The Digestive System
General Anatomy and Digestive Processes
A. Subdivisions of the Digestive System:
The digestive system has two anatomical subdivisions, the accessory organs and
the digestive tract.
1. The accessory organs are the teeth, tongue, salivary glands, liver, gallbladder,
and pancreas.
2. The digestive tract is a tube extending from mouth to anus. It includes the oral
cavity, pharynx, esophagus, stomach, small intestine, and large intestine.
B. Relationship of the Digestive Tract to the Peritoneum:
Most of the digestive tract is within the peritoneal cavity, except for the
duodenum, pancreas, and parts of the large intestine, which are retroperitoneal.
1. Along the dorsal wall of the abdominal cavity, the parietal peritoneum turns
inward and forms a sheet of tissue, the dorsal mesentery, extending to the
digestive tract. The membrane then forms the outer covering, or serosa, of the
stomach and most parts of the intestines. In some places it continues beyond as a
sheet of tissue called the ventral mesentery, which may hang freely in the
abdominal cavity.
2. Along the lesser curvature of the stomach, the serosae of the stomach surfaces
meet and continue as a ventral mesentery, the lesser omentum, extending from the
stomach to the liver.
3. Along the greater curvature of the stomach, the serosae form the greater
omentum, which hangs loosely over the small intestine like an apron.
4. The omenta have a loosely organized, lacy appearance due partly to many holes
in the membranes and partly to an irregular distribution of fatty tissue. They may
also contain lymphatic tissue.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
C. Digestive Functions and Processes:
1. The digestive system has four functions: ingestion, digestion, absorption, and
defecation.
2. The functions of the digestive tract are carried out through three principal
processes: motility, secretion, and membrane transport.
D. Stages of Digestion:
There are two stages of digestion: mechanical and chemical.
1. Mechanical digestion is achieved by the cutting and grinding action of the teeth
and the churning contractions of the stomach and small intestine.
2. Chemical digestion consists solely of hydrolysis reactions that break the dietary
macromolecules into their monomers. It is carried out by digestive enzymes
produced by the salivary glands, stomach, pancreas, and small intestine.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
The Mouth Through Esophagus
I) Oral or buccal cavity
A. The Mouth:
1. The mouth, or oral or buccal cavity, functions in the following: ingestion,
sensory responses, mastication, chemical digestion, swallowing, speech, and
respiration. The mouth is enclosed by the cheeks, lips, palate, and tongue.
2. The Cheeks and Lips
• The cheeks and lips retain food and push it between the teeth for
mastication. They are essential for sucking and blowing actions.
• Externally the lips are divided into a cutaneous area and a red area, the latter
of which is red because of the tall dermal papillae and the proximity of
blood vessels to the surface.
• Each lip is attached to the gum behind it by the labial frenulum.
• The vestibule is the space between the teeth and the cheeks and lips.
3. The Tongue
• The tongue is an agile, muscular organ that moves food for chewing and
swallowing, bears taste receptors, and aids in speech.
• The surface of the tongue is covered with lingual papillae, most of which
have taste buds.
• The body of the tongue is attached to the floor of the mouth by the lingual
frenulum.
• The mass of the tongue is composed mainly of two groups of lingual
muscles composed of skeletal tissue.
4. The Palate
a. The palate, separating the oral cavity from the nasal cavity, makes it possible to
breathe while chewing food.
b. Its anterior portion, the hard palate, is supported by the palatine process of the
maxilla and the palatine bones. Posterior to this is the soft palate.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
c. The soft palate has a conical medial projection called the uvula.
5. The Teeth
a. An adult normally has 32 teeth, 16 in the mandible and 16 in the maxilla.
Collectively, they are called the dentition.
b. Teeth can be grouped into incisors, canines, premolars, and molars, based on
their shape, location, and function.
c. Each tooth is embedded in a socket called an alveolus, forming a gomphosis
between tooth and bone. The alveolus is lined by a periodontal membrane.
d. The gum, or gingiva, covers the alveolar bone. The crown of the tooth is that
which extends above the gum line, the neck is the portion from the margin of the
gum to the alveolar bone, and the root is the portion inserted into the alveolus.
e. Most of a tooth consists of hard yellowish tissue called dentin, covered with
enamel in the crown and cementum in the root.
f. Internally a tooth has a dilated pulp cavity in the crown and a narrow root canal
in the root. These spaces are occupied by pulp, composed of loose connective
tissue, blood and lymphatic vessels, and nerves.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
B. Mastication:
Mastication breaks food into pieces small enough to be swallowed and exposes
more surface to the action of digestive enzymes. It is the first step in mechanical
digestion.
C. Saliva and the Salivary Glands:
Saliva moistens the mouth, digests a small amount of starch and fat, cleanses the
teeth, inhibits bacterial growth, dissolves
dissolves molecules so they can stimulate taste
buds, and moistens food and binds particles together to aid in swallowing. It is
secreted by the salivary glands.
1. Saliva
a. Saliva is a hypotonic solution composed of 97% to 99.5% water and salivary
amylase, lingual lipase, mucus, lysozyme, immunoglobulin A, and electrolytes.
b. Saliva has a pH of 6.8 to 7.0.
2. The Salivary Glands
a. The intrinsic salivary glands, located within the oral tissues, include the lingual
glands embedded in the tongue, labial glands on the inner aspect of the lips, and
buccal glands on the inside of the cheeks. They secrete relatively small amounts of
saliva all the time to keep the mouth moist and
and inhibit bacterial growth.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
b. The extrinsic glands are situated outside the oral cavity but convey saliva to it
through ducts. These are the parotid glands, submandibular glands, and sublingual
glands..
3. Salivation
a. The extrinsic salivary glands secrete 1.0 to 1.5 L of saliva per day.
b. Food stimulates tactile, pressure, and taste receptors, which transmit signals to
the salvatory nuclei in the medulla oblongata and pons. These nuclei also receive
input from higher brain centers so even the odor, sight, or thought of food
stimulates salivation.
c. The salvatory nuclei send autonomic signals: sympathetic stimulation reduces
saliva output, and parasympathetic stimulation causes the production of thinner
saliva with more salivary amylase.
d. Salivary amylase begins to digest starch as the food is chewed, while the mucus
of saliva binds food particles into a soft, slippery, easily swallowed mass called a
bolus.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
II) The Pharynx
. The pharynx has a deep layer of longitudinally oriented skeletal muscle and a
superficial layer of circular skeletal muscle.
. The circular muscle is divided into superior, middle, and inferior pharyngeal
constrictors, which force food downward during swallowing.
III) The Esophagus
1. The esophagus is a straight muscular tube extending from the larynx to the
stomach at the cardiac orifice.
2. The wall of the esophagus consists of four layers: mucosa, submucosa,
muscularis externa, and serosa or adventitia. These layers and their components are
shown in the Figure.
3. The esophagus, stomach, and intestines have a nervous network called the
enteric nervous system, which regulates the system's motility, secretion, and blood
flow.
4. The inferior end of the esophagus is more constricted than the rest, forming a
gastroesophageal sphincter. This is a physiological (not anatomical) constriction
that helps close the cardiac orifice.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
F. Swallowing:
1. Swallowing, or deglutition, is a complex action involving over 22 muscles in the
mouth, pharynx and esophagus. They are coordinated by the swallowing center, a
nucleus in the medulla oblongata and pons.
2. Swallowing occurs in stages called the buccal and pharyngeal-esophageal
phases.
a. In the buccal stage, the tongue collects food, forms a bolus, and pushes it back
into the oropharynx. Here the bolus stimulates tactile receptors and activates the
pharyngeal-esophageal phase.
b. In the pharyngeal-esophageal phase, the root of the tongue blocks the oral
cavity, the soft palate rises and blocks the nasopharynx, and the infrahyoid muscles
pull the larynx up, the epiglottis covers its opening, and vestibular folds close off
the airway. The food bolus is driven downward by constriction of the upper, then
the middle, and finally the lower pharyngeal sphincters. As the bolus enters the
esophagus, it stretches it and triggers peristalsis.
c. In the esophagus, the circular muscle behind the bolus constricts and pushes it
downward. Ahead of the bolus, the circular muscle relaxes while the longitudinal
muscle contracts.
d. Liquid normally reaches the stomach in 1-2 seconds and a food bolus in 4-8
seconds. As a bolus reaches the lower end of the esophagus, the gastroesophageal
sphincter relaxes to let it pass into the stomach.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
IV. The Stomach
The stomach is a muscular sac in the upper left abdominal cavity immediately
inferior to the diaphragm. It functions primarily as a food storage organ. When
empty, is has a volume of 50 mL. When very full, it may hold up to 4L.
The stomach mechanically breaks up food particles, liquefies the food, and begins
the chemical digestion of proteins and a small amount of fat, producing a mixture
of semidigested food called chyme.
Gross Anatomy:
1. The stomach is J-shaped, with a lesser curvature on its medial margin, and a
greater curvature along its lateral margin.
2. The stomach is divided into four regions: the cardiac region, fundic region, body
antrum, and pyloric region. The pyloric region terminates into a pyloric canal and
pylorus, a narrow passage leading to the duodenum. It is guarded with a pyloric
sphincter.
Innervation and Circulation:
1. The stomach receives parasympathetic and sympathetic stimulation.
2. It is supplied with blood from the celiac artery. All blood leaving the stomach
enters hepatic portal circulation before returning to the heart.
The Stomach Wall:
The stomach wall has tissue layers similar to those of the esophagus, with some
variations.
a. When the stomach is empty, the mucosa and submucosa form conspicuous
longitudinal wrinkles called rugae.
b. The gastric mucosa is pocked with depressions called gastric pits. Cells near the
bottom of the pits divide repeatedly, providing a new source for epithelial cells.
c. At the bottom of the pits lie glands. In the cardiac and pyloric regions, these are
called the cardiac and pyloric glands, respectively, and secrete mucus only.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
d. In the rest of the stomach, the glands are called gastric glands and have a greater
variety of cell types and secretions: mucous neck cells secrete mucus, chief cells
secrete renin and lipase in infancy and pepsinogen throughout the remainder of
life, parietal cells secrete hydrochloric acid and intrinsic factor, and
enteroendocrine cells secrete hormones and paracrines that regulate digestion.
Stomach anatomy
Structure of the Stomach Mucosa
Gastric Secretions:
1. The gastric glands produce 2-3 L of gastric juice daily, composed mainly of
water, hydrochloric acid, and pepsin.
2. Hydrochloric Acid
a. Gastric juice has a high concentration of hydrochloric acid (HCl) and a pH as
low as 0.8.
b. HCl secretion does not affect the pH of the parietal cells that manufacture it
because H+ is pumped out as fast as it is generated. The bicarbonate ions are
exchanged for chloride ions from the blood plasma and the Cl- is pumped into the
lumen of the gastric gland. HCl accumulates in the stomach while bicarbonate ions
accumulate in the blood.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
c. Stomach acid has several functions: (1) It activates the enzymes pepsin and
lingual lipase. (2) It breaks up connective tissues and plant cell walls. (3) It
converts ferric ions to ferrous ions. (4) It contributes to nonspecific disease
resistance by destroying ingested bacteria and other pathogens.
3. Intrinsic Factor
a. Parietal cells also secrete a glycoprotein called intrinsic factor that is essential to
the absorption of vitamin B12 by the small intestine.
b. The secretion of intrinsic factor is the only indispensable function of the
stomach.
4. Pepsin
a. Several enzymes are secreted as inactive proteins called zymogens. Chief cells
secrete the zymogen called pepsinogen.
b. Hydrochloric acid removes some of the amino acids from pepsinogen and
converts it to pepsin. The function of pepsin is to digest dietary proteins to shorter
peptide chains.
5. Other Enzymes
In infants, the chief cells also secrete gastric lipase and renin.
6. Chemical Messengers
a. Gastric glands have various kinds of enteroendocrine cells that collectively
produce as many as 20 secretions, most of which behave as hormones or paracrine
secretions.
b Gastrin travels in the bloodstream and stimulates motility of the large intestine
and it diffuses to nearby parietal and chief cells and stimulates the secretion of
hydrochloric acid and enzymes.
Gastric Motility:
1. During swallowing, signals from the swallowing center of the medulla oblongata
stimulate the stomach to relax in preparation of the arrival of food. When food
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
enters the stomach, it is stimulated to stretch further, a phenomenon known as the
stress-relaxation response.
2. Next, the stomach shows a rhythm of peristaltic contractions, governed by
pacemaker cells in the longitudinal layer of muscle of the greater curvature.
3. The antrum of the stomach holds about 30 mL of chyme. As a peristaltic wave
passes down the antrum, it squirts about 3 mL of chyme into the duodenum.
Allowing only small amounts into the duodenum at a time enables the duodenum
to neutralize stomach acid and digests nutrients little by little.
Vomiting:
Vomiting is induced by excessive stretching of the stomach, psychological stimuli,
and chemical irritants. These factors stimulate the emetic center in the medulla
oblongata, which in turn stimulates the gastroesophageal sphincter to relax and the
diaphragm and abdominal muscles to contract.
Digestion and Absorption:
- Protein, starch, and fat are partially digested in the stomach by salivary and
gastric enzymes and then passed to the small intestine, where most digestion and
nearly all nutrient absorption occur.
- The stomach does not absorb a significant amount of nutrients but does absorb
aspirin and some lipid-soluble drugs.
Protection of the Stomach:
The living stomach is protected in three ways from the harsh chemical of its
interior.
a. It has a highly alkaline mucous coat.
b. Epithelial cells are replaced every 3-6 days.
c. Tight junctions between epithelial cells prevent gastric juice from seeping
between cells.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
Regulation of Gastric Function:
1. Gastric activity is divided into three stages called the cephalic, gastric, and
intestinal phases.
2. The Cephalic Phase
a. The cephalic phase is stimulated by the sight, smell, taste, or mere thought of
food.
b. Sensory and mental images converge on the hypothalamus, which transmits
signals to the stomach by way of the medulla oblongata and vagus nerves.
c. Gastric secretion begins when food is swallowed.
3. The Gastric Phase
a. The gastric phase is stimulated by food in the stomach and accounts for twothirds of gastric secretion.
b. Gastric secretion is also under hormonal and paracrine control. Injected food
raises the pH in the stomach, which stimulates G cells to secrete gastrin. Gastrin, in
turn, stimulates pepsinogen and HCl secretion.
4. The Intestinal Phase
a. The intestinal phase is stimulated by chyme entering the duodenum. For a short
time, enteroendocrine cells secrete intestinal gastrin, which initially enhances
gastric secretion and motility.
b. Signals from the small intestine soon become inhibitory. Hydrochloric, fats, and
peptides in the duodenum trigger the enterogastric reflex. With this reflex,
sympathetic signals inhibit gastric motility and secretion.
c. The effects of all this are that gastric secretion declines and the pyloric sphincter
contracts tightly to limit the addition of more chyme into the duodenum, giving the
duodenum more time to work on the chyme.
d. Chyme in the duodenum also stimulates its enteroendocrine cells to release
secretin, cholecystokinin, and gastric inhibitory peptide. The first two hormones
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
stimulate the pancreas and gallbladder, but all three suppress gastric action and
motility.
The Liver, Gallbladder, and Pancreas
The small intestine receives not only chyme from the stomach but also secretions
from the liver and pancreas.
A. The Liver:
The liver is a reddish brown gland located inferior to the diaphragm in the right
hypochondriac and epigastric regions. It is the body's largest gland and performs a
tremendous variety of functions, including the secretion of bile for digestive
purposes.
1. Gross Anatomy
a. The liver has four lobes called the right, left, quadrate, and caudate lobes. Its
ligaments and lobes can be seen in the Figure.
b. The gallbladder adheres to a depression on the inferior surface of the liver
between the right and quadrate lobes.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
2. Microscopic Anatomy
a. The liver parenchyma consists mostly of hepatocytes arranged in cylinders
called hepatic lobules. Each lobule is about 1 mm in diameter and 2 mm long and
has a central vein passing through its core.
b. The liver secretes bile into narrow channels, the bile canaliculi, between sheets
of hepatocytes. Bile passes from there into the small bile ductules and then into the
right and left hepatic ducts. These two ducts converge to form the common hepatic
duct, which then joins the cystic duct coming from the gallbladder.
c. The common bile duct descends through the lesser omentum and joins the duct
of the pancreas, forming the hepatopancreatic ampulla. This ampulla contains the
hepatopancreatic sphincter, which regulates the passage of bile and pancreatic
secretion into the duodenum.
B. The Gallbladder and Bile:
The gallbladder is a greenish sac about 10 cm long.
a. When bile is not needed for digestion, the hepatopancreatic sphincter is closed.
Bile then fills up the common bile duct and spills over into the gallbladder, which
absorbs water and stores the bile for later use.
b. The liver produces 500-1,000 mL of bile per day. It is a yellow-green fluid
containing minerals, bile pigments, bile salts, cholesterol, neutral fats, and
phospholipids.
c. The principal bile pigment is bilirubin from the decomposition of hemoglobin.
d. Bile salts are steroids synthesized from cholesterol. Bile salts are phospholipids
that aid in fat digestion and absorption. They are not excreted in the feces but are
reabsorbed in the ileum and returned to the liver.
C. The Pancreas:
1. The pancreas is a soft, spongy, pink gland posterior to the greater curvature of
the stomach and outside the peritoneal cavity. It has both endocrine and exocrine
functions.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
a. Most of the pancreas is exocrine tissue, which secretes 1,200-1,500 mL of
pancreatic juice per day, which ends up in the main pancreatic duct running
lengthwise through the gland. It empties into the small intestine through the
hepatopancreatic ampulla or by way of a smaller accessory pancreatic duct in some
people.
2. Pancreatic juice is an alkaline mixture of water, electrolytes (bicarbonate
mostly), enzymes, and zymogens.
a. The acinar cells secrete the enzymes and zymogens, whereas the gland ducts
secrete the sodium bicarbonate. Bicarbonate buffers the hydrochloric acid from the
stomach.
b. Pancreatic zymogens are trypsinogen, chymotrypsinogen, and
procarboxypeptidase.
c. Other pancreatic enzymes include pancreatic amylase, pancreatic lipase,
ribonuclease and deoxyribonuclease.
d. Exocrine secretions of the pancreas are used.
Regulation of Secretion
Bile and pancreatic juice are secreted in response to similar stimuli. During the
cephalic and gastric phases of gastric secretion, the vagus nerves also stimulate
pancreatic secretion.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
a. As chyme enters the duodenum laden with acid and fat, it stimulates the
duodenal mucosa to secrete cholecystokinin (CCK).
b. CCK stimulates the contraction of the gallbladder, forcing more bile into the
common bile duct, secretion of pancreatic enzymes, and relaxation of the
hepatopancreatic sphincter.
c. Acidic chyme also stimulates the duodenum to release secretin which stimulates
the production of bicarbonate ions by both the hepatic and pancreatic ducts to
neutralize stomach acid in the duodenum.
V. The Small Intestine
Nearly all chemical digestion and nutrient absorption occur in the small intestine.
The term "small" applies to its diameter, not its length. Circular folds, villi, and
microvilli all serve to increase the surface area inside the small intestine.
A. Gross Anatomy:
The small intestine is divided into three regions.
1. The duodenum constitutes the first 25 cm. It receives the stomach contents,
pancreatic juice, and bile. Stomach acid is neutralized here, pepsin is inactivated by
the elevated pH, and pancreatic enzymes take over the job of chemical digestion.
2. The jejunum comprises the next 2.5 m.
3. The ileum forms the last 3.6 m and ends at the ileocecal junction.
B. Microscopic Anatomy:
1. The largest folds of the intestinal wall are transverse to spiral ridges called
circular folds. They occur from the duodenum to the middle of the ileum, where
they cause the chyme to flow in a spiral path along the intestine. This slows its
progress, causes more contact with the mucosa, and promotes more thorough
mixing and nutrient absorption.
2. The mucosa also possesses villi, with fingerlike shapes. The largest villi are in
the duodenum and progressively become smaller in more distal regions of the
small intestine.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
a. A villus is covered with two kinds of epithelial cells: columnar absorptive cells
and mucus-secreting goblet cells.
b. The core of the villus is filled with areolar tissue containing a capillary network,
an arteriole, a venule, and a lymphatic capillary called a lacteal.
3. Each epithelial cell of a villus has a fuzzy brush border of microvilli containing
brush border enzymes that carry out contact digestion.
4. On the floor of the small intestine between the villi are the intestinal crypts that
are similar in structure and function to the gastric glands.
5. The duodenum has prominent duodenal (Brunner) glands in the submucosa.
They secrete bicarbonate-rich mucus that neutralizes stomach acid while
simultaneously protecting the mucosa.
6. Throughout the small intestine the lamina propria and submucosa have a large
population of lymphocytes. Other lymphatic tissue, Peyers patches, are found in
the ileum on one side of the intestinal wall.
C. Intestinal Secretion:
The intestinal crypts secrete 1-2 L of intestinal juice per day, especially in response
to acid, hypertonic chyme, and intestinal distension.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
D. Intestinal Motility:
1. Contractions of the small intestine serve three functions: (1) to mix chyme with
intestinal juice, bile, and pancreatic juice; (2) to churn chyme and bring it in
contact with the brush border for digestion and absorption; and (3) to move residue
toward the large intestine.
2. Segmentation is the most common type of movement of the small intestine.
Ring-like constrictions appear at several places along the intestine and then relax
while constrictions occur elsewhere. The effect is to churn the contents of the
intestine.
a. The intensity (but not frequency) of the contractions is modified by nervous and
hormonal influences.
3. When most nutrients have been absorbed and little remains but residue,
segmentation slows and peristalsis begins.
4. At the ileocecal junction, the muscularis of the ileum is thickened to form a
sphincter, the ileoceal valve. The valve is usually closed.
a. Food in the stomach triggers the release of gastrin as well as the gastroileal
reflex, both of which enhance segmentation in the ileum and relax the valve.
b. As the cecum fills with residue, the pressure pinches the valve shut, preventing
the reflux of cecal contents into the ileum.
VI. The Large Intestine
A. Gross Anatomy:
1. The large intestine begins with the cecum, a blind pouch inferior to the ileocecal
valve. Attached to its lower end is the vermiform appendix.
- The appendix is densely populated with lymphocytes and is an important source
of immune cells.
2. The ascending colon begins at the ileocecal valve and passes up the right side of
the abdominal cavity near the right lobe of the liver, and becomes the transverse
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
colon. This passes
es horizontally across the upper abdominal cavity and turns 90o
downward and becomes the descending colon.
3. At the pelvic inlet, the colon turns medially and downward, forming a roughly
S-shaped
shaped portion called the sigmoid colon. Within the pelvic cavity, the colon
straightens and becomes the rectum.
- The rectum has three internal folds called rectal valves that enable it to retain
feces while passing gas.
4. The final 3 cm of large intestine is the anal canal, which passes through the
levator ani muscle and terminates at the anus. Here the mucosa forms longitudinal
ridges (anal columns) with depressions between them (anal sinuses).
- Large hemorrhoidal veins form superficial plexuses in the anal columns and,
since they lack valves, are especially subject to distention, leading to hemorrhoids.
5. The anus is regulated by two sphincters: an internal one composed of smooth
muscle, and an external anal sphincter composed of skeletal muscle.
B. Microscopic
pic Anatomy:
1. The mucosa of the large intestine has a simple
simple columnar epithelium in all
regions except the anal canal, where it is stratified squamous.
2. There are no circular folds or villi in the large intestine, but there are intestinal
crypts. They are deeper than in the small intestine and have a greater density
d
of
goblet cells. Mucus is their only significant secretion.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
C. Bacterial Flora and Intestinal Gas:
1. The large intestine is densely populated with several species of bacteria referred
to collectively as the bacterial flora. They ferment cellulose and other undigested
carbohydrates and synthesize B vitamins and vitamin K, which are absorbed by the
colon.
2. The average person expels about 500 mL of flatus per day. It is composed of
nitrogen, carbon dioxide, hydrogen, methane, hydrogen sulfide, and indole and
skatole.
D. Absorption and Motility:
1. Each day, about 500 mL of food residue enters the large intestine. It undergoes
no further chemical digestion, but its volume is reduced as it passes through the
large intestine. The average adult voids about 150 mL of feces per day, consisting
of 75% water and 25% solid matter, of which 30% is bacteria, and 30% undigested
fiber.
2. Strong contractions called mass movements occur one to three times a day, last
about 15 minutes each, and occur especially an hour after breakfast.
E. Defecation:
1. In the intrinsic defecation reflex, stretch signals travel by the mesenteric nerve
plexus to the muscularis of the descending and sigmoid colons and the rectum.
This triggers a peristaltic wave that drives the feces downward, and it relaxes the
internal anal sphincter. Defecation occurs only if the external anal sphincter is
voluntarily relaxed at the same time.
2. The intrinsic reflex is relatively weak and usually require the cooperative action
of a stronger parasympathetic defecation reflex involving the spinal cord. This
intensifies the peristaltic response and helps relax the internal anal sphincter.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
Chemical Digestion and Absorption
A. Carbohydrates:
Most digestible dietary carbohydrate is starch.
1. Carbohydrate Digestion
a. Starch is digested first to oligosaccharides two to eight glucose residues long,
then into the disaccharide maltose, and finally to glucose, which is absorbed by the
small intestine.
b. The process of starch digestion begins in the mouth. Salivary amylase
hydrolyzes starch into oligosaccharides, and functions best at the pH of the mouth
cavity. It is denatured quickly upon contact with stomach acid.
c. Starch digestion resumes in the small intestine when the chyme mixes with
pancreatic amylase that entirely converts it to oligosaccharides and maltose within
10 minutes. Its digestion is completed by brush border enzymes.
2. Carbohydrate Absorption
a. In the plasma membrane adjacent to the brush border enzymes, there are
transport proteins that absorb monosaccharides as soon as they are produced. Most
of the absorbed sugar is glucose, which is taken up by a sodium-dependent glucose
transporter (SGLT).
b. After a high-carbohydrate meal, two to three times as much glucose is absorbed
by solvent drag as by SGLT.
B. Proteins:
1. Enzymes that digest proteins are called proteases or peptidases. They are
absent from the saliva but begin their job in the stomach.
a. In the stomach, pepsin hydrolyzes any peptide bond between tyrosine and
phenylalanine, thus digesting 10-15% of the dietary protein into shorter
polypeptides.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
b. In the small intestine, the pancreatic enzymes trypsin and chymotrypsin take
over protein digestion by hydrolyzing polypeptides into even shorter oligopeptides.
Finally these are taken apart one amino acid at a time by carboxypeptidase,
aminopeptidase, and dipeptidase. All three of these enzymes are found on the
brush border.
2. Amino acid absorption is similar to that of monosaccharides. There are several
sodium-dependent amino acid cotransporters for different classes of amino acids.
C. Lipids:
1. Fats are digested by enzymes called lipases.
a. Lingual lipase from the intrinsic salivary glands is activated by stomach acid,
where it digests as much as 10% of the ingested fat.
b. Most fat digestion occurs in the small intestine through the action of pancreatic
lipase.
c. In the small intestine, fats are first broken up into smaller emulsification droplets
by lecithin and bile salts in the bile. When lipase digests fats, the products are two
fatty acids (FFAs) and a monoglyceride. Bile salts coat these and other lipids and
form droplets called micelles.
2. Micelles pass between the microvilli of the brush border, and upon reaching the
surface of the epithelial cell, they release their lipids. The lipids diffuse freely
across the phospholipid plasma membrane.
3. Within the cell, the FFAs and monoglycerides are resynthesized into
triglycerides. They are coated with a thin film of protein, forming droplets called
chylomicrons.
4. Although some free fatty acids enter the blood capillaries, chylomicrons are too
large to do so and must be first transported in the lymphatic lacteal.
D. Nucleic Acids:
Nucleic acids are generally present in small quantities compared to the other
polymers. The nucleases of pancreatic juice hydrolyze these to their component
nucleotides. Nucleosidases and phosphatases of the brush border further break
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬
them down, and the products are transported across the intestinal epithelium by
membrane carriers.
E. Vitamins:
Vitamins are absorbed unchanged.
1. The fat-soluble vitamins are absorbed with other lipids.
2. Water soluble vitamins are absorbed by simple diffusion, with the exception of
vitamin B12. This is an unusually large molecule that can only be absorbed if it
binds to intrinsic factor from the stomach.
F. Minerals:
Minerals (electrolytes) are absorbed along the entire length of small intestine.
- Iron and calcium are unusual in that they are absorbed in proportion to the body's
need, whereas other minerals are absorbed at fairly constant rates regardless of
need.
G. Water:
1. The digestive system is one of several systems involved in water balance.
2. The digestive tract receives about 9 L of water per day: 0.7 L in food, 1.6 L in
drink, and 6.7 L in gastrointestinal secretions. About 8 L of this is absorbed by the
small intestine and 0.8 L by the large intestine. Water is absorbed by osmosis.
a. Diarrhea occurs when the large intestine absorbs too little water from the feces.
b. Constipation occurs when fecal movement is slow, too much water is
reabsorbed, and the feces become hardened.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬/ ‫كلية العلوم‬/ ‫حقوق الطبع و النسخ خاصة لقسم علم الحيوان‬