle layer

Muscle-like noncontractile cells depolarize and repolarize
spontaneously 3x/min.

Coupled to smooth muscle cells via gap junctions

Depolarization is brought to threshold by neural and
hormonal factors, the same factors that enhance gastric
secretory activity

E.g. stretch receptors & gastrin secreting cells
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Regulation of Gastric Emptying

Stomach empties w/i 4hrs after a meal

Gastric emptying is regulated by:


The neural enterogastric reflex

Hormonal (enterogastrone) mechanisms
These mechanisms inhibit gastric secretion and
duodenal filling
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Regulation of Gastric Emptying

The rate of gastric emptying depends more on the
contents of the duodenum than those of the
stomach

Carbohydrate-rich chyme quickly moves
through the duodenum

Fat-laden chyme is digested more slowly
causing food to remain in the stomach longer
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Small Intestine: Modifications for Absorption

Large surface area due to length (13 ft. in a living person, 30 ft.
in a cadaver)

Microscopic structures like circular folds, villi and microvilli
that further increase the surface area

Most absorption occurs
in the proximal region
of the s. intestine
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Small Intestine: Microscopic Anatomy

Structural modifications of the small intestine wall increase surface
area



Circular folds: 1 cm. Deep circular folds of the mucosa and
submucosa
Villi – 1 mm. Fingerlike extensions of the mucosa

Capillary bed is found in the core of each villus as
well as a lacteal (lymph capillary bed)

Smooth muscle is also found in the core of each
villus allowing it to shorten & lengthen as needed
Microvilli – tiny projections of absorptive mucosal cells’ plasma
membranes

Brush border: projections of the microvilli

Contain enzymes that complete the breakdown of
proteins & carbohydrates
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Duodenum and Related Organs
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Figure 23.20
Small Intestine: Histology of the Wall

The epithelium of the mucosa is made up of:

Absorptive cells and goblet cells (mucus producing
cells)

Enteroendocrine cells

Interspersed T cells called intraepithelial
lymphocytes (IELs)

IELs immediately release cytokines upon
encountering Ag
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Small Intestine: Histology of the Wall

Cells of intestinal crypts secrete intestinal juice

Paneth cells release antimicrobial agents and
protect the s. intestine from breaching bacteria


Peyer’s patches: lymphoid follicles
Duodenal (Brunner’s) glands in the duodenum
secrete alkaline mucus that neutrilizes acidic
chyme
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Intestinal Juice

Intestinal juice is alkaline: pH 7.4 - 7.8

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
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Liver and Gallbladder

Accessory organs associated w/ the s. intestine

Livers digestive function is to produce bile for
export to the duodenum

Bile is a fat emulsifier (breaks fat globules into
smaller fat globules)
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Gallbladder and Associated Ducts
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Figure 23.20
Liver: Microscopic Anatomy

Hexagonal-shaped liver lobules are the structural
and functional units of the liver

Composed of hepatocyte (liver cell) plates
radiating outward from a central vein

Portal triads are found at each of the six corners of
each liver lobule
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Figure 23.24c
Liver: Microscopic Anatomy

Portal triads consist of a bile duct and


Hepatic artery – supplies oxygen-rich blood to the
liver
Hepatic portal vein – carries venous blood with
nutrients from digestive viscera
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Figure 23.24d
Liver: Microscopic Anatomy


Liver sinusoids – enlarged, leaky capillaries
located between hepatic plates
Kupffer cells – hepatic macrophages found in liver
sinusoids
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Liver: Microscopic Anatomy

Hepatocytes’ functions include:

Production of bile

Store glucose & glycogen

Processing bloodborne nutrients

Storage of fat-soluble vitamins

Detoxification: convert ammonia to urea

Secreted bile flows in bile canaliculi towards bile ducts

Bile in the bile ducts flows in the opposite direction of
blood flow
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Composition of Bile

A yellow-green, alkaline solution containing bile
salts, bile pigments, cholesterol, neutral fats,
phospholipids, and electrolytes

Only bile salts and phospholipids contribute to
digestive process

Bile salts are cholesterol derivatives that:

Emulsify fat (the way dish soap works)

Facilitate fat and cholesterol absorption

Help solubilize cholesterol
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Composition of Bile

Enterohepatic circulation recycles bile salts

1) bile salts are reabsorbed into the blood by the
ileum

2) returned to the liver via hepatic portal blood

3) resecreted in newly formed bile

Bilirubin: The chief bile pigment, a waste product of
heme

Bile salts are the major stimulus for enhanced bile
secretion
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Regulation of Bile Release
4 Vagal stimulation causes
weak contractions of
gallbladder
3 Bile salts
and secretin
transported via
bloodstream
stimulate liver
to produce bile
more rapidly
5 Cholecystokinin
(via bloodstream)
causes gallbladder
to contract and
hepatopancreatic
sphincter to relax;
bile enters
duodenum
1 Acidic, fatty chyme
entering duodenum causes
release of cholecystokinin
and secretin from
duodenal wall
enteroendocrine cells
2 Cholecystokinin
and secretin enter the
bloodstream
6 Bile salts reabsorbed into blood
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Figure 23.25
The Gallbladder

Thin-walled, green muscular sac on the ventral
surface of the liver

Stores bile that is not immediately needed for
digestion and concentrates it by absorbing water
and ions

Upon muscular contraction (vagal stimulation),
bile is secreted thru the cystic duct than the bile
duct and into the duodenum
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Regulation of Bile Release

During periods of fasting, the hepatopancreatic sphincter is
closed, backing up bile into the gall bladder

Acidic, fatty chyme causes the duodenum to release:


Cholecystokinin (CCK) is the major stimulus for
gallbladder contraction.

CCK also stimulates secretion of pancreatic juice

CCK also relaxes the hepatopancreatic sphincter
RESULT: Bile enters the duodenum
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Pancreas

Accessory digestive organ

Location

Lies deep to the greater curvature of the stomach

The head is encircled by the duodenum and the tail
abuts the spleen
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Pancreas

Exocrine function

Secretes pancreatic juice which breaks down all categories of foodstuff

Acini (clusters of secretory cells) contain zymogen granules with
digestive enzymes
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Pancreas

Pancreatic juice drains into the main pancreatic
duct which in turn fuses w/ the bile duct

The pancreatic accessory duct drains directly into
the duodenum

Acini manufacture the digestive enzymes
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Pancreatic Juice
pH of 8.0: helps neutralize acidic chyme
Proteases made by the pancreas become active
in the duodenum (prevents self-digestion)
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Figure 23.27
Composition and Function of Pancreatic Juice


Water solution of enzymes and electrolytes
(primarily HCO3–)

Neutralizes acid chyme

Provides optimal environment for pancreatic
enzymes
Enzymes are released in inactive form and
activated in the duodenum
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Composition and Function of Pancreatic Juice


Examples include

Trypsinogen is activated to trypsin

Procarboxypeptidase is activated to
carboxypeptidase
Active enzymes secreted

Amylase, lipases, and nucleases

These enzymes require ions or bile for optimal
activity
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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
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Regulation of Pancreatic Secretion
During cephalic and gastric
phases, stimulation by
vagal nerve fibers causes
release of pancreatic juice
and weak contractions of
the gallbladder.
1 Acidic chyme entering
duodenum causes the
enteroendocrine cells of
the duodenal wall to release
secretin, whereas fatty,
protein-rich chyme induces
release of cholecystokinin.
2 Cholecystokinin
and secretin enter
bloodstream.
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3 Upon reaching the
pancreas, cholecystokinin
induces the secretion of
enzyme-rich pancreatic juice;
secretin causes copious
secretion of bicarbonate-rich
pancreatic juice.
Figure 23.28
Digestion in the Small Intestine

As chyme enters the duodenum:

Carbohydrates and proteins are only partially
digested

No fat digestion has taken place
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Digestion in the Small Intestine

Digestion continues in the small intestine

Chyme is released slowly into the duodenum

Because it is hypertonic and has low pH, mixing is
required for proper digestion

Required substances needed are supplied by the
liver

Virtually all nutrient absorption takes place in the
small intestine

Most H2O absorption also occurs in the s. intestine
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Requirements for Optimal Intestinal Digestive Activity

Most substances required for digestion are
imported from the liver and pancreas

Chyme must be released into the intestine slowly


Chyme is hypertonic

If chyme is released to fast, low blood volume
will result
Low pH of chyme must be adjusted by bile and
pancreatic juice
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Motility in the Small Intestine

The most common motion of the small intestine is segmentation

It is initiated by intrinsic pacemaker cells (Cajal cells) in the
longitudinal smooth muscle layer

Depolarize frequently (12-14x/min) vs. ileum (8-9x/min)

Intensity of segmentation is altered by long and short reflexes
(Parasymp. enhances; Symp. Decreses)

The hormone motilin is released by duodeneal mucosa and
initiates the peristaltic waves in the proximal duodenum

Migrating Motility Complex: sweeping waves of contraction
along the s. intestine

Local enteric neurons coordinate these intestinal motility patterns

Moves contents steadily toward the ileocecal valve
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Control of Motility

Impulses sent proximally by cholinergic effector
neurons cause:


Contraction and shortening of the circular muscle
layer
Impulses sent distally by cholinergic effector
neurons cause:

Shortening of longitudinal muscle

Distension of the intestine
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Control of Motility

Two mechanisms (hormonal & neural) cause
ileocecal sphincter to relax and allow food residues
to enter the cecum

1) Gastroileal reflex: enhanced activity of
stomach forces segmentation in the ileum

2) Gastrin: released by the stomach, gastrin
increases motility in the ileum and relaxes the
iliocecal sphincter

Back-pressure of chyme in the cecum
forces the sphincter closed
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Large Intestine

Has three unique features:



Teniae coli – three bands of longitudinal smooth
muscle in its muscularis
Haustra – pocketlike sacs caused by the tone of the
teniae coli
Epiploic appendages – fat-filled pouches of
visceral peritoneum
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Large Intestine

Is subdivided into the cecum, appendix, colon, rectum, and
anal canal

1.5 m in length

Primary function is to absorb H2O from indigestable food
residues, temporarily store them, and finally excrete them

No breakdown of foodstuff occurs in the L. intestine

The saclike cecum:

Lies below the ileocecal valve in the right iliac fossa

Contains a wormlike vermiform appendix
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Large Intestine
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Figure 23.29a
Large Intestine: Microscopic Anatomy

Contains no villi and no cells that secrete digestive
enzymes

Colon mucosa is simple columnar epithelium
except in the anal canal

Has numerous deep crypts lined with goblet cells

Abundant supply of mucus eases the passage of
feces and protects the colon against acids and
gases released by resident bacteria
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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
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Bacterial Flora


The bacterial flora (over 700 species!) 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 colon

Metabolize some host derived proteins

Ferment indigestible carbohydrates

Release irritating acids and gases (flatus)

Synthesize B complex vitamins and vitamin K used by the
liver to make clotting proteins
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Containment of Bacterial Flora

The gut mucosa release chemicals that recruit
dendritic cells

Dendritic cells sample the lumen with cytoplasmic
extensions and present antigens to T cells

T cells initiate an IgA antibody response restricted
to the gut lumen and keeps bacteria in check if
they were to breach the mucosa
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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
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Motility of the Large Intestine

Generally poor contractile ability

Haustral contractions


Slow segmenting movements of the transverse and
descending colon that move the contents of the colon (last
about 1 min. and occur every 30 sec.)

Haustra sequentially contract as they are stimulated by
distension
Presence of food in the stomach:

Activates the gastrocolic reflex

Initiates peristalsis that forces contents toward the rectum
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Defecation



Distension of rectal walls caused by feces:

Stimulates contraction of the rectal walls

Relaxes the internal anal sphincter
Voluntary signals stimulate relaxation of the
external anal sphincter and defecation occurs
Valsalva’s maneuver helps force fecal material out
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Chemical Digestion

Large food products are broken down to monomers
small enough to be absorbed by the GI tract lining

Accomplished by enzymes secreted into the lumen

Breakdown is called hydrolysis and involves the
addition of H2O
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Chemical Digestion: Carbohydrates

Simple sugars (monosaccharides) are absorbed
immediately, e.g. glucose, fructose, galactose

Breakdown must occur w/ disaccharides, e.g.
sucrose, lactose, maltose & w/ polysaccharides,
e.g. starch and glycogen
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Chemical Digestion: Carbohydrates

Digestion begins in the mouth

Salivary amylase: splits starch into oligosaccharides (2-8
linked glucose molecules) and is completed in the stomach
until salivary amylase is denatured and digested by
enzymes

Pancreatic amylase: breaks down starch completely to
oligosaccharides in the S. intestine

Intestinal brush border enzymes further digest products to
monosaccharides

Chemical digestion ends in the S. Intestine
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Carbohydrate Absorption

Glucose and galactose are transported by
secondary active transport into epithelial cells by
protein carriers
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Chemical Digestion: Proteins

Source of digested proteins: food, enzymes released into
the GI tract, sloughed cells into the lumen of the GI tract

Protein digestion begins in the stomach when pepsinogen
is activated to pepsin which is functionally active between
pH 1.5-2.5

Pepsin is then inactivated at high pH in the duodenum

Protein is further broken down by trypsin and
chymotrypsin secreted by the pancreas into the S. Intestine

Brush border enzymes carboxypepsidase &
aminopepsidase split off 1 amino acid at a time from the
carboxy and amino ends
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Protein Absorption

Carriers (active transport) are used to transport
amino acids into epithelial cells and enter the
capillary blood by diffusion
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Figure 23.34
Chemical Digestion: Fats

S. Intestine is the sole site for lipid digestion

Pancreas is the only significant source of lipases, fat
digesting enzymes

Triglycerides are treated with bile salts before being
digested (in duodenum)

Bile salts allow triglycerides to interact with H2O forming
emulsions.

Does not break chemical bonds, but increases the number
of triglycerides exposed to pancreatic lipases

Lipases break down fats by cleaving off 2 fatty acid chains
leaving free fatty acids and monoglycerides
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Chemical Digestion: Fats

Glycerol and short chain fatty
acids are:

Absorbed into the capillary
blood in villi

Transported via the hepatic
portal vein
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Fatty Acid Absorption

Fatty acids and monoglycerides associate with bile salts
forming micelles and diffuse between the microvilli

Lipids leave the micelles and enter intestinal cells via
diffusion

Free fatty acids and monoglycerides are resynthesized into
triglycerides in the epithelial cells

Triglycerides are then combined with proteins forming
chylomicrons that enter lacteals and are transported to the
circulation via lymph

Once back in the blood, triglycerides are hydrolyzed to
free fatty acids and glycerol and used by tissue cells
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Fatty Acid Absorption
Fatty acids and
monoglycerides
associated with
micelles in
lumen of intestine
Lumen of
intestine
1 Fatty acids and
monoglycerides
resulting from fat
digestion leave
micelles and enter
epithelial cell by
diffusion.
Absorptive
epithelial cell
cytoplasm
ER
Golgi
apparatus
2 Fatty acids are
used to synthesize
triglycerides in
smooth endoplasmic reticulum.
3 Fatty globules are
combined with
proteins to form
chylomicrons
(within Golgi
apparatus).
4 Vesicles containing
chylomicrons
migrate to the
basal membrane,
are extruded from
the epithelial cell,
and enter a lacteal
(lymphatic capillary).
Chylomicron
Lacteal
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5 Lymph in the
lacteal transports
chylomicrons away
from intestine.
Figure 23.36
Electrolyte Absorption


Most ions are actively absorbed along the length of
small intestine

Na+ is coupled with absorption of glucose and
amino acids

Ionic iron is transported into mucosal cells where it
binds to ferritin
Anions passively follow the electrical potential
established by Na+
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Electrolyte Absorption

K+ diffuses across the intestinal mucosa in
response to osmotic gradients

Ca2+ absorption:

Is related to blood levels of ionic calcium

Is regulated by vitamin D and parathyroid hormone
(PTH)
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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
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