Download Digestion

The
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
Alimentary canal
Accessory digestive organs
Digestive Process
•
The GI tract is a “disassembly” line
–
•
Nutrients become more available to the body in
each step
There are six essential activities:
1.
2.
3.
4.
5.
6.
Ingestion
Propulsion
Mechanical digestion
Chemical digestion
Absorption
Defecation
Digestive
Process
GI Tract
•
The GI tract is
controlled at three
levels:
1. Mechanical
and chemical
stimuli
2. Extrinsic
control by CNS
centers
3. Intrinsic control
by local centers
1. Mechanical and chemical stimuli
• 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
2. Extrinsic and 3.
Intrinsic Controls
• 2. Extrinsic controls
– Long reflexes arising within
or outside the GI tract
– Involve CNS centers and
extrinsic autonomic nerves
• 3. Intrinsic controls
– Nerve plexuses near the GI tract initiate short
reflexes
– Short reflexes are mediated by local enteric
plexuses (gut brain)
Enteric Nervous System
• Composed of two major intrinsic nerve
plexuses that control glands (submucosal)
and mobility (myenteric).
• Segmentation and peristalsis are largely
automatic involving local reflex arcs
• Linked to the CNS via long autonomic
reflex arc
Pathology: Colic
• What causes colic?
No one is really sure, but there are a few
suspected causes, such as intestinal gas,
food sensitivity or allergy, or an immature
nervous system.
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
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
Peritoneum and
Peritoneal Cavity
• 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
• Peritoneal organs (intraperitoneal) –
organs surrounded by peritoneum
• Retroperitoneal organs – organs
outside the peritoneum
Pathology: Peritonitis
• Inflammation
of the
peritoneum.
Pathology: Hernia
• A hernia is
a protrusion of a tissue,
structure, or part of an
organ through the
muscular tissue or the
membrane by which it is
normally contained. The
hernia has three parts:
the orifice through which
it herniates, the hernial
sac, and its contents.
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
1.
2.
3.
4.
•
Mucosa
Submucosa
Muscularis externa
Serosa
Each tunic has a predominant tissue type
and a specific digestive function
Histology of the Alimentary
Canal
Mucosa: Other
Sublayers
• Mucosa: Moist epithelial layer that
lines the lumen of the alimentary
canal
1. Secretion of mucus
2. Absorption of the end products
3. Protection against invaders
• 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
Mucosa: Other
Sublayers
• Mucosa: Moist epithelial layer that
lines the lumen of the alimentary
canal
1. Secretion of mucus
2. Absorption of the end products
3. Protection against invaders
• 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
Review of Digestive Tract
Mucosa - Absorbs food
Submucosa - Contains
blood vessels
and nerves
Muscularis -Propels food
Serosa - Provides protection
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
• Labial frenulum – median fold that
joins the internal aspect of each lip
to the gum
• Vestibule – bounded by the lips
and cheeks externally, and teeth
and gums internally
• Oral cavity proper – area that lies
within the teeth and gums
Lips and Cheeks
• Have a core of skeletal muscles
• Labial frenulum – median fold that
joins the internal aspect of each lip
to the gum
• Vestibule – bounded by the lips
and cheeks externally, and teeth
and gums internally
• Oral cavity proper – area that lies
within the teeth and gums
Oral Cavity
and
Pharynx:
Anterior
View
Palate
• There are two palates:
– 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 and
when touched, causes the soft palate to widen.
If you forget which is which, use your tongue.
Tongue
•
•
Occupies the floor of the mouth
and fills the oral cavity when
mouth is closed
Functions include:
1. Gripping and repositioning food
during chewing
2. Mixing food with saliva and forming
the bolus
3. Initiation of swallowing, and speech
Tongue
• Intrinsic muscles change the
shape of the tongue
• Extrinsic muscles alter the
tongue’s position
• Lingual frenulum secures
the tongue to the floor of the
mouth
Tongue
• Superior surface bears three
types of 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
Tongue
Salivary Glands
•
Produce and secrete saliva that:
1.
2.
3.
4.
•
•
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
Salivary Glands
• 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
Pathology: Mumps
• A viral disorder infection of the
parotid salivary gland.
Iowa to launch mass clinics for mumps
shots
State to target young adults caught up
in growing epidemic (4/2006)
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 – 20 deciduous teeth
that erupt 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
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
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
Tooth Structure
• Two main regions – crown and
the root (and the intervening
neck)
• Crown – exposed part of the
tooth above the gingiva (gum)
• Root – portion of the tooth
embedded in the jawbone
Tooth Structure
• Cementum – calcified connective
tissue
– Covers the root
– Attaches it to the periodontal
ligament
• Enamel – acellular, brittle material
composed of calcium salts and
hydroxyapatite crystals is the
hardest substance in the body
– Encapsules the crown of the tooth
Tooth Structure
• Dentin – bonelike material
deep to the enamel cap that
forms the bulk of the tooth
• Pulp – connective tissue, blood
vessels, and nerves
• Root canal – portion of the pulp
cavity that extends into the root
Tooth
Structure
Tooth and Gum
Disease
•
Dental caries – gradual
demineralization of enamel
and dentin by bacterial action
1. Dental plaque, a film of sugar, bacteria, and
mouth debris, adheres to teeth
2. Acid produced by the bacteria in the plaque
dissolves calcium salts
3. Without these salts, organic matter is
digested by proteolytic enzymes
4. Daily flossing and brushing help prevent
caries by removing forming plaque
Tooth and Gum
Disease: Periodontitis
•
Gingivitis –
1. as plaque accumulates, it calcifies
and forms calculus, or tartar
2. Calculus disrupts the seal
between the gingivae and the
teeth
3. Puts the gums at risk for infection
•
Periodontitis –Immune system
attacks intruders as well as
body tissues, carving pockets
around the teeth and dissolving
bone
Normal
Pharynx: The Basics
– Nasopharynx:
just air
– Oropharynx:
food comes in to
epiglottis
– Laryngopharynx
: epiglottis to the
larynx
Pathology: Cranial Nerve
Lesions
• Cranial nerve
lesions can lead to
paralysis of the
tongue(11) or
pharynx (10).
• Damage to 5, 9 or
10 can lead to
difficulty
swallowing.
Polio, encephalitis, botulism, MD, mysasthenia gravis
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
Pathology:
Esophageal Cancer
• May occlude the
esophagus
• May impose on the
trachea as well.
Pathology: GERD
(gastroesophageal reflux disorder)
Gas is trapped in the
fundus of the stomach.
– This gas may
cause the pyloric
valve to become
misshapen and
open.
Either this alone will
allow acid access to the
sphinchter and
esophagus or the
escaping gas will carry
acid with it.
Pathology: Achalasia
• The lower
esophageal
sphincters fails
to relax during
swallowing.
– Minutes to
hours and
then up until it
will hold 1 L
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
Pathology:
Choking
Stomach
• Chemical breakdown of proteins begins
and food is converted to chyme
• Cardiac region – surrounds the cardiac
orifice
• Fundus – dome-shaped region beneath
the diaphragm
• Body – midportion of the stomach
• Pyloric region – made up of the antrum
and canal which terminates at the pylorus
• The pylorus is continuous with the
duodenum through the pyloric sphincter
Stomach
Stomach
• Greater curvature – entire extent of the
convex lateral surface
• Lesser curvature – concave medial
surface
• Lesser omentum – group of ligaments that
run from the liver to the lesser curvature
• Greater omentum – drapes inferiorly from
the greater curvature to the small intestine
Omentums
Lesser
Greater
Stomach
• Nerve supply – sympathetic and
parasympathetic fibers of the autonomic
nervous system
• Blood supply – celiac trunk, and
corresponding veins (part of the hepatic
portal system)
Stomach
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
– Gastric pits contain gastric glands that secrete
gastric juice, mucus, and gastrin
Microscopic
Anatomy of
the Stomach
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 (basic barrier)
– Epithelial cells that are joined by tight
junctions (no holes for acid to leak through)
– Gastric glands that have cells impermeable to
HCl (the cells don’t let the acid through)
• Also, damaged epithelial cells are quickly
replaced
Digestion in the Stomach
•
The stomach:
1. Holds ingested food
2. Degrades this food both physically and
chemically
3. Delivers chyme to the small intestine
4. Enzymatically digests proteins with pepsin
5. Secretes intrinsic factor required for
absorption of vitamin B12
Release of Gastric Juice
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
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
Gastric Contractile Activity
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 (Atkins diet).
Regulation
of Gastric
Emptying
Digestive
Problems
• Ulcers
– Peptic ulcer
– Helicobactor pylorii infection
– Use of certain anti-inflammatory drugs
– Disorders that cause excessive acid
secretion
– Poor mucous production form poor
nutrition or circulation.
Pathology:
Peptic Ulcer
• 80% of ulcers are caused
by helicobactor pylori.
• About 25% of us have H.
Pylori in our stomach
right now.
Gastritis
• Inflammation
of the
stomach.
• May be mild
to severe.
Gastric Atrophy
• With prolonged gastritis, it can lead to
atrophy.
• This can lead to nutritional deficiencies
because the stomach makes intrinsic
factor, an enzyme necessary to absorbe
B12 (and B12 is methionine)
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
empty into the duodenum.
• 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: Microscopic
Anatomy
Plicae circulares
Sprue
• Several diseases can caused decreased
absorption of the small intestine mucosa. These
are termed sprue.
– Celiac Disease
– Chrohn’s
– Ischemic Death of Microvilli
• All will cause…
– sever nutritional deficiency
– Possible osteomalacia
– Lack of blood clotting due to loss of Vit. K
– Anemia due to lack of Vit. B12
Pathology:
Coeliac/Crohn’sDisease
Pathology:
Coeliac/Crohn’sDisease
• Due to a “error in
design” the tips of
microvilli usually get
less oxygen.
– The O2 diffuses
straight from artery to
vein before reaching
the tip.
• In systemic ischemia,
the microvilli can be
damaged.
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)
Small Intestine: Histology of the
Wall
• Cells of intestinal crypts secrete intestinal
juice
• Peyer’s patches are found in the
submucosa (lymphatics follicles)
• Brunner’s glands in the duodenum secrete
alkaline mucus
Liver
• The largest gland in the body
• Superficially has four lobes – right, left,
caudate, and quadrate
Liver
• The falciform ligament:
– Separates the right and left lobes anteriorly
– Suspends the liver from the diaphragm and
anterior abdominal wall
Liver: Associated Structures
• The lesser omentum anchors the liver to
the stomach
• The hepatic blood vessels enter the liver
at the porta hepatis
• The gallbladder rests in a recess on the
inferior surface of the right lobe
Liver: Associated Structures
• Bile leaves the liver via:
– Bile ducts, which fuse into the common
hepatic duct
1. Liver
2. Common Bile
Duct
3. Gall Stones
4. Gall Bladder
Jaundice
Unconjugated
Urobilinogen
Blood
Conjugated
Urobilinogen
Jaundice
Unconjugated
Urobilinogen
Blood
Conjugated
Urobilinogen
High Unconjugated
Biliruben means…
1. Excess hemolysis
• Hemolytic
jaundice
2. Liver isn’t converting
to conjugated
biliruben.
• Hepatic
Jaundice
3. Blockage of bile
duct
• Biliary
obstruction
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
Microscopic Anatomy of the Liver
Liver: Microscopic Anatomy
•
Hepatocytes’ functions include:
1.
2.
3.
4.
Production of bile
Processing bloodborne nutrients
Storage of fat-soluble vitamins
Detoxification
Liver: Microscopic Anatomy
•
Hepatocytes’ functions include:
1.
2.
3.
4.
Production of bile
Processing bloodborne nutrients
Storage of fat-soluble vitamins
Detoxification
Liver Function
Gallbladder and Associated
Ducts
Pathology:
Cirrhosis
• Cirrhosis is
most commonly
caused by
alcoholism,
hepatitis B and
fatty liver
disease but has
many other
possible
causes.
Hepatitis C is
also an
important
cause.
Pathology:
Cirrhosis
• Cirrhosis is
most commonly
caused by
alcoholism,
hepatitis B and
fatty liver
disease but has
many other
possible
causes.
Hepatitis C is
also an
important
cause.
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
• 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
• Releases bile via the cystic duct, which
flows into the bile duct
Pathology:
Cholecystitis
• Inflammation of the
gall bladder.
• Choleliths (gall
stones block the
cystic duct.
– Leads to inspissation
(thickening) of bile,
bile stasis, and
secondary infection
Regulation
of Bile
Release
Pancreas
• Location
– Lies deep to the greater curvature of the
stomach
– The head is encircled by the duodenum and
the tail abuts the spleen
Pancreas
• 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 bicarbonate (HCO3–))
– Neutralizes acid chyme
– Provides optimal environment for pancreatic
enzymes
• Enzymes are released in inactive form and
activated in the duodenum
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
Pancreatic Excretions
Regulation of Pancreatic
Secretion
Pancreatitis
• Caused by
excessive
alcohol or
gallstones.
Digestion in the Small Intestine
• As chyme enters the duodenum:
– Carbohydrates and proteins are only partially
digested
– No fat digestion has taken place
Digestion in the Small Intestine
•
Digestion continues in the small intestine
1. Chyme is released slowly into the
duodenum
2. Because it is hypertonic and has low pH,
mixing is required for proper digestion
3. Required substances needed are supplied
by the liver
4. 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
Control of Motility
• 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
Large Intestine
Large Intestine
• Is subdivided into the cecum, appendix,
colon, rectum, and anal canal
• The saclike cecum:
– Lies below the ileocecal valve in the right iliac
fossa
– Contains a wormlike vermiform appendix
Large Intestine
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
Large Intestine
Constipation
• Slow movement of
feces through the
large intestine.
• Increased time to
absorb bicarbonate,
which results in
alkalosis.
• Can lower K+ levels
and thus affect heart,
neurons, muscles
and respiration.
•
Diarrhea
Fast movement of feces
through the large
intestine.
• Electrolytes released to
intestine and water
follows.
• Decreased time to
absorb bicarbonate,
which results in
alkalosis.
• Can raise K+ levels and
thus affect heart,
neurons, muscles and
respiration.
Hirschsprung’s/Megacolon
• http://aliveandwellcoloncare.com/_wsn/pa
ge9.html
Valves and Sphincters of the
Rectum and Anus
• 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
Mesenteries of Digestive
Organs
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
Defecation
Digestion
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
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:
Proteins
Chemical Digestion: Fats
• Absorption: Diffusion into intestinal cells
where they:
– Combine with proteins and extrude
chylomicrons
– 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
Chemical
Digestion: Fats
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
Fatty Acid
Absorption
Chemical Digestion: Nucleic
Acids
• Absorption: active transport via
membrane carriers
• Absorbed in villi and transported to liver
via hepatic portal vein
• Enzymes used: pancreatic ribonucleases
and deoxyribonuclease in the small
intestines
Electrolyte Absorption
• Most ions are actively absorbed along the
length of small intestine
– Na+ is coupled with absorption of glucose and
amino acids
– Ionic iron is transported into mucosal cells
where it binds to ferritin
• Anions passively follow the electrical
potential established by Na+
Electrolyte Absorption
• K+ diffuses across the intestinal mucosa in
response to osmotic gradients
• Ca2+ absorption:
– Is related to blood levels of ionic 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)
Cancer
• Stomach and colon cancers rarely have
early signs or symptoms
• Metastasized colon cancers frequently
cause secondary liver cancer
• Prevention is by regular dental and
medical examinations
Cancer
• Colon cancer is the 2nd largest cause of
cancer deaths in males (lung cancer is 1st)
• Forms from benign mucosal tumors called
polyps whose formation increases with
age
• Regular colon examination should be done
for all those over 50
Appendicitis
•Usually caused by fecal obstruction
or anatomical “kinking” of the
appendix.
•A rupture leads to peritonitis.
Diverticulitis
•Small herniations of the mucosa in the
large intestine.
•These areas can become inflamed and
possibly rupture.
•Prevention is the treatment of choice. A
diet high in fiber will help prevent
diverticulitis.
Diarrhea
•Movement of fecal material through
the GI Tract too rapidly.
•May be caused by microbes, spicy
foods, stress, etc.
Constipation
•Infrequent defecation of fecal
material.
•Usually caused by a diet low in
fiber and water.
Buliemia Nervosa
•Condition where a patient binges on
food and then purges with either
laxatives or vomiting.
•Considered a psychological disorder
where the patient has a fear of gaining
weight.
•Treated with psychotherapy.
Anorexia Nervosa
•Psychological disorder where the patient
has a false perception of their own weight.
•Patient denies their own appetite.
•Patients are usually 15 - 20 % below normal
body weight.
•Extreme cases are lethal.
•Closely associated with bulimia nervosa.
Flatulence
•Excessive intestinal gas resulting
from bacteria in the intestines, diet,
or swallowing air.
Cystic Fibrosis
•Genetic disorder where excess mucous
is produced.
•Causes a blocking of the pancreatic duct,
therefore enzymes cannot enter the
duodenum from the pancreas.
•Treated by giving digestive enzymes
orally.
Nutrition,
Metabolism,
and Body
Temperature
Regulation
Nutrition
• Nutrient – a substance that
promotes normal growth,
maintenance, and repair
• Major nutrients – carbohydrates, lipids,
and proteins
• Other nutrients – vitamins and minerals
(and technically speaking, water)
Carbohydrates
• Complex carbohydrates (starches) are
found in bread, cereal, flour, pasta, nuts,
and potatoes
• Simple carbohydrates (sugars) are found
in soft drinks, candy, fruit, and ice cream
• Glucose is the molecule ultimately used by
body cells to make ATP
• Neurons and RBCs rely almost entirely
upon glucose to supply their energy needs
• Excess glucose is converted to glycogen
or fat and stored
Lipids
• The most abundant dietary lipids,
triglycerides, are found in both
animal and plant foods
• Essential fatty acids – linoleic and linolenic acid,
found in most vegetables, must be ingested
• Dietary fats:
– Help the body to absorb vitamins
– Are a major energy fuel of hepatocytes and skeletal
muscle
– Are a component of myelin sheaths and all cell
membranes
Proteins
• Proteins supply:
– Essential amino acids, the building
blocks for nonessential amino acids
– Nitrogen for nonprotein nitrogen-containing
substances
• Daily intake should be approximately
0.8g/kg of body weight
Vitamins
• Organic (carbon) compounds
needed for growth and good
health
• They are crucial in helping the body use
nutrients and often function as coenzymes
• Only vitamins D, K, and B are synthesized
in the body; all others must be ingested
• Water-soluble vitamins (B-complex and C)
are absorbed in the gastrointestinal tract
– B12 additionally requires gastric intrinsic factor
to be absorbed
Minerals
• Seven minerals are required
in moderate amounts
– Calcium, phosphorus, potassium, sulfur,
sodium, chloride, and magnesium
• Dozens are required in trace amounts
• Minerals work with nutrients to ensure
proper body functioning
• Calcium, phosphorus, and magnesium
salts harden bone
Minerals
• Sodium and chloride help
maintain normal osmolarity,
water balance, and are essential in nerve
and muscle function
• Uptake and excretion must be balanced to
prevent toxic overload
Metabolism
• Metabolism – all chemical reactions
necessary to maintain life
• Cellular respiration – food fuels are broken
down within cells and some of the energy
is captured to produce ATP
– Anabolic reactions – synthesis of larger
molecules from smaller ones
– Catabolic reactions – hydrolysis of complex
structures into simpler ones
Metabolism
• Enzymes shift the high-energy phosphate
groups of ATP to other molecules
• These phosphorylated molecules are
activated to perform cellular functions
Stages of Metabolism
•
Energy-containing nutrients are
processed in three major stages
1. Digestion – breakdown of food; nutrients are
transported to tissues
2. Anabolism and formation of catabolic intermediates
where nutrients are:
1. Built into lipids, proteins, and glycogen
2. Broken down by catabolic pathways to pyruvic acid and
acetyl CoA
3. Oxidative breakdown – nutrients are catabolized to
carbon dioxide, water, and ATP
Stages of
Metabolism
Carbohydrate Metabolism
•
Since all carbohydrates are transformed
into glucose, it is essentially glucose
metabolism
• Oxidation of glucose is shown by the
overall reaction:
C6H12O6 + 6O2  6H2O + 6CO2 + 36 ATP +
heat
• Glucose is catabolized in three pathways
1. Glycolysis
2. Krebs cycle
3. The electron transport chain and oxidative
phosphorylation
Carbohydrate Catabolism
Glycolysis
•
A three-phase pathway in which:
1. Glucose is oxidized into pyruvic acid
2. NAD+ is reduced to NADH + H+
3. ATP is synthesized by substrate-level
phosphorylation
•
Pyruvic acid:
– Moves on to the Krebs cycle in an aerobic
pathway (with O2)
– Is reduced to lactic acid in an anaerobic
environment (if no O2)
3 Outcomes of Glycolysis
3. ATP is
synthesized by
substrate-level
phosphorylation
2. NAD+ is reduced
to NADH + H+
1. Glucose is
oxidized
into pyruvic
acid
Glycolysis: Phase 1 and 2
• Phase 1: Sugar activation
– Two ATP molecules activate glucose into
fructose-1,6-diphosphate
• Phase 2: Sugar cleavage
– Fructose-1,6-bisphosphate is cleaved into two
3-carbon isomers
• Bishydroxyacetone phosphate
• Glyceraldehyde 3-phosphate
Glycolysis: Phase 3
• Phase 3: Oxidation and ATP formation
– The 3-carbon sugars are oxidized (reducing
NAD+)
– Inorganic phosphate groups (Pi) are attached
to each oxidized fragment
– The terminal phosphates are cleaved and
captured by ADP to form four ATP molecules
Glycolysis: Phase 3
•
The final products are:
1. Two pyruvic acid molecules
2. Two NADH + H+ molecules (reduced NAD+)
3. A net gain of two ATP molecules
Krebs Cycle: Preparatory Step
• Occurs in the mitochondrial matrix and is
fueled by pyruvic acid and fatty acids
Krebs Cycle: Preparatory Step
•
Pyruvic acid is converted to acetyl CoA in three
main steps:
1. Decarboxylation
•
•
Carbon is removed
Carbon dioxide is released
2. Oxidation
•
•
Hydrogen atoms are removed from pyruvic acid
NAD+ is reduced to NADH + H+
3. Formation of acetyl CoA – the resulting acetic acid is
combined with coenzyme A, a sulfur-containing
coenzyme, to form acetyl CoA
Krebs Cycle
•
An eight-step cycle in which each acetic
acid is decarboxylated and oxidized,
generating:
1.
2.
3.
4.
•
Three molecules of NADH + H+
One molecule of FADH2
Two molecules of CO2
One molecule of ATP
For each molecule of glucose entering
glycolysis, two molecules of acetyl CoA
enter the Krebs cycle
Krebs Cycle
Electron Transport Chain
•
Food (glucose) is oxidized and the released
hydrogens:
1. Are transported by coenzymes NADH and
FADH2
2. Enter a chain of proteins bound to metal
atoms (cofactors)
3. Combine with molecular oxygen to form
water
4. Release energy
• The energy released is harnessed to attach
inorganic phosphate groups (Pi) to ADP,
making ATP by oxidative phosphorylation
Mechanism of Oxidative
Phosphorylation
• The hydrogens delivered to the chain are
split into protons (H+) and electrons
– The protons are pumped across the inner
mitochondrial membrane by:
• NADH dehydrogenase (FMN, Fe-S)
• Cytochrome b-c1
• Cytochrome oxidase (a-a3)
– The electrons are shuttled from one acceptor
to the next
Mechanism of Oxidative
Phosphorylation
1. Electrons are delivered to oxygen,
forming oxygen ions
2. Oxygen ions attract H+ to form water
3. H+ pumped to the intermembrane space:
– Diffuses back to the matrix via ATP
synthase
– Releases energy to make ATP
Mechanism of Oxidative
Phosphorylation
1. Hydrogens are
split into protons
and electrons
Mechanism of Oxidative
Phosphorylation
2. Protons are shuttled across membrane
Mechanism of Oxidative
Phosphorylation
3. Electrons cross
membrane as well but fall
back sooner
Mechanism of Oxidative
Phosphorylation
4. When electrons fall back, you have
2 gradients, a pH gradient and a
strong charge gradient
Mechanism of Oxidative
Phosphorylation
5. When H comes back, it can give
off energy that is used to make ATP
Summary of ATP Production
Lipid Metabolism
• Most products of fat metabolism are
transported in lymph as chylomicrons
• Lipids in chylomicrons are hydrolyzed by
plasma enzymes and absorbed by cells
• Catabolism of fats involves two separate
pathways
Glycerol
Fatty Acids
– Glycerol pathway
– Fatty acids pathway
Lipid
Metabolism
Lipogenesis and Lipolysis
• Excess dietary glycerol and fatty acids
undergo lipogenesis to form triglycerides
• Glucose is easily converted into fat since
acetyl CoA is:
– An intermediate in glucose catabolism
– The starting molecule for the synthesis of fatty
acids
Lipogenesis and Lipolysis
• Lipolysis, the breakdown of stored fat, is
essentially lipogenesis in reverse
• Oxaloacetic acid is necessary for the
complete oxidation of fat
– Without it, acetyl CoA is converted into
ketones (ketogenesis)
Lipogenesis and Lipolysis
The main point here is that lipids and sugars are interconvertible
Protein Metabolism
• Excess dietary protein results in amino
acids being:
– Oxidized for energy
– Converted into fat for storage
• Amino acids must be deaminated prior to
oxidation for energy
Protein Metabolism
Synthesis of Proteins
• Amino acids are the most important
anabolic nutrients, and they form:
– All protein structures
– The bulk of the body’s functional molecules
State of the Body
• The body exists in a dynamic catabolicanabolic state
• Organic molecules (except DNA) are
continuously broken down and rebuilt
• The body’s total supply of nutrients
constitutes its nutrient pool
Absoprtive and Postabsorptive
States
• Metabolic controls equalize blood
concentrations of nutrients between two
states
• Absorptive
– The time during and shortly after nutrient
intake
• Postabsorptive
– The time when the GI tract is empty
– Energy sources are supplied by the
breakdown of body reserves
Absorptive State (Full Stomach)
• The major metabolic thrust is anabolism
and energy storage
– Amino acids become proteins
– Glycerol and fatty acids are converted to
triglycerides
– Glucose is stored as glycogen
• Dietary glucose is the major energy fuel
• Excess amino acids are deaminated and
used for energy or stored as fat in the liver
Absorptive State
Principal Pathways of the
Absorptive State
• In muscle:
– Amino acids become protein
– Glucose is converted to glycogen
• In the liver:
– Amino acids become protein or are
deaminated to keto acids
– Glucose is stored as glycogen or converted to
fat
Principal Pathways of the
Absorptive State
• In adipose tissue:
– Glucose and fats are converted and stored as
fat
• All tissues use glucose to synthesize ATP
Principal Pathways of the
Absorptive State
Insulin Effects on Metabolism
• Insulin controls the absorptive state and its
secretion is stimulated by:
– Increased blood glucose
– Elevated amino acid levels in the blood
– Gastrin, CCK, and secretin
• Insulin enhances:
– Active transport of amino acids into tissue
cells
– Facilitated diffusion of glucose into tissue
Insulin
Effects on
Metabolism
Diabetes Mellitus
• A consequence of inadequate insulin
production (Type I) or abnormal insulin
receptors (Type II)
• Glucose becomes unavailable to most
body cells
• Metabolic acidosis, protein wasting, and
weight loss result as fats and tissue
proteins are used for energy
Postabsorptive State
• The major metabolic thrust is catabolism and
replacement of fuels in the blood
– Proteins are broken down to amino acids
– Triglycerides are turned into glycerol and fatty acids
– Glycogen becomes glucose
• Glucose is provided by glycogenolysis and
gluconeogenesis
• Fatty acids and ketones are the major energy
fuels
• Amino acids are converted to glucose in the liver
Postabsorptive State
Principle Pathways in the
Postabsorptive State
• In muscle:
– Protein is broken down to amino acids
– Glycogen is converted to ATP and pyruvic
acid (lactic acid in anaerobic states)
Principle Pathways in the
Postabsorptive State
• In the liver:
– Amino acids, pyruvic acid, stored glycogen,
and fat are converted into glucose
– Fat is converted into keto acids that are used
to make ATP
• Fatty acids (from adipose tissue) and
ketone bodies (from the liver) are used in
most tissue to make ATP
• Glucose from the liver is used by the
nervous system to generate ATP
Principle Pathways in the
Postabsorptive State
Hormonal and Neural Controls
of the Postabsorptive State
• Decreased plasma glucose concentration
and rising amino acid levels stimulate alpha
cells of the pancreas to secrete glucagon
(the antagonist of insulin)
• Glucagon stimulates:
– Glycogenolysis and gluconeogenesis
– Fat breakdown in adipose tissue
– Glucose sparing
Hormonal and Neural Controls
of the Postabsorptive State
• In response to low plasma glucose, the
sympathetic nervous system releases
epinephrine, which acts on the liver,
skeletal muscle, and adipose tissue to
mobilize fat and promote glycogenolysis
Cholesterol
• Is the structural basis of bile salts, steroid
hormones, and vitamin D
• Makes up part of the hedgehog molecule
that directs embryonic development
• Is transported to and from tissues via
lipoproteins
Cholesterol
• Lipoproteins are classified as:
– HDLs – high-density lipoproteins have more
protein content
– LDLs – low-density lipoproteins have a
considerable cholesterol component
– VLDLs – very low density lipoproteins are
mostly triglycerides
Lipoproteins
• High levels of HDL are thought to protect
against heart attack
• High levels of LDL, especially lipoprotein
(a), increase the risk of heart attack
Metabolic Rate
• Rate of energy output (expressed per
hour) equal to the total heat produced by:
– All the chemical reactions in the body
– The mechanical work of the body
• Measured directly with a calorimeter or
indirectly with a respirometer
Metabolic Rate
• Basal metabolic rate (BMR)
– Reflects the energy the body needs to
perform its most essential activities
• Total metabolic rate (TMR)
– Total rate of kilocalorie consumption to fuel all
ongoing activities
Factors that Influence BMR
• Surface area, age, gender, stress, and
hormones
• As the ratio of surface area to volume
increases, BMR increases
• Males have a disproportionately high BMR
• Stress increases BMR
• Thyroxine increases oxygen consumption,
cellular respiration, and BMR