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DIGESTIVE SYSTEM LECTURE
I. General Introduction
The digestive system is the system of the body that mechanically and chemically breaks
down food. It takes about 12 to 24 hours to completely break down food.
The digestive organs are usually divided into two main groups: the digestive tract and the
accessory structures.
A. Digestive Tract (Gastrointestinal Tract)
The digestive tract is also called the gastrointestinal (GI) tract or the alimentary canal.
It is a continuous tube running from the mouth to the anus. It measures about 9 m (30 ft)
in length.
The GI tract includes the oral cavity, pharynx, esophagus, stomach, small intestine, and
large intestine.
B. Accessory Organs
The second group of organs consists of the accessory organs. These include the teeth,
tongue, salivary glands, pancreas, liver, and gallbladder.
II. Functions of the Digestive System
A. Ingestion
This is just the taking in of food through the mouth.
B. Mechanical Processing
This is the physical breakdown of solid foods. Masses of food are broken into smaller
parts.
First the tongue and teeth physically break down the food and then the digestive tract
mixes the food. The small intestine also physically breaks down food when the secretion
of bile emulsifies or breaks up fat globules into smaller droplets.
C. Digestion
This is the chemical breakdown of food into smaller components that can be absorbed by
the epithelium of the digestive tract. For example starch is broken down into the
disaccharide maltose in the oral cavity. Later the disaccharides are broken down into
monosaccharides in the small intestine.
All of this occurs through the process of hydrolysis. Recall that in hydrolysis water is
used to break a covalent bond and larger molecules are reduced to smaller molecules.
This occurs in the oral cavity, stomach, and small intestine.
D. Secretion
This is the release of water, acids, enzymes, and buffers by the digestive tract and by the
accessory organs into the lumen of the digestive tract.
E. Absorption
This is the movement of small organic molecules, electrolytes, vitamins, and water across
the digestive epithelium and into the blood and lymph.
F. Excretion
This is the removal of waste products which are first compacted and then discharged
during defecation.
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III. Histology of the Digestive Tract
Most of the digestive tract are made up of the same 4 basic tissue layers.
A. Mucosa
The mucosa is the innermost layer or lining of the digestive tract. It is a mucous
membrane made of
1. Epithelium that is in direct contact with the contents of the digestive tract.
2. Lamina Propria- This is loose areolar loose connective tissue under the epithelial
layer.
Have many blood and lymphatic vessels to absorb nutrients. It also binds the mucosa
to the next layer, the submucosa. It also has lots of lymphatic nodules.
The lamina propria extends into the villi (we’ll talk about later)
3. Muscularis mucosa- Sometimes it includes a layer of smooth muscle which throws
the mucous membrane of the stomach and small intestine into many folds called
rugae (RU jee), which increase the surface area for digestion and absorption. Hard
to i.d. on slides.
B. Submucosa
The next layer going out is the submucosa. It consists of a thick layer of connective tissue
that binds the mucosa to the muscularis.
It contains many blood and lymphatic vessels that receive absorbed food molecules.
Also has nerve plexus that regulate movements and secretions of the digestive tract.
The submucosa may also contain glands and lymphatic tissue.
C. Muscularis Externa
Skeletal or smooth muscle.
When smooth, it is generally found in two sheets, an inner sheet of circular fibers and
(you can see spindle shaped cells and nuclei) and outer sheet of longitudinal fibers. The
stomach has a 3rd layer (oblique)
There are also another nerve plexus here that controls the frequency and strength of
contraction of smooth muscle.
D. Adventitia or Serosa
Adventitia = areolar connective tissue with dispersed collagen and elastic fibers
(retroperitoneal organs, e.g. ascending organ)
Serosa = same as adventitia but covered by a visceral peritoneum (intraperitoneal, e.g.
stomach)
IV. Components of the Digestive System
A. Digestive Tract (Gastrointestinal (GI) Tract)
1. Oral Cavity
a. Location/Gross Anatomy
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It includes the lips, hard and soft palates, uvula, oral cavity, tongue, teeth,
frenulum (mucous membrane that secures the tongue to the floor of the mouth),
and salivary glands.
b. Function
The oral cavity has several functions. It is involved in
1. mechanical processing using the teeth and tongue
2. lubricating food by mixing it with mucus and saliva
3. digesting carbohydrates with salivary enzymes.
c. Microscopic Anatomy
Has mucosa, submucosa, and muscularis but no serosa.
Epithelium = stratified squamous (protects food as it goes through the mouth).
Muscularis = skeletal muscle (voluntary swallowing).
2. Pharynx
a. Location/Gross Anatomy
Has 3 regions: nasopharynx, oropharynx, and laryngopharynx
b. Function
Muscule contractions during swallowing move the food bolus along to the
esophagus.
c. Microscopic Anatomy
Does not have serosa.
Epithelium = stratified squamous to protect as food goes through the mouth.
Muscularis = skeletal muscle (swallowing).
3. Esophagus
a. Location/Gross Anatomy
It is a muscular, collapsible tube posterior to the trachea. It is 23 to 25 cm (10 in.)
long and extends from the laryngopharynx through the esophageal hiatus in the
diaphragm and ends in the superior portion of the stomach. The cardiac
sphincter is b/w esophagus and stomach and keeps stomach contents from
going back into esophagus
b. Function
The esophagus carries solid and liquids from the pharynx to the stomach .
It moves the food through a process called peristalsis.
(peri = around; stalsis = contractions)
These are rhythmic contractions of the wall which squeeze food along.
c. Microscopic Anatomy
Does not have a serosa.
Epithelium = stratified squamous
Muscularis = skeletal muscle in upper 1/3 of esophagus; smooth in lower 1/3
both in middle.
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4. Stomach
(chyme = juice)
a. Location/Gross Anatomy
This is a J-shaped organ that lies under the diaphragm.
The superior part is connected to the esophagus. The inferior part empties into
the duodenum of the small intestine.
Regions have names: cardia (area near heart); fundus (dome-shaped part);
body (mid-portion); pylorus (near duodenum).
Pyloric sphincter near duodenum controls movement of partially digested food
(called chyme) into duodenum.
b. Microscopic Anatomy
Epithelium = simple columnar but no goblet cells. Instead has mucous cells that
secrete mucus
Lining has depressions called gastric pits. At the base of each pit are the
openings of several gastric glands which secrete hormones, enzymes, and
buffers.
The mucosa in stomach has a thin layer of muscle fibers that throw the mucous
membrane into many small folds called rugae which increase surface area for
absorption. Movements of this muscle ensure all contents touch all cells.
Muscularis = smooth but with 3 layers: circular, longitudinal, oblique
c. Functions:
1. Stores ingested food and then passes it on in small increments to the small
intestine.
2. Mechanical breakdown of food by peristalsis.
3. Breaks down food. The chemical digestion of proteins begins in this chamber.
The enzyme pepsin works in an acid enviroment to speed up this digestion.
HCl secreted into the stomach accounts for this acidity.
5. Small Intestine
a. Location/Gross Anatomy
The small intestine includes the duodenum (the first 10 to 12 inches), the jejunum (8
feet long), and the ileum (12 feet long).
It is a total of about 6.35 m (21 ft) long.)
b. Function
1. Enzymes from pancreas and bile from liver enter a common duct
(hepatopancreatic ampulla) that empties into the duodenum. Bile emulsifies
lipids by physically breaking down large fat globules into smaller ones. Then
pancreatic fat-digesting enzymes continue to break fats donw.
2. Almost all food, water, and mineral ion absorption occurs here. Nutrients first
go to liver via hepatic portal system.
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The chemical digestion of carbohydrates and proteins is completed here. The
chemical digestion of lipids is started and completed here.
c. Microscopic Anatomy
epithelium = simple columnar.
Mucosa = smooth muscle fibers so have folds that can move and ensure all
contents touch all cells.
Mucosa also has villi, finger like projections that greatly increase the surface
area of this lining. (villus = shaggy hair)
In addition the villi have microvilli that add even more surface areas for
absorption. Microvilli are hair-like projections of the membrane of cells.
Between some of the villi are invaginations of mucosa called intestinal glands
(release hormones) with goblet cells that secrete mucin.
Submucosa = has Brunner's glands (also called submucosal glands) that
secrete alkaline mucin to protect and lubricate the lining of the small intestine.
6. Large Intestine
a. Location/Gross Anatomy
The large intestine is about 1 ½ m (5 ft.) long and extends from the ileum to the anus.
It also has villi, but not as many as in the small intesting.
The large intestine has 4 regions: cecum, colon, rectum, and anal canal.
The cecum is a blind pouch below the ileum.
The appendix is a thin hollow finger-like sac lined by lymphatic nodules. It is
attached to the cecum by an extension of the visceral peritoneum.
The colon is divided into the ascending colon, transverse colon, descending colon,
and sigmoid colon.
The rectum is the last 20 cm (7 to 8 in.) of the colon. Solid waste passes through the
rectum. It does not have serosa.
The anus is the opening of the rectum to the exterior. The muscularis layer has
skeletal muscle so can voluntarily control defecation.
b. Function
a. reabsorbs water, vitamins, bilirubin products, bile salts, and toxins.
b. it compacts feces when water is absorbed
b. absorbing vitamins liberated by bacteria
c. storing fecal material before it is defecated out. The fecal material consists of some
fluid, undigested food molecules, dead cells, and bacteria that normally inhabit
this lower portion of the tract. This mass passes through the rectum when
eliminated.
c. Microscopic Anatomy
Epithelium = simple columnar with goblet cells.
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Muscularis = The longitudinal muscle layer is reduced to 3 bands of smooth muscle
called the teniae coli (TE nee ee). They cause the wall of the colon to draw up into
pocket like sacs called haustra.
B. Accessory Organs
1. Tongue
a. Location/Gross Anatomy
Mostly skeletal muscle. Covered by stratified squamous epithelium
b. Functions
1. mechanical processing of food
2. manipulating food into a ball or bolus as it is chewed to prepare it to be
swallowed
3. contains receptors for the sense of taste.
2. Teeth
a. Location/Gross Anatomy
Have 32 permanent teeth in adults. Will learn details in lab.
b. Function:
1. mechanically break down food in a process called mastication (the teeth cut,
tear, crush, and grind the food).
3. Salivary Glands
a. Location/Gross Anatomy
Many small salivary glands are dispersed in the tongue, inside of lips, and inside
of cheek.
Also have 3 large ones: parotid (anterior to earlobe) (para = beside; ot = ear),
submandibular (under mandible); sublingual (near base of tongue).
Mucous cells secrete mucin (becomes mucus when mixes with water). Serous
cells secrete a watery fluid containng electrolytes and salivary amylase.
b. Function:
1. Release saliva through ducts into the oral cavity.
The saliva lubricates the mouth, secretes amylase, an enzyme that breaks down
starch, and flushes the oral surfaces and helps to control bacteria..
Saliva is 99.5% water and 0.5% other solutes. Those other solutes include ions
(e.g. Na+, K+, Cl-, HCO3-, PO4-), gases, lysozyme (antibacterial enzyme), and
salivary amylase (enzyme that breaks down starch), mucus, and antibodies.
4. Pancreas
a. Location/Gross Anatomy
Located in “C” of duodenum.
b. Functions
1. it secretes the hormones insulin and glucagon into the blood which regulate
blood glucose levels.
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2. it secretes water and digestive enzymes into the small intestine which digest
carbos, fats, proteins, and nucleic acids.
3. it secretes bicarbonate ions which buffer the acidic chime coming from the
stomach.
The pancreatic juice is delivered from the pancreas to the duodenum by the
pancreatic duct. The pancreatic duct unites with the common bile duct from
the liver and pancreas and enters the duodenum in a common duct called the
hepatopancreatic ampulla
c. Microscopic Anatomy
It is made of exocrine glands that secrete enzymes and endocrine cells that
secrete hormones. The exocrine glands are called acini (sing. acinus). Are made
of simple cuboidal cells.
5. Liver
(hepaticus = liver)
a. Location/Gross Anatomy
The liver is located inferior to the diaphragm. Weighs about 3 pounds. It has 4
lobes: right, left, quadrate, and caudate.
b. Functions
1. The liver makes and secretes bile, which is important for digestion of lipids in
the small intestine (emulsifies fat globules).
2. The liver also stores nutrients. The liver converts glucose into stored glycogen.
3. The liver also detoxifies poisons.
The hepatic ducts secrete bile made by the liver. The right hepatic duct is found
in the right lobe of the liver and the left hepatic duct is found in the left lobe of the
liver
The left and right hepatic ducts unite outside the liver to form a single common
hepatic duct. This duct joins the cystic duct from the gall bladder to form the
common bile duct.
c. Microscopic Anatomy
Is organized into 6 sided hepatic lobes that contain plates of hepatocytes.
The lobules are arranged around a central vein that drains the blood flow to the
lobule.
At each corner of the lobule is a portal triad (hepatic artery, hepatic portal veins,
bile duct)
Between the plates are leaky capillaries called hepatic sinusoids which are
bordered by cords of hepatocytes (like spokes on a wheel). Blood from the
hepatic artery and hepatic portal vein percolates through the sinusoids and
empties into the central veins.
The blood from the central veins empties into hepatic veins that empty into the
inferior vena cava.
6. Gallbladder
a. Location/Gross Anatomy
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This is a pear-shaped sac in a fossa along the undersurface of the liver.
b. Functions
It stores and concentrates bile for release into the duodenum.
c. Microscopic Anatomy
The cystic duct carries bile between the gallbladder and the common bile duct.
The common bile duct is formed when the cystic duct and common hepatic duct
unite outside the liver.
V. Digestion
A. Oral Cavity
Here salivary amylase is an enzyme that accelerates the hydrolysis of starch into
maltose.
B. Stomach
The hydrolysis of proteins begins here. The enzyme pepsin catalyzes this reaction. Large
protein molecules are broken down into peptides.
C. Small Intestine
The hydrolysis of starch continues here. The enzyme pancreatic amylase speed this up.
Maltose and other disaccharides are further broken down.
Maltose is broken down into 2 glucoses by the enzyme maltase.
Sucrose is broken down into fructose and glucose by the enzyme sucrase.
Lactose is broken down into galactose and glucose by the enzyme lactase.
Peptides and broken down into amino acids, step by step. Enzymes such as trypsin and
chymotrypsin speed up this process.
After fat globules are emulsified by bile, the fat molecules (triglycerides) are broken
down into fatty acids and glycerol. These can simply diffuse across the cell membrane
because they are soluble in membrane lipids.
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