Digestive System
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
Digestive System:
Overview
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
Gastrointestinal
Tract Activities
•
Ingestion – taking food into the digestive tract
•
Propulsion – swallowing and peristalsis
– Peristalsis
–
waves of contraction and relaxation of muscles in the organ walls
•
Mechanical digestion – chewing, mixing, and churning food
Peristalsis and
Segmentation
Gastrointestinal
Tract Activities
•
Chemical digestion – catabolic breakdown of food
•
Absorption – movement of nutrients from the GI tract to the blood or lymph
•
Defecation – elimination of indigestible solid wastes
GI Tract
•
External environment for the digestive process
•
Regulation of digestion involves:
– Mechanical
and chemical stimuli – stretch receptors, osmolarity, and presence of substrate in the lumen
– Extrinsic
control by CNS centers
– Intrinsic
control by local centers
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
– Mix
lumen contents and move them along
Nervous Control of
the GI Tract
•
Intrinsic controls
– Nerve
plexuses near the GI tract initiate short reflexes
– Short
reflexes are mediated by local enteric plexuses (gut brain)
•
Extrinsic controls
– Long
reflexes arising within or outside the GI tract
– Involve
CNS centers and extrinsic autonomic nerves
Nervous Control of
the GI Tract
Peritoneum and
Peritoneal Cavity
•
Peritoneum – serous membrane of the abdominal cavity
– Visceral
–
covers external surface of most digestive organs
– Parietal
–
lines the body wall
•
Peritoneal cavity
– Lubricates
digestive organs
– Allows
them to slide across one another
•
Mesentery – double layer of peritoneum that provides:
– Vascular
and nerve supplies to the viscera
– A
means to hold digestive organs in place and store fat
•
Retroperitoneal organs – organs outside the peritoneum
•
Peritoneal organs (intraperitoneal) –
organs surrounded by peritoneum
Blood Supply:
Splanchnic Circulation
•
Arteries and the organs they serve include
– The
hepatic, splenic, and left gastric: spleen, liver, and stomach
– Inferior
and superior mesenteric: small and large intestines
•
Hepatic portal circulation:
– Collects
nutrient-rich venous blood from the digestive viscera
– Delivers
this blood to the liver for metabolic processing and storage
Histology of the
Alimentary Canal
•
From esophagus to the anal canal the walls of the GI tract have the same
four tunics
– 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
Mucosa
•
Moist epithelial layer that lines the lumen of the alimentary canal
•
Its three major functions are:
– Secretion
of mucus
– Absorption
of the end products of digestion
– Protection
against infectious disease
•
Consists of three layers: a
lining epithelium, lamina propria, and muscularis mucosae
Mucosa: Epithelial
Lining
•
Consists of simple columnar epithelium and mucus-secreting goblet cells
•
The 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)
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
Mucosa: Other
Sublayers
•
Submucosa – dense connective tissue containing elastic fibers, blood and lymphatic
vessels, lymph nodes, and nerves
•
Muscularis externa – responsible for segmentation and peristalsis
•
Serosa – the protective visceral peritoneum
– Replaced
by the fibrous adventitia in the esophagus
– Retroperitoneal
organs have both an adventitia and serosa
Mouth
•
Oral or buccal cavity:
– Is
bounded by lips, cheeks, palate, and tongue
– Has
the oral orifice as its anterior opening
– Is
continuous with the oropharynx posteriorly
•
To withstand abrasions:
– The
mouth is lined with stratified squamous epithelium
– The
gums, hard palate, and dorsum of the tongue are slightly keratinized
Lips and Cheeks
•
Have a core of skeletal muscles
– Lips:
orbicularis oris
– Cheeks:
buccinators
•
Vestibule – bounded by the lips and cheeks externally, and teeth and gums internally
•
Oral cavity proper – area that lies within the teeth and gums
•
Labial frenulum – median fold that joins the internal aspect of each lip to the gum
Palate
•
Hard palate – underlain by palatine bones and palatine processes of the maxillae
– Assists
the tongue in chewing
– Slightly
corrugated on either side of the raphe (midline ridge)
•
Soft palate – mobile fold formed mostly of skeletal muscle
– Closes
off the nasopharynx during swallowing
– Uvula
projects downward from its free edge
•
Palatoglossal and palatopharyngeal arches form the borders of the fauces
Tongue
•
Occupies the floor of the mouth and fills the oral cavity when mouth is
closed
•
Functions include:
– Gripping
and repositioning food during chewing
– Mixing
food with saliva and forming the bolus
– Initiation
of swallowing, and speech
•
Intrinsic muscles change the shape of the tongue
•
Extrinsic muscles alter the tongue’s
position
•
Lingual frenulum secures the tongue to the floor of the mouth
•
Superior surface bears three types of papillae
– Filiform
– give
the tongue roughness and provide friction
– Fungiform
–
scattered widely over the tongue and give it a reddish hue
– Circumvallate
–
V-shaped row in back of tongue
•
Sulcus terminalis – groove that separates the tongue into two areas
– Anterior
2/3 residing in the oral cavity
– Posterior
third residing in the oropharynx
Salivary Glands
•
Produce and secrete saliva that:
– Cleanses
the mouth
– Moistens
and dissolves food chemicals
– Aids
in bolus formation
– Contains
enzymes that break down starch
•
Three pairs of extrinsic glands –
parotid, submandibular, and sublingual
•
Intrinsic salivary glands (buccal glands) –
scattered throughout the oral mucosa
•
Parotid – lies anterior to the ear between the masseter muscle and skin
– Parotid
duct –
opens into the vestibule next to the second upper molar
•
Submandibular – lies along the medial aspect of the mandibular body
– Its
ducts open at the base of the lingual frenulum
•
Sublingual – lies anterior to the submandibular gland under the tongue
– It
opens via 10-12 ducts into the floor of the mouth
Saliva: Source and
Composition
•
Secreted from serous and mucous cells of salivary glands
•
A 97-99.5% water, hypo-osmotic, 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
Control of
Salivation
•
Intrinsic glands keep the mouth moist
•
Extrinsic salivary glands secrete serous, enzyme-rich saliva in response
to:
– Ingested
food which stimulates chemoreceptors and pressoreceptors
– The
thought of food
•
Strong sympathetic stimulation inhibits salivation and results in dry
mouth
Teeth
•
Primary and permanent dentitions have formed by age 21
•
Primary – 20 deciduous teeth that erupt at intervals between 6 and 24 months
•
Permanent – enlarge and develop causing the root of deciduous teeth to be resorbed
and fall out between the ages of 6 and 12 years
–
All but the third molars have erupted by the end of adolescence
–
There are usually 32 permanent teeth
Deciduous Teeth
Permanent Teeth
Classification of
Teeth
•
Teeth are classified according to their shape and function
– Incisors
–
chisel-shaped teeth adapted for cutting or nipping
– Canines
–
conical or fanglike teeth that tear or pierce
– Premolars
(bicuspids) and molars – have broad crowns with rounded tips and are best suited for grinding or
crushing
•
During chewing, upper and lower molars lock together generating crushing
force
Dental Formula:
Permanent Teeth
•
A shorthand way of indicating the number and relative position of teeth
– Written
as ratio of upper to lower teeth for the mouth
– Primary:
2I (incisors), 1C (canine), 2M (molars)
– Permanent:
2I, 1C, 2PM (premolars), 3M
Tooth Structure
•
Two main regions – crown and the root
•
Crown – exposed part of the tooth above the gingiva (gum)
•
Enamel – acellular, brittle material composed of calcium salts and hydroxyapatite
crystals is the hardest substance in the body
– Encapsules
the crown of the tooth
•
Root – portion of the tooth embedded in the jawbone
•
Neck – constriction where the crown and root come together
•
Cementum – calcified connective tissue
–
Covers the root
–
Attaches it to the periodontal ligament
•
Periodontal ligament
–
Anchors the tooth in the alveolus of the jaw
–
Forms the fibrous joint called a gomaphosis
•
Gingival sulcus – depression where the gingiva borders the tooth
•
Dentin – bonelike material deep to the enamel cap that forms the bulk of the
tooth
•
Pulp cavity – cavity surrounded by dentin that contains pulp
•
Pulp – connective tissue, blood vessels, and nerves
•
Root canal – portion of the pulp cavity that extends into the root
•
Apical foramen – proximal opening to the root canal
•
Odontoblasts – secrete and maintain dentin throughout life
Tooth and Gum
Disease
•
Dental caries – gradual demineralization of enamel and dentin by bacterial action
– Dental
plaque, a film of sugar, bacteria, and mouth debris, adheres to teeth
– Acid
produced by the bacteria in the plaque dissolves calcium salts
– Without
these salts, organic matter is digested by proteolytic enzymes
– Daily
flossing and brushing help prevent caries by removing forming plaque
Tooth and Gum
Disease: Periodontitis
•
Gingivitis – as
plaque accumulates, it calcifies and forms calculus, or tartar
•
Accumulation of calculus:
– Disrupts
the seal between the gingivae and the teeth
– Puts
the gums at risk for infection
•
Periodontitis – serious gum disease resulting from an immune response
•
Immune system attacks intruders as well as body tissues, carving pockets
around the teeth and dissolving bone
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
Esophagus
•
Muscular tube going from the laryngopharynx to the stomach
•
Travels through the mediastinum and pierces the diaphragm
•
Joins the stomach at the cardiac orifice
Esophageal
Characteristics
•
Esophageal mucosa – nonkeratinized stratified squamous epithelium
•
The empty esophagus is folded longitudinally and flattens when food is
present
•
Glands secrete mucus as a bolus moves through the esophagus
•
Muscularis changes from skeletal (superiorly) to smooth muscle
(inferiorly)
Digestive
Processes in the Mouth
•
Food is ingested
•
Mechanical digestion begins (chewing)
•
Propulsion is initiated by swallowing
•
Salivary amylase begins chemical breakdown of starch
•
The pharynx and esophagus serve as conduits to pass food from the mouth
to the stomach
Deglutition
(Swallowing)
•
Involves the coordinated activity of the tongue, soft palate, pharynx,
esophagus and 22 separate muscle groups
•
Buccal phase – bolus is forced into the oropharynx
•
Pharyngeal-esophageal phase –
controlled by the medulla and lower pons
– All
routes except into the digestive tract are sealed off
•
Peristalsis moves food through the pharynx to the esophagus
Stomach
•
Chemical breakdown of proteins begins and food is converted to chyme
•
Cardiac region – surrounds the cardiac orifice
•
Fundus – dome-shaped region beneath the diaphragm
•
Body – midportion of the stomach
•
Pyloric region – made up of the antrum and canal which terminates at the pylorus
•
The pylorus is continuous with the duodenum through the pyloric sphincter
•
Greater curvature – entire extent of the convex lateral surface
•
Lesser curvature – concave medial surface
•
Lesser omentum – runs from the liver to the lesser curvature
•
Greater omentum – drapes inferiorly from the greater curvature to the small intestine
•
Nerve supply – sympathetic and parasympathetic fibers of the autonomic nervous system
•
Blood supply – celiac trunk, and corresponding veins (part of the hepatic portal
system)
Microscopic
Anatomy of the Stomach
•
Muscularis – has an additional oblique layer that:
– Allows
the stomach to churn, mix, and pummel food physically
– Breaks
down food into smaller fragments
•
Epithelial lining is composed of:
– Goblet
cells that produce a coat of alkaline mucus
• The
mucous surface layer traps a bicarbonate-rich fluid beneath it
•
Gastric pits contain gastric glands that secrete gastric juice, mucus,
and gastrin
Glands of the
Stomach Fundus and Body
•
Gastric glands of the fundus and body have a variety of secretory cells
– Mucous
neck cells –
secrete acid mucus
– Parietal
cells –
secrete HCl and intrinsic factor
– Chief
cells –
produce pepsinogen
• Pepsinogen
is activated to pepsin by:
–
HCl in the stomach
–
Pepsin itself via a
positive feedback mechanism
– Enteroendocrine
cells –
secrete gastrin, histamine, endorphins, serotonin, cholecystokinin (CCK), and
somatostatin into the lamina propria
Stomach Lining
•
The stomach is exposed to the harshest conditions in the digestive tract
•
To keep from digesting itself, the stomach has a mucosal barrier with:
–
A thick coat of bicarbonate-rich mucus on the stomach wall
–
Epithelial cells that are joined by tight junctions
–
Gastric glands that have cells impermeable to HCl
•
Damaged epithelial cells are quickly replaced
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
– Secretes
intrinsic factor required for absorption of vitamin B12
Regulation of
Gastric Secretion
•
Neural and hormonal mechanisms regulate the release of gastric juice
•
Stimulatory and inhibitory events occur in three phases
– Cephalic
(reflex) phase: prior to food entry
– Gastric
phase: once food enters the stomach
– Intestinal
phase: as partially digested food enters the duodenum
Cephalic Phase
•
Excitatory events include:
– Sight
or thought of food
– Stimulation
of taste or smell receptors
•
Inhibitory events include:
– Loss
of appetite or depression
– Decrease
in stimulation of the parasympathetic division
Gastric Phase
•
Excitatory events include:
–
Stomach distension
–
Activation of stretch receptors (neural activation)
–
Activation of chemoreceptors by peptides, caffeine, and rising pH
–
Release of gastrin to the blood
•
Inhibitory events include:
–
A pH lower than 2
–
Emotional upset that overrides the parasympathetic division
Intestinal Phase
•
Excitatory phase – low pH; partially digested food enters the duodenum and encourages
gastric gland activity
•
Inhibitory phase – distension of duodenum, presence of fatty, acidic, or hypertonic chyme,
and/or irritants in the duodenum
–
Initiates inhibition of local reflexes and vagal nuclei
–
Closes the pyloric sphincter
–
Releases enterogastrones that inhibit gastric secretion
Release of Gastric
Juice
Regulation and
Mechanism of HCl Secretion
•
HCl secretion is stimulated by ACh, histamine, and gastrin through
second-messenger systems
•
Release of hydrochloric acid:
– Is
low if only one ligand binds to parietal cells
– Is
high if all three ligands bind to parietal cells
•
Antihistamines block H2 receptors and decrease HCl release
Response of the
Stomach to Filling
•
Stomach pressure remains constant until about 1L of food is ingested
•
Relative unchanging pressure results from reflex-mediated relaxation and
plasticity
•
Reflex-mediated events include:
– Receptive
relaxation – as
food travels in the esophagus, stomach muscles relax
– Adaptive
relaxation – the
stomach dilates in response to gastric filling
•
Plasticity – intrinsic ability of smooth muscle to exhibit the stress-relaxation
response
Gastric
Contractile Activity
•
Peristaltic waves move toward the pylorus at the rate of 3 per minute
•
This basic electrical rhythm (BER) is initiated by pacemaker cells (cells
of Cajal)
•
Most vigorous peristalsis and mixing occurs near the pylorus
•
Chyme is either:
– Delivered
in small amounts to the duodenum or
– Forced
backward into the stomach for further mixing
Regulation of
Gastric Emptying
•
Gastric emptying is regulated by:
– The
neural enterogastric reflex
– Hormonal
(enterogastrone) mechanisms
•
These mechanisms inhibit gastric secretion and duodenal filling
•
Carbohydrate-rich chyme quickly moves through the duodenum
•
Fat-laden chyme is digested more slowly causing food to remain in the
stomach longer
Small Intestine:
Gross Anatomy
•
Runs from pyloric sphincter to the ileocecal valve
•
Has three subdivisions: duodenum, jejunum, and ileum
•
The bile duct and main pancreatic duct:
– Join
the duodenum at the hepatopancreatic ampulla
– Are
controlled by the sphincter of Oddi
•
The jejunum extends from the duodenum to the ileum
•
The ileum joins the large intestine at the ileocecal valve
Small Intestine:
Microscopic Anatomy
•
Structural modifications of the small intestine wall increase surface
area
– Plicae
circulares: deep circular folds of the mucosa and submucosa
– Villi
–
fingerlike extensions of the mucosa
– Microvilli
– tiny
projections of absorptive mucosal cells’
plasma membranes
Small Intestine:
Histology of the Wall
•
The epithelium of the mucosa is made up of:
– Absorptive
cells and goblet cells
– Enteroendocrine
cells
– Interspersed
T cells called intraepithelial lymphocytes (IELs)
• IELs
immediately release cytokines upon encountering Ag
•
Cells of intestinal crypts secrete intestinal juice
•
Peyer’s patches are found in the submucosa
•
Brunner’s glands in the duodenum secrete alkaline mucus
Intestinal Juice
•
Secreted by intestinal glands in response to distension or irritation of
the mucosa
•
Slightly alkaline and isotonic with blood plasma
•
Largely water, enzyme-poor, but contains mucus
Liver
•
The largest gland in the body
•
Superficially has four lobes –
right, left, caudate, and quadrate
•
The falciform ligament:
–
Separates the right and left lobes anteriorly
–
Suspends the liver from the diaphragm and anterior abdominal wall
•
The ligamentum teres:
–
Is a remnant of the fetal umbilical vein
–
Runs along the free edge of the falciform ligament
Liver: Associated
Structures
•
The lesser omentum anchors the liver to the stomach
•
The hepatic blood vessels enter the liver at the porta hepatis
•
The gallbladder rests in a recess on the inferior surface of the right
lobe
•
Bile leaves the liver via:
– Bile
ducts, which fuse into the common hepatic duct
– The
common hepatic duct, which fuses with the cystic duct
• These
two ducts form the bile duct
Gallbladder and
Associated Ducts
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
•
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
•
Liver sinusoids – enlarged, leaky capillaries located between hepatic plates
•
Kupffer cells – hepatic macrophages found in liver sinusoids
•
Hepatocytes’ functions include:
– Production
of bile
– Processing
bloodborne nutrients
– Storage
of fat-soluble vitamins
– Detoxification
•
Secreted bile flows between hepatocytes toward the bile ducts in the
portal triads
Composition of
Bile
•
A yellow-green, alkaline solution containing bile salts, bile pigments,
cholesterol, neutral fats, phospholipids, and electrolytes
•
Bile salts are cholesterol derivatives that:
– Emulsify
fat
– Facilitate
fat and cholesterol absorption
– Help
solubilize cholesterol
•
Enterohepatic circulation recycles bile salts
•
The chief bile pigment is bilirubin, a waste product of heme
The Gallbladder
•
Thin-walled, green muscular sac on the ventral surface of the liver
•
Stores and concentrates bile by absorbing its water and ions
•
Releases bile via the cystic duct, which flows into the bile duct
Regulation of Bile
Release
•
Acidic, fatty chyme causes the duodenum to release:
– Cholecystokinin
(CCK) and secretin into the bloodstream
•
Bile salts and secretin transported in blood stimulate the liver to
produce bile
•
Vagal stimulation causes weak contractions of the gallbladder
•
Cholecystokinin causes:
– The
gallbladder to contract
– The
hepatopancreatic sphincter to relax
•
As a result, bile enters the duodenum
Pancreas
•
Location
–
Lies deep to the greater curvature of the stomach
–
The head is encircled by the duodenum and the tail abuts the spleen
•
Exocrine function
–
Secretes pancreatic juice which breaks down all categories of foodstuff
–
Acini (clusters of secretory cells) contain zymogen granules with
digestive enzymes
•
The pancreas also has an endocrine function –
release of insulin and glucagon
Acinus of the
Pancreas
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
•
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
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
Digestion in the
Small Intestine
•
As chyme enters the duodenum:
– Carbohydrates
and proteins are only partially digested
– No
fat digestion has taken place
•
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
Motility in the
Small Intestine
•
The most common motion of the small intestine is segmentation
– It
is initiated by intrinsic pacemaker cells (Cajal cells)
– Moves
contents steadily toward the ileocecal valve
•
After nutrients have been absorbed:
– Peristalsis
begins with each wave starting distal to the previous
– Meal
remnants, bacteria, mucosal cells, and debris are moved into the large intestine
Control of
Motility
•
Local enteric neurons of the GI tract coordinate intestinal motility
•
Cholinergic neurons cause:
–
Contraction and shortening of the circular muscle layer
–
Shortening of longitudinal muscle
–
Distension of the intestine
•
Other impulses relax the circular muscle
•
The gastroileal reflex and gastrin:
–
Relax the ileocecal sphincter
–
Allow chyme to pass into the large intestine
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
•
Is subdivided into the cecum, appendix, colon, rectum, and anal canal
•
The saclike cecum:
– Lies
below the ileocecal valve in the right iliac fossa
– Contains
a wormlike vermiform appendix
Colon
•
Has distinct regions: ascending
colon, hepatic flexure, transverse colon, splenic flexure, descending colon, and
sigmoid colon
•
The transverse and sigmoid portions are anchored via mesenteries called
mesocolons
•
The sigmoid colon joins the rectum
•
The anal canal, the last segment of the large intestine, opens to the
exterior at the anus
Valves and
Sphincters of the Rectum and Anus
•
Three valves of the rectum stop feces from being passed with gas
•
The anus has two sphincters:
– Internal
anal sphincter composed of smooth muscle
– External
anal sphincter composed of skeletal muscle
•
These sphincters are closed except during defecation
Mesenteries of
Digestive Organs
Large Intestine:
Microscopic Anatomy
•
Colon mucosa is simple columnar epithelium except in the anal canal
•
Has numerous deep crypts lined with goblet cells
•
Anal canal mucosa is stratified squamous epithelium
•
Anal sinuses exude mucus and compress feces
•
Superficial venous plexuses are associated with the anal canal
•
Inflammation of these veins results in itchy varicosities called
hemorrhoids
Structure of the
Anal Canal
Bacterial Flora
•
The bacterial flora of the large intestine consist of:
–
Bacteria surviving the small intestine that enter the cecum and
–
Those entering via the anus
•
These bacteria:
–
Colonize the colon
–
Ferment indigestible carbohydrates
–
Release irritating acids and gases (flatus)
–
Synthesize B complex vitamins and vitamin K
Functions of the
Large Intestine
•
Other than digestion of enteric bacteria, no further digestion takes
place
•
Vitamins, water, and electrolytes are reclaimed
•
Its major function is propulsion of fecal material toward the anus
•
Though essential for comfort, the colon is not essential for life
Motility of the
Large Intestine
•
Haustral contractions
– Slow
segmenting movements that move the contents of the colon
– 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
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
Physiology of
Chemical Digestion
Chemical
Digestion
•
Chemical Digestion is a catabolic pricess in which large food molecules
are brocen down to monomers (building blocks), which are small enough to be
absorbed by the GI lining.
•
The enzymatic breakdown of any type of food is hydrolysis.
Chemical
Digestion: Carbohydrates
•
Absorption: via cotransport with Na+, and facilitated
diffusion
– Enter
the capillary bed in the villi
– Transported
to the liver via the hepatic portal vein
•
Enzymes used: salivary
amylase, pancreatic amylase, and brush border enzymes (dextrinase,
glucoamylase, maltase, sucrase and lactase)
Chemical
Digestion: Proteins
•
Absorption: similar to carbohydrates
•
Enzymes used: pepsin in the stomach
•
Enzymes acting in the small intestine
– Pancreatic
enzymes – trypsin,
chymotrypsin, and carboxypeptidase
– Brush
border enzymes
– aminopeptidases,
carboxypeptidases, and dipeptidases
Chemical
Digestion: Fats
•
Bile salts play very important role in emulsification of fat.
•
Emulsification does not break chemical bonds. It reduces attraction between fat
molecules, so they can be widely dispersed.
•
Bile salt associate with the products of lipase activity –
monoglycerides and free fatty acids to form micells.
•
Absorption: Diffusion into intestinal cells where they:
– Combine
with proteins and form chylomicrons (water soluble lipoprotein droplets)
– Enter
lacteals and are transported to systemic circulation via lymph
•
Glycerol and short chain fatty acids are:
– Absorbed
into the capillary blood in villi
– Transported
via the hepatic portal vein
•
Enzymes/chemicals used: bile salts and pancreatic lipase
Fatty Acid
Absorption
•
Fatty acids and monoglycerides enter intestinal cells via diffusion
•
They are combined with proteins within the cells
•
Resulting chylomicrons are extruded
•
They enter lacteals and are transported to the circulation via lymph
Chemical
Digestion: Nucleic Acids
•
Absorption: active transport
via membrane carriers
•
Absorbed in villi and transported to liver via hepatic portal vein
•
Enzymes used: pancreatic
ribonucleases and deoxyribonuclease in the small intestines
Electrolyte
Absorption
•
Most ions are actively absorbed along the length of small intestine
– Na+
is coupled with absorption of glucose and amino acids
– Ionic
iron is transported into mucosal cells where it binds to ferritin
•
Anions passively follow the electrical potential established by Na+
•
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)
Water Absorption
•
95% of water is absorbed in the small intestines by osmosis
•
Water moves in both directions across intestinal mucosa
•
Net osmosis occurs whenever a concentration gradient is established by
active transport of solutes into the mucosal cells
•
Water uptake is coupled with solute uptake, and as water moves into
mucosal cells, substances follow along their concentration gradients
Malabsorption of
Nutrients
•
Results from anything that interferes with delivery of bile or pancreatic
juice
•
Factors that damage the intestinal mucosa (e.g., bacterial infection)
•
Gluten enteropathy (adult celiac disease) –
gluten damages the intestinal villi and reduces the length of microvilli
– Treated
by eliminating gluten from the diet (all grains but rice and corn)