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BIOLOGY 206: CHAPTER 23 DIGESTIVE SYSTEM: LAUREL SPRING 2007
I.
II.
III.
Overview of the Digestive System
A. Overall Function
1.
Digestive system takes in food, breaks it down into nutrient molecules,
absorbs those molecules into the bloodstream and rids the body of
indigestible remains.
B. Organization: 2 major components
1.
GI tract/alimentary canal: organs through which food actually passes. In
order includes: oral cavity, pharynx, esophagus, stomach, small intestine,
and large intestine.
2.
Accessory organs: assist in digestion, but food does not pass through
them.
a.
Include: teeth, tongue, salivary glands, pancreas, liver, and gall bladder.
Digestive Process
A. Overview
1.
Digestive tract is a disassembly line where food becomes less complex at
each processing step.
2.
The 6 essential activities of digestion follow
B.
Ingestion
1. taking of food into the body.
C.
Propulsion
1. movement of food through the digestive system
2. Peristalsis is the major means of propulsion and is involuntary waves of
contraction and relaxation of smooth muscles of organ wall.
D. Mechanical Digestion
1. Prepares food for chemical digestion
2. Includes chewing, mixing and segmentation (local rhythmic contractions)
E.
Chemical Digestion
1. Chemical breakup of food into its smallest building blocks
2. Accomplished by digestive enzymes
F.
Absorption
1. passage of digested end products, vitamins, minerals and water from the
lumen of the GI tract into the blood or lymph
2. The small intestine is the major organ of absorption
G. Defecation
1. elimination of indigestible substances through the anus as feces
Peritoneum = serous membrane of abdominopelvic cavity
A. Visceral peritoneum
1.
covers external surface of most digestive organs
B. Parietal peritoneum
1.
Continuous with visceral peritoneum
2.
Lines abdominal and pelvic cavities.
1
3.
IV.
Separated from visceral peritoneum by the peritoneal cavity with contains
serous fluid.
C. Folds of the peritoneum
1.
Mesentery is a double layer of peritoneum that extends to the digestive
organs from the body wall.
a. route for blood and lymphatic vessels and nerves
b. hold organs in place
c. store fat
D. Retroperitoneal
1.
refers to organs that adhere to the dorsal wall and lie behind the
peritoneum.
2.
Includes portions of duodenum, pancreas and rectum
3.
May help the alimentary canal from kinking
Histology of GI tract wall: 4 layer plan
A. Mucosa/mucous membrane:
1.
moist innermost layer that lines the lumen.
2.
Functions to 1) secrete enzymes, hormones and mucus, 2) absorption and
3) protection against disease
3. Consists of 3 sublayers.
a.
epithelium of simple columnar epithelium and goblet cells with
hormone and enzyme-secreting cells
b.
Lamina propria: areolar connective tissue with blood and lymph
vessels and nodes.
c. . Muscularis mucosa: small amount of smooth muscle and creates
local movement of mucosa and produces folds to increase surface
area
B. Submucosa
1.
Connective tissue that is deep to mucosa.
2.
Connects mucosa to muscularis and contains nerves, lymph and blood
vessels.
C. Muscularis Externa
1.
Mixes food and propels it through tract.
2.
Mouth, pharynx, and upper third of esophagus = skeletal muscle for
voluntary control for swallowing.
3.
Rest has 2 layers of smooth muscle:
a.
Circular muscle: innermost
b.
Longitudinal: outermost
c.
Stomach has a third layer of oblique muscles
4.
In several places the circular layer thickens to form sphincters that act as
valves to regulate food flow between organs.
D. Serosa (visceral peritoneum)
1.
Outermost layer with protective function
2.
areolar connective tissue and simple squamous epithelium
2
2.
V.
In the esophagus the serosa is replaced by an adventitia of fibrous
connective tissue
Anatomy of Oral cavity/buccal cavity/mouth
A. General anatomy
1.
Bounded by lips anteriorly and cheeks laterally which keeps food between
teeth and assists in speech.
2.
Bounded superiorly by palate.
a.
Hard palate: composed of palatine and maxillary bones (palatine
process) against which tongue forces food in chewing.
b.
Soft palate: posterior to hard palate, separate oral cavity from
nasopharynx and closes off the nasopharynx in swallowing. Uvula
is posterior portion of soft palate.
3.
Vestibule: area between lips, cheeks, and gums.
4.
Oral cavity proper lies within teeth and gums.
B. Tongue
1.
Occupies floor of mouth
2.
Composed of skeletal muscle, connective tissue, glands, and sense
receptors.
3.
Functions in speech, mixes food, forms food into bolus (ball) for
swallowing, initiates swallowing.
4.
Lingual Frenulum: fold of mucosa that secures tongue to floor of mouth
and limits its posterior movement.
5.
Labial frenulum is median fold that joins the internal aspect of each lip to
gum.
6.
Extrinsic muscles attach tongue to bone (hyoid, mandible, styloid process
of temporal) and move tongue within the mouth.
7.
Intrinsic muscles are not attached to bone and change the shape of the
tongue (curl)
8.
Papilla: projections of mucosa on superior surface
a.
Aid in licking
b.
Contain taste buds
C. 3 pairs of Salivary glands
1.
Parotid: large, anterior and inferior to ears, superficial to masseter.
a.
Inflamed by mumps
b.
Ducts empty by upper molars.
2.
Submandibular: lie medial to mandibular angle near base of tongue.
a.
Ducts empty saliva at frenulum.
3.
Sublingual: anterior to submandibular and under tongue
a.
Ducts empty along floor of mouth.
4.
Above 3 are extrinsic salivary glands because they are outside the oral
cavity
5.
Intrinsic salivary glands are small and scattered throughout oral cavity
6.
Saliva composed of
3
D.
a.
Water (99%) to moisten and dissolve food
b.
with enzyme salivary amylase for starch digestion
c.
lysozyme to destroy bacteria. Are also other antibacterial agents
d.
Inorganic salts to buffer food for optimum pH for amylase
e.
Electrolytes such as Na+, Cl-, HCO3- etc.
f.
Mucin: mixes with water to form mucus for lubrication
7.
Functions:
a.
Moisten food
b.
Cleanse teeth
c.
Dissolve food for tasting
d.
Begin chemical breakdown of starch
Control of Salivation
1.
Intrinsic glands secrete continuously to keep mouth moist but extrinsic are
activated to deal with food.
2.
Secretion of saliva is reflex response.: and controlled by the
PARASYMPATHETIC division of the ANS
3.
Salivation center between medulla and pons stimulated activated by
chemoreceptors and pressoreceptors in mouth  increased
parasympathetic firing down facial and glossopharyngeal nerves
increased salivary secretion rich in enzymes
4.
Higher brain centers (smell, sight, hearing, thought of food)  increased
salivation
5.
Stomach and SI can also increase salivation, especially if irritated
6.
Sympathetic nerves  decreased salivation as occurs in stage fright
VI.
The Pharynx
(Review on your own the anatomy of the pharynx as learned in the Respiratory System)
VII.
Anatomy of Esophagus
A. Structure and Function
1.
25 cm (10 in.) long muscular tube that acts as a passageway for food from
pharynx to the stomach
2.
Upper third: skeletal muscle, middle third: skeletal and smooth, lower
third: all smooth.
3.
Collapsed when not swallowing
4.
Pierces diaphragm at esophageal hiatus
5.
Joins stomach at cardiac orifice/ gastroesophageal sphincer
B. Deglutition: swallowing
1.
Buccal phase is voluntary. A bolus is formed and pushed into
oropharynx
2.
Pharyngeal – esophageal phase: bolus moves through pharynx and
esophagus by peristaltic waves via reflex action coordinated in the medulla
and pons
a.
Soft palate/uvula lifts to close opening to nasopharynx
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VIII.
b.
Larynx rises so epiglottis closes glottis
C. Peristalsis
1.
wave of contractions of smooth muscle that propels food forward through
the GI tract.
Anatomy of the Stomach
A. Functions
1.
Mechanical mixing to produce a creamy paste called chyme.
2.
Enzymatic digestion
3.
Storage of up to 4 L through distension
4.
Absorption
5.
Destroy foreign substances with HCl.
B. Gross Anatomy
1.
J-shaped when empty
2.
Rugae are longitudinal folds of mucosa to increase surface area.
3.
Regions of the stomach
a.
Cardiac region: small area that surrounds the cardiac orifice
through which food enters from the esophagus.
b.
Fundus: dome-shaped part projecting above the cardiac orifice
c.
Body: main section
d.
Pyloric region: funnel-shaped inferior end which ends in the
pylorus where the stomach joins the duodenum through the pyloric
sphincter
4.
Greater curvature: convex lateral surface and anchors greater omentum
5.
Lesser curvature: the on the concave medial surface and anchors lesser
omentum
6.
Both omenta are mesentaries that tether the stomach to the body wall and
other digestive organs.
7.
Lesser omentum: fold that connects stomach to liver.
8.
Greater omentum: large fold that hangs from inferior portion of the
stomach, interconnects stomach, duodenum, and large intestine.
a.
Called fatty apron as has much adipose tissue
b.
Also contains many lymph nodes that protects the peritoneal cavity
and peritoneal organs
C. Microscopic Anatomy
1.
In addition to the circular and longitudinal smooth muscles that run
through the digestive system, the stomach has an extra layer that runs
obliquely. This allows for extra mixing, churning, and physical
breakdown of food.
2.
The mucosa is composed of simple columnar epithelilum of alkaline
mucous-producing goblet cells
3.
Gastric pits are deep grooves in mucosa that lead into the gastric glands
which produce the stomach secretion called gastric juice.
4.
Glands composed of different types of cells as follows:
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5.
D.
IX.
Parietal cells secrete:
a.
HCl (pH = 1.5-3.5  = 2)
(1) Converts pepsinogen to pepsin
(2) Kills Microorganisms
(3) Disintegrates plasma membranes/disintegrates intermolecular
bonds
b.
Intrinsic factor: required for absorption of vitamin B12 and
formation of RBCs.
6.
Chief cells which secrete pepsinogen, a precursor of the enzyme pepsin.
7.
Enteroendocrine cells: secrete hormones such as gastrin, a hormone
which increases gastric secretion and motility plus contracts the cardiac
sphincter and relaxes the pyloric sphincter.
Emesis
1.
vomiting
2.
Generally caused by excess stretching of stomach or small
intestines or chemical irritants
3.
4.
Force generated by the diaphragm and abdominal muscles
Excess vomiting can lead to dehydration and acid-base
imbalances
Digestive Processes Occurring in the Stomach
A. Activities
1.
Only enzymatic digestion in adults is protein digestion accomplished by
the enzyme pepsin
2.
Recall the intrinsic factor is also secreted by stomach and prevents
pernicious anemia
3.
Lipid-soluble substances like alcohol and aspirin are absorbed in stomach
B. Regulation of Gastric Secretions (recall parasympathetic is the stimulator!)
1.
Gastric juices and motility regulated by nerves and hormones in 3 phases
2.
Cephalic phase: nervous stimulation
a.
Smell, taste, sight, thought of food stimulates cerebral cortex
feeding center in hypothalamus  impulses to medulla
increases parasympathetic input to gastric glands via vagus nerve
 increased rate of gastric secretion
3.
Gastric phase: both neural and hormonal stimulation once food is in
stomach
a.
Nervous control begins with distension of stomach wall which
activates stretch receptors  increased vagal firing which
stimulate the medulla increased parasympathetic (vagal)
stimulation of gastric glands  increased gastric secretions.
a.
The secretion of the hormone gastrin from stomach mucosa is
6
X.
stimulated locally by presence of food in stomach, especially
proteins, caffeine, alcohol and rising pH . It circulates in
blood stream, returns to stomach and increase gastric
secretions, gastric motility and increase gastric emptying.
4
Intestinal phase
a.
The first component is excitatory and mimics gastrin so the hormone
is called enteric gastrin. It is initiated when chyme enters first part of the duodenum. Is
very brief.
b.
The second component is inhibitory and is called the enterogastric
reflex . It inhibits gastric secretions, gastric motility and
decrease gastric emptying and is stimulated by chyme in
lower duodenum (low pH, distension of duodenum, protein
digestion products).
c.
Allows more time for duodenal digestion and absorption.
d.
Stimulation  increased vagal afferent input to medulla increased
sympathetic stimulation to stomach decreased gastric secretions,
decreased motility, and decreased emptying
e.
Chyme in duodenum also causes release of many intestinal
hormones that also inhibit gastric peristalsis. 2 examples are
a.
Secretin
b.
Cholecystokinin (CCK)
Small Intestine and Associated Structures
A. Functions
1.
Major organ of digestion and absorption
B. Gross Anatomy of the Small Intestine
1.
Major organ of both digestion and absorption
2.
2 valves
a.
Pyloric: between stomach and duodenum
b.
Ileocecal: between ileum of Small intestine and Large Intestine
3.
6-7 m long (17-20') and 2.5 cm (1inch) in diameter.
4.
It has 3 segments: duodenum, jejunum and ileum
5.
Duodenum:
a.
First straight section of 25 cm. (10 in.) and much is retroperitoneal
b.
Receives chyme from stomach
c.
Receives secretions from pancreas and gall bladder whose ducts
dump into common area, the hepatopancreatic ampulla, and then
enter the duodenum at the duodenal papilla, controlled by
hepatopancreatic sphincter
6.
Jejunum: second segment of 2.5 m (8')
7.
Ileum: last segment of 3.5 m (12') which joins Large Intestine at the
ileocecal valve.
8.
The ileum and jejunum are sometimes called ileojejunum and its parts are
suspended from the posterior abdominal wall by the fan-shaped
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C.
mesentary
Microscopic anatomy of S.I is adapted for absorption and secretion
1.
Plicae circulares: deep, permanent folds of mucosa and submucosa
2.
Villi: finger like projections of mucosa approximately 1 mm tall.
a.
Each contains a blood capillary bed and lymphatic capillary bed, a
lacteal, for absorption.
3.
Microvilli: microscopic projections of plasma membranes which form a
brush border
4.
D.
E..
F.
All 3 of the above structures function to increase surface area for
absorption (600X).
5.
Between the villi are openings into intestinal glands which secrete
intestinal juice of water, mucus, lysozyme and digestive enzymes.
6.
Peyer’s Patches are lymphoid structures in the submucosa and become
more abundant toward the large intestine where there are huge numbers of
bacteria.
Digestive Processes Occurring in the Small Intestine
1.
Food spends 3-6 hours in the small intestine
2.
Most digestion and absorption take place here
3.
The major type of movement in the small intestine is segmentation
a.
Rhythmic contractions form segments along SI and function to push
the food back and forth a few centimeters at a time for maximum
contact with the mucosa and digestive enzymes.
b.
Occurs as often as 12 X/min.
4.
Peristaltic contractions move food through small intestine only after
most of the nutrients have been absorbed.
Overall Functions of the Liver
1.
Liver is one of the body’s most important organs, but it’s primary
digestive function is the production of bile
2.
Bile is stored in the gallbladder and secreted into duodenum.
3.
Bile emulsifies fats
a.
Mechanically breaks up fat into small droplets to increase its surface
area for increased speed of digestion.
Gross anatomy of liver
1.
Largest gland in body and weighs 1.4 kg (3 lbs)
2.
Runs all across body under diaphragm.
3.
4 lobes:
a.
Large right and left lobes separated by falciform ligament. (Right
= 6x bigger than left)
b.
Right lobe has inferior quadrate lobe and posterior caudate lobe
associated with it.
4.
Gallbladder is on inferior surface of right lobe.
5.
Each lobe of liver is divided into lobules that contain hepatic cells
8
G.
H.
I.
radiating out from a central vein.
a.
Hepatic cells secrete bile into small canals called bile canaliculi that
drain into bile ducts between lobes. These converge to form the
b.
right and left hepatic ducts, that fuse to form the common hepatic
duct.
c.
These then join with the cystic duct draining the gall bladder
d.
and form the common bile duct which join with the
Hepatopancreatic ampulla where the pancreatic duct joins the
common bile duct and dump into the duodenum controlled by the
hepatopancreatic sphincter.
Microscopic Anatomy of the Liver
1.
Liver is divided into functional units called lobules that are 6-sided
structures of hepatocytes (liver cells).
2.
Hepatocytes radiate outward from a central vein that runs longitudinally
3.
At each of the 6 corners of a lobule is a portal triad composed of an
hepatic artery (for oxygen), a branch of the hepatic portal vein (nutrientrich), and a bile duct taking bile out of the liver.
4.
Blood flows into the liver via an hepatic artery and the hepatic portal veins
and form the equivalent of capillaries in liver called sinusoids.
a.
These dump into central veins  hepatic veins inferior vena cava
5.
Bile flows into bile canaliculi bile ducts in the opposite direction of
blood flow
The gallbladder
1.
Located on ventral surface of liver
2.
Stores bile
3.
Bile contains bile salts, bile pigments, cholesterol, fats and
phospholipids
4.
Hormone Cholecystokinin (CCK) from duodenum is the major
stimulus for gall bladder contraction.
a.
Duodenum is stimulated to produce CCK by fatty acids and
protein digestion products in duodenum
b.
CCK also inhibits gastric secretions
c.
CCK also stimulates secretion of pancreatic juice high in
enzymes from acinar cells in pancreas
e.
CCK also stimulates secretion of intestinal juice and
f.
Decreases GI motility.
Pancreas
1.
Retroperitoneal, elongated (15 cm) organ
2.
Head in C-shaped curve of duodenum.
3.
Body and tail lie behind greater curvature of stomach.
4.
Functions as both in endocrine and exocrine gland.
5.
Endocrine glands secretes hormones (insulin and glucagon) from
9
XI.
clusters of secretory cells called Islets of Langerhans . Hormones go
directly into bloodstream.
a.
Alpha cells secrete glucagons and increase blood glucose
b.
Beta cells secrete insulin and decrease blood glucose
6.
Exocrine glands secretes pancreatic juice of proenzymes into ducts that
lead into pancreatic duct which joins the common bile duct 
hepatopancreatic Ampulla  duodenum.
a.
acinar cells are arranged in cluster around duct and secrete
proenzymes.
b.
Duct cells (in ducts) secrete sodium bicarbonate to neutralize acidic
chyme.
a.
pH = 7-8 and activates proenzymes of pancreas and small intestine.
7.
Pancreas produces enzymes that digest all the food groups in the small
intestine
J.
Regulation of Pancreatic Secretions
1.
Hormone Secretin from duodenum
a.
Stimulated by acids in duodenum
b.
Produces an increase in secretion of bicarbonates from duct cells in
pancreas (recall CCK stimulates acini to produce enzyme-rich juice)
c. Increases rate of bile secretion from liver (recall CCK stimulates
contraction of gall baldder)
d.
It is like CCK in that it decreases gastric secretion and decreases
gastric motility while stimulating production of intestinal juice
secretions
Anatomy of Large intestine
A. Functions
1.
Absorption of water from indigestible food residues
2.
Elimination of feces.
B. Gross anatomy
1.
1.5 m (5')long and 6.5 cm (2.5 in) in diameter.
2.
Extends from ileocecal valve to anus.
3.
Major subdivisions:
a.
Cecum
b.
Appendix
c.
Colon
d.
Rectum
e.
Anal canal
4.
Cecum: saclike portion that lies below ileocecal valve is first part of large
intestine
5.
Vermiform appendix hangs down from cecum
a.
On right side of body
c.
Composed of lymphatic tissue meant to contain bacteria of large
intestine.
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6.
7.
8.
9.
10.
11.
C.
D.
E.
The colon has several distinct regions
a.
Ascending colon travels up right side to right kidney.
b.
Transverse colon runs horizontally across abdomen from hepatic
flexure to splenic flexure.
c.
Descending colon runs down left side from liver to
d.
Sigmoid colon , an S-shaped curve in pelvis to
Rectum which runs inferiorly in front of sacrum.
a.
It has 3 valves that separate feces from gas
b.
A digital rectal exam can be used to examine other pelvic organs
such as the prostate or ovaries
Last 3 cm is the anal canal
a.
Hemorrhoids = weakened/varicose veins in anal canal
Anus is the opening to the exterior and is controlled by
a.
Internal anal sphincter that is involuntary and an
b.
External anal sphincter that is voluntary
2 unique features of the large intestine are the
a.
Teniae coli and
b.
Haustra
The teniae coli are the remnants of the longitudinal muscle that form
longitudinal bands. Their muscle tone puckers the large intestine into
pouches called haustra.
Microscopic Anatomy
1.
Villi and enzyme-secreting glands are absent as most food is already
absorbed
2.
There are huge quantities of goblet cells for mucus to assist in elimination
of feces
Bacterial Flora
1.
Bacteria colonize the large intestine. Some survived the trip come from
ingestion, but most enter through the anus.
2.
Some ferment the indigestible carbohydrates like cellulose and release
acids and a variety of gases, some of which are very stinky.
3.
The flora also synthesizes the B complex vitamins and the vitamin K the
liver needs for making the clotting proteins
Digestive Processes Occurring in the Large Intestine
1.
Substances remain in the large intestine 12 to 24 hours while water,
electrolytes and vitamins are absorbed.
2.
Primary role, however is defecation or removal of feces from the body
3.
Haustral contractions occur every 30 minutes or so and move material
from one haustra to another. This aids in mixing and water reabsorption.
4.
Mass movement are strong peristaltic waves that move feces en masse
over long distances into the rectum.
c.
Occur 3-4 X/day, generally following a meal and stimulated by
11
XII.
stretch receptors in a gastrocolic reflex .
5.
Fiber in the diet strengthens colon contractions and softens the stool.
6.
Defecation: removal of feces from rectum.
7.
Feces contain undigested food residues such as cellulose, millions of
bacteria, epithelial cells, pigments and just enough water for it to pass out
of the body.
8.
The Defecation reflex is initiated by distension of rectum following mass
movement.
a.
Stretch receptors  stimulate sacral spinal cord increase
parasympathetic impulses  strong peristaltic contractions of lower
LI  relaxation of internal anal sphincter voluntary relaxation of
external anal sphincter defecation.
Chemical Digestion (At your discretion)
A. Carbohydrate Digestion
1.
Accomplished through salivary amylase in the mouth, pancreatic
amylase from the pancreas (works in the small intestine), and brush
border enzymes from the small intestine
2.
The end products of carbohydrate digestion are simple sugars such as
glucose, and galactose
B. Protein digestion
1.
Accomplished through pepsin, pancreatic enzymes (works in the small
intestine.
2.
The end products of protein digestion are amino acids
C. Lipid Digestion
1. Accomplished through the enzyme lipase secreted by the pancreas and
working in the small intestine.
2.
Recall bile from the liver only emulsifies fats and does not chemically
digest it.
3.
The end products of lipid digestion are glycerol and fatty acids
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