The Digestive System
OVERVIEW OF THE DIGESTIVE SYSTEM
The organs of the digestive system fall into two main groups:
1- Alimentary canal, also called the gastrointestinal (GI) tract
or gut,
It digests food—breaks it down into smaller fragments (digest =
dissolved)—and absorbs the digested fragments through its
lining into the blood.
-The organs of the alimentary canal are the mouth, pharynx,
esophagus, stomach, small intestine, and large intestine.
-In a cadaver, the alimentary canal is approximately 9 m (about
30 ft) long, but in a living person, it is considerably shorter
because of its muscle tone.
.
- Food material in this tube is technically
outside the body because the canal is open to
the external environment at both ends.
2- The accessory digestive organs are the teeth,
tongue, gallbladder, and a number of large
digestive glands—the salivary glands, liver,
and pancreas.
-The teeth and tongue are in the mouth, or
oral cavity, while the digestive glands and
gallbladder lie outside the GI tract and
connect to it by ducts.
Digestive System Organs
The Mouth
The mouth, a mucosa-lined cavity, is also called the
oral cavity, or buccal cavity (buk′al).
- Its boundaries are the lips (labia) anteriorly, cheeks
laterally, palate superiorly, and tongue inferiorly .
- Its anterior opening is the oral orifice. Posteriorly, the
oral cavity is continuous with the oropharynx.
- It is lined with stratified squamous epithelium which
can withstand considerable friction.
- The space bounded externally by the lips and cheeks
and internally by the gums and teeth is called the
vestibule .
- The area that lies within the teeth and gums is the
oral cavity proper.
• The palate, forming the roof of the mouth, has two
distinct parts: the hard palate anteriorly and the
soft palate posteriorly.
• The hard palate is formed by the palatine bones
and the palatine processes of the maxillae, and it
forms a rigid surface against which the tongue
forces food during chewing.
• The soft palate is a mobile fold formed mostly of
skeletal muscle. Projecting downward from its free
edge is the fingerlike uvula (u′vu-lah). The soft
palate rises reflexively to close off the nasopharynx
when we swallow.
The Tongue
The tongue occupies the floor of the mouth and fills
most of the oral cavity when the mouth is closed.
• A fold of mucosa, called the lingual frenulum,
secures the tongue to the floor of the mouth and
limits posterior movements of the tongue.
• Children born with an extremely short lingual
frenulum are often referred to as “tongue-tied”
because of speech distortions that result when
tongue movement is restricted. This congenital
condition, called ankyloglossia (“fused tongue”), is
corrected surgically by snipping the frenulum.
• The Pharynx
From the mouth, food passes posteriorly
into the oropharynx and then the
laryngopharynx ,both common
passageways for food, fluids, and air. (The
nasopharynx has no digestive role.)
The mucosa contains a friction-resistant
stratified squamous epithelium well
supplied with mucus-producing glands.
• The Esophagus
A muscular tube about 25 cm (10 inches) long, is
collapsed when not involved in food propulsion.
The presence of the stratified squamous epithelium
indicates the esophagus is to accommodate high
friction.
The esophagus takes a fairly straight course
through the mediastinum of the thorax and pierces
the diaphragm at the esophageal hiatus to enter
the abdomen. It joins the stomach at the cardiac
orifice. The cardiac orifice is surrounded by the
gastro-esophageal or cardiac sphincter acts as a
valve. The muscular diaphragm, which surrounds
this sphincter, helps keep it closed when food is not
being swallowed.
• Histology of the Alimentary Canal
From the esophagus to the anal canal, the walls of
the alimentary canal have the same four basic
layers :
1-The mucosa, or mucous membrane—the
innermost layer—is a moist epithelial membrane
that lines the alimentary canal lumen from mouth
to anus. Its major functions are (1) secretion of
mucus, digestive enzymes, and hormones, (2)
absorption of the end products of digestion into the
blood, and (3) protection against infectious disease.
2- The Submucosa
just external to the mucosa, is a moderately dense
connective tissue containing blood and lymphatic
vessels, lymphoid follicles, and nerve fibers.
3-The Muscularis Externa
This layer is responsible for segmentation and
peristalsis. It typically has an inner circular layer and
an outer longitudinal layer of smooth muscle cells
.In several places along the tract, the circular layer
thickens, forming sphincters that act as valves to
prevent backflow and control food passage from
one organ to the next.
4-The Serosa
The outermost layer which is the visceral
peritoneum. It is formed of a single layer of
squamous epithelial cells.
In the esophagus, which is located in the thoracic
instead of the abdominopelvic cavity, the serosa is
replaced by an adventitia (ad″ven-tish′e-ah).
• HOMEOSTATIC IMBALANCE
Heartburn, the first symptom of gastroesophageal reflux
disease (GERD), is the burning, radiating substernal
pain that occurs when the acidic gastric juice
regurgitates into the esophagus. Heartburn is most
likely to happen in conditions that force abdominal
contents superiorly, such as extreme obesity,
pregnancy, and running, which causes stomach
contents to splash upward with each step (runner’s
reflux). It is also common in those with a hiatal hernia,
in which the superior part of the stomach protrudes
slightly above the diaphragm.
If the episodes are frequent and prolonged, esophagitis
(inflammation of the esophagus) and esophageal ulcers
may result.
• The Stomach
Below the esophagus, the GI tract expands to form
the stomach ,a temporary “storage tank” where
chemical breakdown of proteins begins and food is
converted to a creamy paste called chyme (kīm;
“juice”).
The stomach lies in the upper left quadrant of the
peritoneal cavity, nearly hidden by the liver and
diaphragm. Specifically, it lies in the left
hypochondriac, epigastric, and umbilical regions of
the abdomen. Though relatively fixed at both ends,
the stomach is quite movable in between.
• The adult stomach varies from 15 to 25 cm (6
to 10 inches) long, but its diameter and
volume depend on how much food it
contains. An empty stomach has a volume of
about 50 ml and a cross-sectional diameter
only slightly larger than the large intestine,
but when it is really distended it can hold
about 4 L (1 gallon) of food and may extend
nearly all the way to the pelvis! When empty,
the stomach collapses inward, throwing its
mucosa (and submucosa) into large,
longitudinal folds called rugae .
• The small cardiac region, or cardia (“near the heart”),
surrounds the cardiac orifice through which food enters
the stomach from the esophagus. The fundus is its
dome-shaped part, that bulges superolaterally to the
cardia. The body, the midportion of the stomach, is
continuous inferiorly with the funnel-shaped pyloric
region. The pylorus is continuous with the duodenum
through the pyloric sphincter, which controls stomach
emptying (pylorus = gatekeeper).
• The convex lateral surface of the stomach is its greater
curvature, and its concave medial surface is the lesser
curvature. Extending from these curvatures are two
mesenteries, called omenta (o-men′tah), that help
tether the stomach to other digestive organs and the
body wall
• The lesser omentum runs from the liver to the
lesser curvature of the stomach, where it becomes
continuous with the visceral peritoneum covering
the stomach.
• The greater omentum from the greater curvature
of the stomach to cover the coils of the small
intestine. The greater omentum is riddled with fat
deposits. It also contains large collections of lymph
nodes.
• The immune cells and macrophages in these nodes
“police” the peritoneal cavity and intraperitoneal
organs.
• Microscopic Anatomy of the stomach
The stomach wall contains the four layers typical of
most of the alimentary canal .Besides the usual circular
and longitudinal layers of smooth muscle, there is a
layer that runs obliquely .
The lining epithelium of the stomach mucosa is a
simple columnar epithelium composed entirely of
goblet cells, which produce a protective coat of alkaline
mucus .This otherwise smooth lining is dotted with
millions of deep gastric pits which lead into the gastric
glands that produce the stomach secretion called
gastric juice. The cells forming the walls of the gastric
pits are primarily goblet cells. The glands in these pits
contain a variety of secretory cells, including the four
types described here:
1. Mucous neck cells, found in the upper, or “neck,”
regions of the glands, produce mucus .
2. Parietal cells, secrete hydrochloric acid (HCl) and
intrinsic factor.
-HCl makes the stomach contents extremely acidic
(pH 1.5–3.5), a condition necessary for activation
and optimal activity of pepsin. The acidity also helps
in food digestion by denaturing proteins and
breaking down cell walls of plant foods, and can kill
many of the bacteria ingested with foods.
- Intrinsic factor is a glycoprotein required for
vitamin B12 absorption in the small intestine.
3. Chief cells produce pepsinogen (pep-sin′o-jen),
the inactive form of the protein-digesting enzyme
pepsin.
4. Enteroendocrine cells (en″ter-o-en′do-krin; “gut
endocrine”) release a variety of chemical
messengers. Some of these, for example histamine
and serotonin, act locally. Others, such as
somatostatin, act both locally and as hormones .
Gastrin, a hormone, plays essential roles in
regulating stomach secretion and motility.
• The Small Intestine
The small intestine is the body’s major digestive
organ. Within its twisted passageways, digestion is
completed and virtually all absorption occurs.
Gross Anatomy
The small intestine is a convoluted tube extending
from the pyloric sphincter in the epigastric region to
the ileocecal valve in the right iliac region where it
joins the large intestine .It is the longest part of the
alimentary tube, but is only about half the diameter
of the large intestine, ranging from 2.5 to 4 cm (1–
1.6 inches). Although it is 6–7 m long in a cadaver,
the small intestine is only about 2–4 m (7–13 ft)
long during life because of muscle tone.
• The small intestine has three subdivisions:
-the duodenum, is relatively immovable “twelve
finger widths long”, which curves around the head
of the pancreas ,is about 25 cm (10 inches) long.
Although it is the shortest intestinal subdivision, the
duodenum has the most features of interest. The
bile duct, delivering bile from the liver, and the
main pancreatic duct, carrying pancreatic juice from
the pancreas, unite in the wall of the duodenum in
the hepatopancreatic ampulla . The ampulla opens
into the duodenum and is controlled by a muscular
valve called the hepatopancreatic sphincter, or
sphincter of Oddi.
• The jejunum ,about 2.5 m (8 ft) long, extends from
the duodenum to the ileum.
• The ileum ,approximately 3.6 m (12 ft) in length,
joins the large intestine at the ileocecal valve. The
jejunum and ileum hang in sausagelike coils in the
central and lower part of the abdominal cavity,
suspended from the posterior abdominal wall by
the fan-shaped mesentery .These more distal parts
of the small intestine are encircled and framed by
the large intestine.
The nutrient-rich venous blood from the small
intestine drains into the hepatic portal vein, which
carries it to the liver.
• Modifications for Absorption :
- Its length alone provides a huge surface area, and
- its wall has three structural modifications that
amplify its absorptive surface enormously (by a
factor of more than 600 times). Most absorption
occurs in the proximal part of the small intestine, so
these specializations decrease in number toward its
distal end.
1- The circular folds, or plicae circulares are deep,
permanent folds of the mucosa and submucosa .
Nearly 1 cm tall, these folds force chyme to spiral
through the lumen, slowing its movement and
allowing time for full nutrient absorption.
2-Villi are fingerlike projections of the mucosa .In
the core of each villus is a dense capillary bed and a
wide lymph capillary called a lacteal . The villi are
large and leaflike in the duodenum (the intestinal
site of most active absorption) and gradually
narrow and shorten along the length of the small
intestine.
3- Microvilli, tiny projections of the plasma
membrane of the absorptive cells of the mucosa,
give the mucosal surface an appearance called the
brush border .The plasma membranes of the
microvilli bear enzymes referred to as brush border
enzymes, which complete the digestion of
carbohydrates and proteins in the small intestine.
The Large Intestine
The large intestine frames the small intestine on
three sides and extends from the ileocecal valve to
the anus. Its diameter, at about 7 cm, is greater
than that of the small intestine (hence, large
intestine), but it is less than half as long (1.5 m
versus 6 m). Its major function is to absorb most of
the remaining water from indigestible food
residues (delivered to it in a fluid state), store the
residues temporarily, and then eliminate them
from the body as semisolid feces (fe′sēz).
• Gross Anatomy
-The large intestine has two features not seen elsewhere :
-the longitudinal muscle layer is reduced to three bands of
smooth muscle called teniae coli .
-Their tone causes the wall of the large intestine to pucker into
pocketlike sacs called haustra .
-The large intestine has the following subdivisions: cecum,
appendix, colon, rectum, and anal canal.
-The saclike cecum (se′kum; “blind pouch”), which lies below
the ileocecal valve in the right iliac fossa, is the first part of
the large intestine .
-Attached to its posteromedial surface is the blind, wormlike
vermiform appendix. The appendix contains masses of
lymphoid tissue, and as part of MALT, it plays an important
role in body immunity. Its twisted structure provides an ideal
location for enteric bacteria to accumulate and multiply.
• The colon has several distinct regions:
-the ascending colon, it travels up the right side of the
abdominal cavity to the level of the right kidney. -Here it
makes a right-angle turn—the right colic, or hepatic, flexure—
and travels across the abdominal cavity as the transverse
colon.
-Directly anterior to the spleen, it bends acutely at the left
colic (splenic) flexure and descends down the left side of the
posterior abdominal wall as the descending colon.
-Inferiorly, it enters the pelvis, where it becomes the S-shaped
sigmoid colon.
• In the pelvis, at the level of the third sacral vertebra, the
sigmoid colon joins the rectum, which runs just in front of the
sacrum. The position of the rectum allows the prostate gland
of males to be examined digitally (with a finger) through the
anterior rectal wall. This is called a rectal exam.
• The anal canal, the last segment of the large
intestine, lies in the perineum, entirely
external to the abdominopelvic cavity. About 3
cm long, it begins where the rectum
penetrates the levator ani muscle of the pelvic
floor and opens to the body exterior at the
anus. The anal canal has two sphincters, an
involuntary internal anal sphincter composed
of smooth muscle and a voluntary external
anal sphincter composed of skeletal muscle.
The sphincters, which act rather like purse
strings to open and close the anus, are
ordinarily closed except during defecation.
Accessory Digestive organs
• The Teeth
The teeth lie in sockets (alveoli) in the gum-covered
margins of the mandible and maxilla. We masticate, or
chew, by opening and closing our jaws and moving
them from side to side while continually using our
tongue to move the food between our teeth.
Teeth are classified according to their shape and
function as incisors, canines, premolars, and molars .
Ordinarily by age 21, two sets of teeth:
- The primary dentition consists of the deciduous
teeth, also called milk or baby teeth. The first teeth to
appear, at about age 6 months, are the lower central
incisors. Additional pairs of teeth erupt at one- to twomonth intervals until about 24 months, when all 20
milk teeth have emerged.
- As the deep-lying permanent teeth enlarge and
develop, the roots of the milk teeth are resorbed
from below, causing them to loosen and fall out
between the ages of 6 and 12 years. Generally, all
the teeth of the permanent dentition (but the
third molars) have erupted by the end of
adolescence.
-The third molars, also called wisdom teeth,
emerge between the ages of 17 and 25 years.
There are usually 32 permanent teeth in a full set,
but sometimes the wisdom teeth are completely
absent.
• Tooth Structure Each tooth has two major regions:
the crown and the root .The enamel-covered crown is
the exposed part of the tooth above the gingiva (jin′jĭ
-vah), or gum, which surrounds the tooth like a tight
collar. Enamel, an acellular, brittle material that
directly bears the force of chewing, is the hardest
substance in the body.
• The crown and root are connected by a constricted
tooth region called the neck. The outer surface of the
root is covered by cementum, a calcified connective
tissue, which attaches the tooth to the thin
periodontal ligament (per″e-o-don′tal; “around the
tooth”). This ligament anchors the tooth in the bony
alveolus of the jaw.
• Dentin, a bonelike material, underlies the enamel
cap and forms the bulk of a tooth. It surrounds a
central pulp cavity containing a number of soft
tissue structures (connective tissue, blood vessels,
and nerve fibers) collectively called pulp. Pulp
supplies nutrients to the tooth tissues and provides
for tooth sensation.
• Where the pulp cavity extends into the root, it
becomes the root canal. At the proximal end of
each root canal is an apical foramen that allows
blood vessels, nerves, and other structures to
enter the pulp cavity.
• In youth, the gingiva adheres to the enamel
covering the crown. But as the gums begin to
recede with age, as a result, the teeth appear to
get longer in old age.
• HOMEOSTATIC IMBALANCE When a tooth remains
embedded in the jawbone, it is said to be
impacted.
Impacted teeth can cause a good deal of pressure
and pain and must be removed surgically. Wisdom
teeth are most commonly impacted .
• The Salivary Glands
A number of glands associated with the oral cavity secrete
saliva which.
(1) cleanses the mouth,
(2) dissolves food chemicals so that they can be
tasted,
(3) moistens food and aids in compacting it into a
bolus, and
(4) contains enzymes that begin the chemical
breakdown of starchy foods.
Three pairs of salivary glands:
1-The large parotid gland lies anterior to the ear between
the masseter muscle and the skin. The prominent parotid
duct opens into the vestibule next to the second upper
molar. Branches of the facial nerve run through the parotid
gland on their way to the muscles of facial expression. For
this reason, surgery on this gland can result in facial
paralysis.
• HOMEOSTATIC IMBALANCE
Mumps, a common children’s disease, is an
inflammation of the parotid glands caused by the
mumps virus (myxovirus), which spreads from
person to person in saliva.
People with mumps complain of pain when
they open their mouth or chew. Mumps in adult
males carry a 25% risk that the testes may become
infected as well, leading to sterility.
2- the submandibular gland lies along the
medial aspect of the mandibular body.
3- The small sublingual gland lies anterior to
the submandibular gland under the tongue
and opens via 10–12 ducts into the floor of
the mouth.
To a greater or lesser degree, the salivary
glands are composed of two types of
secretory cells: mucous and serous. Serous
cells produce a watery secretion containing
enzymes and ions, whereas the mucous cells
produce mucus.
• Composition of Saliva Saliva is largely water—97 to
99.5%.. As a rule, saliva is slightly acidic (pH 6.75 to
7.00), but its pH may vary.
Its solutes include electrolytes (Na+, K+, Cl–, PO4–,
and HCO3–); the digestive enzyme salivary amylase;
the proteins mucin (mu′sin), lysozyme, and IgA; and
metabolic wastes (urea and uric acid).
When dissolved in water, the glycoprotein mucin
forms thick mucus that lubricates the oral cavity
and hydrates foodstuffs.
The Pancreas
The pancreas is a soft, gland that extends across the
abdomen from its tail (abutting the spleen) to its head,
which is encircled by the C-shaped duodenum . Most of the
pancreas is retroperitoneal and lies deep to the greater
curvature of the stomach.
• It produces enzymes that break down all categories of
foodstuffs. This exocrine product, called pancreatic juice,
drains from the pancreas via the centrally located main
pancreatic duct. The pancreatic duct generally fuses with
the bile duct just as it enters the duodenum (at the
hepatopancreatic ampulla).
A smaller accessory pancreatic duct empties directly into
the duodenum just proximal to the main duct.
• Pancreatic islets (islets of Langerhans) are mini-endocrine
glands release insulin and glucagon, hormones that play an
important role in carbohydrate metabolism, as well as
several other hormones .
• Composition of Pancreatic Juice
Approximately 1200 to 1500 ml is produced daily. It consists
mainly of water, and contains enzymes and electrolytes
(primarily bicarbonate ions). The high pH (about pH 8)of
pancreatic fluid helps neutralize acid chyme entering the
duodenum and provides the optimal environment for activity
of intestinal and pancreatic enzymes.
Like pepsin of the stomach, pancreatic proteases (proteindigesting enzymes) are produced and released in inactive
forms, which are activated in the duodenum, where they do
their work. This prevents the pancreas from self-digestion.
For example, within the duodenum, trypsinogen is activated
to trypsin. Trypsin, in turn, activates two other pancreatic
proteases (procarboxypeptidase and chymotrypsinogen) to
their active forms, carboxypeptidase (kar-bok″se-pep′tĭ-dās)
and chymotrypsin (ky″mo-trip′sin), respectively .Other
pancreatic enzymes—amylase, lipases, and nucleases—are
secreted in active form.
The Liver and Gallbladder
The liver and gallbladder are accessory organs
associated with the small intestine. The liver, one of
the body’s most important organs, has many
metabolic and regulatory roles. However, its
digestive function is to produce bile for export to
the duodenum.
Bile is a fat emulsifier; that is, it breaks up fats into
tiny particles so that they are more accessible to
digestive enzymes. The gallbladder is chiefly a
storage organ for bile.
• Gross Anatomy of the Liver
The liver is the largest gland in the body, weighing
about 1.4 kg (3 lb) in the average adult ,it occupies
most of the right hypochondriac and epigastric regions,
extending farther to the right of the body midline than
to the left. Located under the diaphragm, the liver lies
almost entirely within the rib cage, which provides
some protection .
Typically, the liver is said to have four primary lobes.
The largest of these, the right lobe, is visible on all liver
surfaces and separated from the smaller left lobe by a
deep fissure .A mesentery, the falciform ligament,
separates the right and left lobes anteriorly and
suspends the liver from the diaphragm and anterior
abdominal wall .
• Bile leaves the liver through several bile ducts that
ultimately fuse to form the large common hepatic
duct, which travels downward toward the duodenum.
Along its course, that duct fuses with the cystic duct
draining the gallbladder to form the bile duct .
• Composition of Bile
Bile is a yellow-green, alkaline solution containing bile
salts, bile pigments, cholesterol, triglycerides,
phospholipids (lecithin and others), and a variety of
electrolytes. Of these, only bile salts and
phospholipids aid the digestive process.
Bile salts are cholesterol derivatives. Their role is to
emulsify fats .As a result, large fat globules entering
the small intestine are physically separated into
millions of small, more accessible fatty droplets that
provide large surface areas for the fat-digesting
enzymes to work on.
• Bile salts also facilitate fat and cholesterol
absorption .Although many substances secreted in
bile leave the body in feces, bile salts are not among
them. Instead, bile salts are conserved by means of
a recycling mechanism called the enterohepatic
circulation. In this process, bile salts are (1)
reabsorbed into the blood by the ileum, (2)
returned to the liver via the hepatic portal blood,
and then (3) resecreted in newly formed bile.
• The chief bile pigment is bilirubin (bil″ĭ-roo′bin), a
waste product of the heme of hemoglobin formed
during the breakdown of worn-out erythrocytes The
globin and iron parts of hemoglobin are saved and
recycled, but bilirubin is absorbed from the blood
by the liver cells, excreted into bile, and
metabolized in the small intestine by resident
bacteria. One of its breakdown products, stercobilin
(ster′ko-bi″lin), gives feces a brown color. In the
absence of bile, feces are gray-white in color and
have fatty streaks (because essentially no fats are
digested or absorbed).
• The liver produces 500 to 1000 ml of bile daily, and
production is stepped up when the GI tract contains
fatty chyme .
• The gallbladder is a thin-walled green muscular
sac about 10 cm (4 inches) long in a shallow fossa
on the ventral surface of the liver .Its rounded
fundus protrudes from the inferior margin of the
liver. The gallbladder stores bile and concentrates
it by absorbing some of its water and ions. (In
some cases, bile released from the gallbladder is
ten times as concentrated as that entering it.)
When empty, or when storing only small amounts
of bile, its mucosa is thrown into folds like the
rugae of the stomach, allow the organ to expand
as it fills. When its muscular wall contracts, bile is
expelled into its duct, the cystic duct, and then
flows into the bile duct.
• HOMEOSTATIC IMBALANCE Bile is the major vehicle for
cholesterol excretion, and bile salts keep the
cholesterol dissolved within bile. Too much cholesterol
or too few bile salts leads to cholesterol crystallization,
forming gallstones, or biliary calculi which obstruct the
flow of bile. Then, when the gallbladder or its duct
contracts, the sharp crystals cause agonizing pain that
radiates to the right thoracic region.
When the gallbladder is removed, the bile duct enlarges
to assume the bile-storing role.
Bile duct blockage prevents both bile salts and bile
pigments from entering the intestine. As a result,
yellow bile pigments accumulate in blood and
eventually are deposited in the skin, causing it to
become yellow, or jaundiced. Jaundice caused by
blocked ducts is called obstructive jaundice .
Digestive Processes
The processing of food by the digestive system
involves six essential activities:
1. Ingestion is simply taking food into the digestive
tract, usually via the mouth.
2. Propulsion, which moves food through the
alimentary canal, includes :
-swallowing, which is initiated voluntarily, and
- peristalsis (per″ĭ -stal’sis), an involuntary
process. the major means of propulsion, involves
alternate waves of contraction and relaxation of
muscles in the organ walls .Its main effect is to
squeeze food along the tract, but some mixing
occurs as well. In fact, peristaltic waves are so
powerful that, once swallowed, food and fluids
will reach your stomach even if you stand on your
head.
3. Mechanical digestion
Mechanical processes include chewing, mixing of food with
saliva by the tongue, churning food in the stomach, and
segmentation, or rhythmic local constrictions of the intestine
Segmentation mixes food with digestive juices.
4. Chemical digestion is a series of steps in which complex
food molecules are broken down to their chemical building
blocks by enzymes secreted into the lumen of the alimentary
canal. Chemical digestion of foodstuffs begins in the mouth
and is essentially complete in the small intestine.
5. Absorption is the passage of digested end products (plus
vitamins, minerals, and water) from the lumen of the GI tract
through the mucosal cells by active or passive transport into
the blood or lymph. The small intestine is the major
absorptive site.
6. Defecation eliminates indigestible substances from the
body via the anus in the form of feces.
• Digestive Processes Occurring in the Mouth, Pharynx,
and Esophagus
-The mouth (1) ingests, (2) begins mechanical digestion
by chewing, and (3) initiates propulsion by swallowing.
Salivary amylase, the main enzyme in saliva, starts the
chemical breakdown of polysaccharides (starch and
glycogen) into smaller fragments of linked glucose
molecules. (If you chew a piece of bread for a few
minutes, it will begin to taste sweet as sugars are
released.)
Except for a few drugs that are absorbed through the
oral mucosa (for example, nitroglycerine), essentially
no absorption occurs in the mouth.
The pharynx and esophagus single digestive function
is food propulsion, accomplished by the role they play
in swallowing.
• Mastication (Chewing)
As food enters the mouth, its mechanical breakdown
begins with mastication, or chewing. The cheeks and
closed lips hold food between the teeth, the tongue
mixes food with saliva to soften it, and the teeth cut
and grind solid foods into smaller morsels. Mastication
is partly voluntary and partly reflexive. We voluntarily
put food into our mouths and contract the muscles that
close our jaws.
• Deglutition (Swallowing)
To send food on its way from the mouth, it is first
compacted by the tongue into a bolus and then
swallowed. Deglutition (deg″loo-tish′un), or
swallowing, is a complicated process that involves
coordinated activity of over 22 separate muscle groups.
It has two major phases, the buccal and the
pharyngeal-esophageal phases.
-The buccal phase occurs in the mouth and is
voluntary. In the buccal phase, we place the tip of
the tongue against the hard palate, and then
contract the tongue to force the bolus into the
oropharynx . Once food enters the pharynx,
respiration is momentarily inhibited and all routes
except the desired one—into the digestive tract—
are blocked off:
-The tongue blocks off the mouth.
-The soft palate rises to close off the nasopharynx.
-The larynx rises so that the epiglottis covers its
The upper esophageal sphincter relaxes.
HOMEOSTATIC IMBALANCE Vomiting, or emesis, is
caused by extreme stretching of the stomach or
intestine or the presence of irritants such as bacterial
toxins, excessive alcohol, spicy foods, and certain drugs
in those organs.
The diaphragm and abdominal wall muscles contract,,
the cardiac sphincter relaxes, and the soft palate rises
to close off the nasal passages. As a result, the stomach
(and perhaps duodenal) contents are forced upward
through the esophagus and pharynx and out the
mouth.
Before vomiting, an individual typically is pale, feels
nauseated, and salivates. Excessive vomiting can cause
dehydration and may lead to severe disturbances in the
electrolyte and acid-base balance of the body.
HOMEOSTATIC IMBALANCE Watery stools, or
diarrhea, result from any condition that rushes
food residue through the large intestine before
that organ has had sufficient time to absorb the
remaining water. Prolonged diarrhea may result in
dehydration and electrolyte imbalance (acidosis
and loss of potassium). Conversely, when food
remains in the colon for extended periods, too
much water is absorbed and the stool becomes
hard and difficult to pass. This condition, called
constipation, may result from lack of fiber in the
diet, improper bowel habits ,lack of exercise,
emotional upset, or laxative abuse.
Defecation
The rectum is usually empty, but when feces are forced
into it by mass movements, stretching of the rectal wall
initiates the defecation reflex .This is a spinal cord–
mediated parasympathetic reflex that causes the
sigmoid colon and the rectum to contract, the internal
anal sphincter to relax. As feces are forced into the anal
canal, messages reach the brain allowing us to decide
whether the external (voluntary) anal sphincter should
be opened or remain constricted to stop feces passage
temporarily. If defecation is delayed, the reflex
contractions end within a few seconds, and the rectal
walls relax. With the next mass movement, the
defecation reflex is initiated again—and so on, until one
chooses to defecate or the urge to defecate becomes
unavoidable.
METABOLISM
The term metabolism encompasses all of the reactions
that take place in the body. Everything that happens
within us is part of our metabolism. The reactions
of metabolism may be divided into two major categories:
anabolism and catabolism.
Anabolism means synthesis or “formation” reactions,
the bonding together of smaller molecules to
form larger ones.
Synthesis of hemoglobin by cells of the red bone marrow,
synthesis of glycogen by liver cells, and synthesis of fat
to be stored in adipose tissue are all examples of
anabolism. Such reactions require energy, usually in the
form of ATP.
Catabolism means decomposition, the breaking of
bonds of larger molecules to form smaller molecules.
Cell respiration is a series of catabolic reactions that
down food molecules to carbon dioxide and water.
During catabolism, energy is often released and used to
synthesize ATP .The ATP formed during catabolism is
then used for energy-requiring anabolic reactions. Most
of our anabolic and catabolic reactions are catalyzed by
enzymes. Enzymes are proteins that enable reactions to
take place rapidly at body temperature.
The body has thousands of enzymes, and each is
specific, that is, will catalyze only one type of reaction.
• Carbohydrate Metabolism
Because all food carbohydrates are eventually transformed to
glucose, the story of carbohydrate metabolism is really a tale
of glucose metabolism. Glucose enters the tissue cells by
facilitated diffusion, a process that is greatly enhanced by
insulin. Immediately upon entry into the cell, glucose is
phosphorylated to glucose-6-phosphate by transfer of a
phosphate group to its sixth carbon during a coupled reaction
with ATP:
Glucose + ATP→ glucose-6-PO4 + ADP
• Most body cells lack the enzymes needed to reverse this
reaction, so it effectively traps glucose inside the cells.
Because glucose-6-phosphate is a different molecule from
simple glucose, the reaction also keeps intracellular glucose
levels low, maintaining a concentration gradient for glucose
entry. The catabolic and anabolic pathways for carbohydrates
all begin with glucose-6-phosphate.
Glucose is the raw material for the synthesis of
another important monosaccharide, the pentose
Sugars that are part of nucleic acids. Deoxyribose is
the five-carbon sugar found in DNA, and ribose is
found in RNA. This function of glucose is very
important, for without the pentose sugars our cells
could neither produce new chromosomes for cell
division nor carry out the process of protein
synthesis.
Any glucose in excess of immediate energy needs or
the need for pentose sugars is converted to glycogen
in the liver and skeletal muscles.
Glycogen is then an energy source during states of
hypoglycemia or during exercise. If still more
glucose is present, it will be changed to fat and
stored in adipose tissue(lipogenesis).
• Gluconeogenesis is the formation of glucose from
noncarbohydrate (fat or protein) molecules. It
occurs in the liver when blood glucose levels begin
to fall.
CELL RESPIRATION ,the breakdown of food molecules to
release their potential energy d synthesize ATP
the summary reaction of cell respiration is:
C6H12O6(glucose) + O2 → CO2 + H2O + ATP + Heat,
the purpose of which is to produce ATP. Glucose contains
potential energy, and when it is broken down to CO2 and
H2O, this energy is released in the forms of ATP and
heat. The oxygen that is required comes from
breathing, and the CO2 formed is circulated to the
lungs to be exhaled. The water formed is called
metabolic water, and helps to meet our daily need for
water. Energy in the form of heat gives us a body
temperature, and the ATP formed is used for energyrequiring reactions.
Cell respiration of glucose involves three major
stages: glycolysis, the Krebs citric acid cycle, and the
cytochrome (or electron) transport system.
• Glycolysis
A series of ten chemical steps by which glucose is
converted to two pyruvic acid molecules. All steps
except the first, during which glucose entering the cell
is phosphorylated to glucose-6-phosphate, are fully
reversible.
The enzymes for the reactions of glycolysis are found in
the cytoplasm of cells, and oxygen is not required
(glycolysis is an anaerobic process).
In glycolysis, a sixcarbon glucose molecule is broken
down to two threecarbon molecules of pyruvic acid. As
a result of these reactions, a net gain of two ATP
molecules per glucose molecule is obtained.
If no oxygen is present in the cell, as may happen in
muscle cells during exercise, pyruvic acid is converted
to lactic acid, which causes muscle fatigue. If oxygen is
present, however, pyruvic acid continues into the next
stage, the Krebs citric acid cycle (or, more simply, the
Krebs cycle).
• Krebs Citric Acid Cycle
The enzymes for the Krebs cycle (or citric acid
cycle) are located in the mitochondria of cells. This
second stage of cell respiration is aerobic, meaning
that oxygen is required. The Krebs cycle is fueled by
pyruvic acid (and fatty acids). To enter the cycle,
pyruvic acid is converted to acetyl CoA. The acetyl
CoA is then oxidized and decarboxylated. Complete
oxidation of two pyruvic acid molecules yields a net
gain of 2 ATP .
To enter the cycle, pyruvic acid is converted to
acetyl CoA. The acetyl CoA is then oxidized and
decarboxylated. Complete oxidation of two pyruvic
acid molecules yields 6 CO2, 8 NADH + H+, 2 FADH2,
and a net gain of 2 ATP. Much of the energy
originally present in the bonds of pyruvic acid is
now present in the reduced coenzymes.
• In the electron transport chain, reduced
coenzymes are oxidized by delivering hydrogen to a
series of oxidation-reduction acceptors; and
electrons are combined with oxygen to form
water .
• For each glucose molecule oxidized to carbon
dioxide and water, there is a net gain of 38 ATP.
Proteins Metabolism Animal products provide highquality complete protein containing all (10) essential
amino acids. Most plant products lack one or more of
the essential amino acids.
Amino acids are the structural building blocks of the
body and of important regulatory molecules.
Protein synthesis can and will occur if all essential amino
acids are present and sufficient carbohydrate (or fat)
calories are available to produce ATP. Otherwise, amino
acids will be burned for energy.
Nitrogen balance occurs when protein synthesis equals
protein loss.
A dietary intake of 0.8 g of protein per kg of body weight
is recommended for most healthy adults.
Amino Acids
The primary uses for amino acids are the synthesis of
the non-essential amino acids by the liver and the
synthesis of new proteins in all tissues. By way of
review, we can mention some proteins with which you
are already familiar: keratin and melanin in the
epidermis; collagen in the dermis,tendons, and
ligaments; myosin, actin, and myoglobin in muscle
cells; hemoglobin in RBCs; antibodies produced by
WBCs; prothrombin and fibrinogen for clotting;
albumin to maintain blood volume; pepsin and
amylase for digestion; growth hormone and insulin;
and the thousands of enzymes needed to catalyze
reactions within the body.
-Only when the body’s needs for new proteins have
been met are amino acids used for energy
production.
- Excess amino acids; they will be deaminated and
converted to simple carbohydrates and contribute
to glycogen storage or they may be changed to fat
and stored in adipose tissue.
-Amine groups removed during deamination (as
ammonia) are combined with carbon dioxide by the
liver to form urea. Urea is excreted in urine.
-Protein synthesis requires the presence of all ten
essential amino acids. If any are lacking, amino
acids are used as energy fuels.
Lipids Metabolism Most dietary lipids are
triglycerides. The primary sources of saturated fats
are animal products while unsaturated fats are
present in plant products, nuts, and fish.
-The major sources of cholesterol are egg yolk,
meats, and milk products .
Linoleic and linolenic acids are essential fatty
acids.
-Triglycerides provide reserve energy, cushion body
organs, and insulate the body.
-Phospholipids are used to synthesize plasma
membranes and myelin.
-Cholesterol is used in plasma membranes and is the
structural basis of vitamin D, steroid hormones, and
bile salts.
-Fat intake should represent 35% or less of caloric
intake. The liver makes a basal amount of
cholesterol (85%) even when dietary cholesterol
intake is excessive.
Fatty Acids and Glycerol
The end products of fat digestion that are not
needed immediately for energy production but
may be stored as fat (triglycerides) in adipose
tissue. Most adipose tissue is found
subcutaneously and is potential energy for times
when food intake decreases.
Fatty acids and glycerol are also used for the
synthesis of phospholipids, which are essential
components of all cell membranes. Myelin, for
example, is a phospholipid of the membranes of
Schwann cells, which form the myelin sheath of
peripheral neurons.
The liver can synthesize most of the fatty acids
needed by the body. Two exceptions are linoleic
acid and linolenic acid, which are essential fatty
acids and must be obtained from the diet. Linoleic
acid is part of lecithin, which in turn is part of all cell
membranes.
When fatty acids are broken down in the
process of beta-oxidation, the resulting acetyl
groups may also be used for the synthesis of
cholesterol. This takes place primarily in the
liver, although all cells are capable of
synthesizing cholesterol for their cell
membranes.
The liver uses cholesterol to synthesize bile
salts for the emulsification of fats in digestion.
The steroid hormones(Adrenal cortex and sex
hormones) are also synthesized from
cholesterol.
• Cholesterol Metabolism and Regulation of Blood
Cholesterol Levels
Cholesterol, though an important dietary lipid, it is
not used as an energy source. It serves instead as
the structural basis of bile salts, steroid hormones,
and vitamin D and as a major component of plasma
membranes. About 15% of blood cholesterol comes
from the diet. The other 85% is made from acetyl
CoA by the liver.
• Cholesterol is lost from the body when it is
catabolized and secreted in bile salts, which are
eventually excreted in feces.
• Cholesterol Transport Because triglycerides and
cholesterol are insoluble in water, they are
transported to and from tissue cells bound to small
lipid-protein complexes, formed by the liver, called
lipoproteins.
Lipoproteins vary considerably in their relative fatprotein composition, but they all contain
triglycerides, phospholipids, and cholesterol in
addition to protein .
In general, the higher the percentage of lipid in the
lipoprotein, the lower its density; and the greater
the proportion of protein, the higher its density. On
this basis, there are high-density lipoproteins
(HDLs), low-density lipoproteins (LDLs), and very
low density lipoproteins (VLDLs).
• The Metabolic Role of the Liver
1-The liver is the body’s main metabolic organ and it
plays a crucial role in processing (or storing) virtually
every nutrient group.
2-It helps maintain blood energy sources, metabolizes
hormones, and detoxifies drugs and other substances.
3-The liver synthesizes cholesterol, catabolizes
cholesterol and secretes it in the form of bile salts, and
makes lipoproteins.
LDLs transport triglycerides and cholesterol from the
liver to the tissues, whereas HDLs transport cholesterol
from the tissues to the liver (for catabolism and
elimination).
Excessively high LDL levels are implicated in
atherosclerosis, cardiovascular disease, and strokes.
METABOLIC RATE
Although the term metabolism is used to describe all of the
chemical reactions that take place within the body,
metabolic rate is usually expressed as an amount of heat
production. This is because many body processes that utilize
ATP also produce heat. These processes include the
contraction of skeletal muscle, the pumping of the heart,
and the normal breakdown of cellular components.
Therefore, it is possible to quantify heat production as a
measure of metabolic activity.
During sleep, for example, energy expended by a 150-pound
person is about 60 to 70 kcal per hour.
Getting up and preparing breakfast increases energy
expenditure to 80 to 90 kcal per hour.
For mothers with several small children, this value may be
significantly higher.
Clearly, greater activity results in greater energy expenditure.
The energy required for merely living (lying quietly in bed) is the
basal metabolic rate (BMR). A number of factors affect the
metabolic rate:
1- Age—metabolic rate is highest in young children and
decreases with age.
2-Body configuration—more surface area proportional to weight
(tall and thin) means a higher metabolic rate.
3. Sex hormones—men usually have a higher metabolic rate
than do women; men have more muscle proportional to fat
than do women.
4. Sympathetic stimulation—metabolic activity increases in
stress situations.
5. Decreased food intake—metabolic rate decreases to conserve
available energy sources.
6. Climate—people who live in cold climates usually have higher
metabolic rates because of a greater need for heat
production.
7. Exercise—Contraction of skeletal muscle increases energy
expenditure and raises metabolic rate .
To estimate your own basal metabolic rate (BMR),
calculate kilocalories (kcal) used per hour as follows:
For women: use the factor of 0.9 kcal / kg/hour.
For men: use the factor of 1.0 kcal / kg /hour
Then multiply kcal/hour by 24 hours to determine
kcal per day.
Example: A 120-pound woman:
1. Change pounds to kilograms:
120 lb at 2.2 lb/kg = 55 kg
2. Multiply kg weight by the BMR factor:
55 kg x 0.9 kcal/kg/hr = 49.5 kcal/hr
3. Multiply kcal/hr by 24:
49.5 kcal/hr x 24 = 1188 kcal/day (An approximate BMR,
about 1200 kcal/day)
Example: A 160-pound man:
1. 160 lb at 2.2 lb/kg = 73 kg
2. 73 kg x 1.0 kcal/kg/hr = 73 kcal/hr
3. 73 kcal/hr x 24 = 1752 kcal/day
To approximate the amount of energy actually expended(Total
Metabolic Rate,TMR) during an average day (24 hours), the
following percentages may be used:
-Sedentary activity: add 40% to 50% of the BMR to the BMR
-Light activity: add 50% to 65% of the BMR to the BMR
-Moderate activity: add 65% to 75% of the BMR to the BMR
-Strenuous activity: add 75% to 100% of the BMR to the BMR
Using our example of the 120-pound woman with a BMR of 1200
kcal/day:
Sedentary: 1680 to 1800 kcal/day
Light: 1800 to 1980 kcal/day
Moderate: 1980 to 2100 kcal/day
Strenuous: 2100 to 2400 kcal/day
Proteins and Fats—as energy sources
1. Excess amino acids are deaminated in the liver and
converted to pyruvic acid or acetyl groups to enter the
Krebs cycle. Amino acids may also be converted to
glucose to supply the brain .
2. Glycerol is converted to pyruvic acid to enter the Krebs
cycle.
3. Fatty acids, in the process of beta-oxidation in the
liver, are split into acetyl groups to enter the Krebs
cycle; ketones are formed for transport to other cells.
Energy Available from Food
1. Energy is measured in kilocalories (Calories)kcal.
2. There are 4 kcal per gram of carbohydrate, 4 kcal per
gram of protein, 9 kcal per gram of fat.
With reference to food,kilocalories may be called calories.
Synthesis Uses of Foods
1. Glucose—used to synthesize the pentose sugars for
DNA and RNA; used to synthesize glycogen to store
energy in liver and muscles.
2. Amino acids—used to synthesize new proteins and
the non-essential amino acids; essential amino acids
must be obtained in the diet.
3. Fatty acids and glycerol—used to synthesize
phospholipids for cell membranes, triglycerides for
fat storage in adipose tissue, and cholesterol and
other steroids; essential fatty acids must be
obtained in the diet.
4. Any food eaten in excess will be changed to fat and
stored.
Body Temperature
- Normal range is 36° to 38°C with an average of 37°C.
- Temperature regulation in infants and the elderly is
not as precise as it is at other ages.
Heat Production
Heat is one of the energy products of cell respiration.
Many factors affect the total heat actually produced:
1. Thyroxine from the thyroid gland—the most
important regulator of daily heat production. As
metabolic rate decreases, more thyroxine is secreted
to increase the rate of cell respiration.
2. Stress—sympathetic impulses and epinephrine and
norepinephrine increase the metabolic activity of
many organs, increasing the production of ATP and
heat.
3. Active organs continuously produce heat. Skeletal
muscle tone produces 25% of the total body heat at
rest. The liver provides up to 20% of the resting
body heat.
4. Food intake increases the activity of the digestive
organs and increases heat production.
5. Changes in body temperature affect metabolic rate.
A fever increases the metabolic rate, and more heat
is produced.
Heat Loss
1. Most heat is lost through the skin.
2. Blood flow through the dermis determines the amount of heat
that is lost by radiation, conduction, and convection.
3. Vasodilation in the dermis increases blood flow and heat loss;
radiation and conduction are effective only if the environment is
cooler than the body.
4. Vasoconstriction in the dermis decreases blood flow and
conserves heat in the core of the body.
5. Sweating is a very effective heat loss mechanism; excess body
heat evaporates sweat on the skin surface; sweating is most
effective when the atmospheric humidity is low.
Sweating also has a disadvantage in that water is lost and must be
replaced to prevent serious dehydration.
6. Heat is lost from the respiratory tract by the evaporation of water
from the warm respiratory mucosa; water vapor is part of exhaled
air.
7. A very small amount of heat is lost as urine and feces are excreted
at body temperature.
Regulation of Heat Loss
1. The hypothalamus is the thermostat of the body and
regulates body temperature by balancing heat
production and heat loss.It receives information from
its own neurons and from the temperature receptors
in the dermis.
Fever—is controlled hyperthermia. Most often, it results
from infection somewhere in the body, but it may be
caused by cancer or allergic reactions.
1. Pyrogens are substances that cause a fever: bacteria,
foreign proteins, or chemicals released during
inflammation (endogenous pyrogens).
2. Pyrogens raise the setting of the hypothalamic
thermostat; the person feels cold and begins to shiver
to produce heat.
3. When the pyrogen has been eliminated, the
hypothalamic setting returns to normal; the person
feels warm, and sweating begins to lose heat to
lower the body temperature.
-A low fever may be beneficial because it increases
the activity of WBCs and inhibits the activity of
some pathogens.
-A high fever may be detrimental because enzymes
are denatured at high temperatures. This is most
critical in the brain, where cells that die cannot be
replaced.
Heat-Promoting Mechanisms
When the external temperature is low or blood
temperature falls for any reason, the heat-promoting
center is activated and triggers one or more of the
following mechanisms :
1-Constriction of cutaneous blood vessels. As a result,
blood is restricted to deep body areas and largely
bypasses the skin. Because the skin is separated from
deeper organs by a layer of insulating subcutaneous
(fatty) tissue, heat loss from the shell is dramatically
reduced.
HOMEOSTATIC IMBALANCE Restricting blood flow to
the skin is not a problem for a brief period, but if it is
extended (as during prolonged exposure to very cold
weather), skin cells deprived of oxygen and nutrients
begin to die. This extremely serious condition is called
frostbite.
2. Shivering. is involuntary shuddering contractions.
Shivering is very effective in increasing body
temperature because muscle activity produces large
amounts of heat.
3. Increase in metabolic rate. Cold stimulates the
release of epinephrine and norepinephrine by the
adrenal medulla in reponse to sympathetic nerve
stimuli, which elevates the metabolic rate and
enhances heat production.
4. Enhanced thyroxine release. Because thyroid
hormone increases metabolic rate, body heat
production increases.
Beside these involuntary adjustments, we humans
make a number of behavioral modifications to
prevent overcooling of our body core:
-Putting on more or warmer clothing to restrict heat
loss (a hat, gloves, and “insulated” outer garments)
-Drinking hot fluids
-Changing posture to reduce exposed body surface
area (clasping the arms across the chest).
-Increasing physical activity to generate more heat
(jumping up and down, clapping the hands)
• Heat-Loss Mechanisms
Heat-loss mechanisms protect the body from
excessively high temperatures. Most heat loss occurs
through the skin via radiation, conduction, convection,
and evaporation. Heat-loss mechanisms include one or
both of the following :
1. Dilation of cutaneous blood vessels. As the blood
vessels swell with warm blood, heat is lost from the
shell by radiation, conduction, and convection.
2. Enhanced sweating. The sweat glands are strongly
activated. Evaporation of perspiration is an efficient
means of ridding the body of heat as long as the air is
dry. However, when the relative humidity is high,
evaporation occurs much more slowly. In such cases,
the heat-liberating mechanisms cannot work well, and
we feel miserable and irritable.
Behavioral or voluntary measures commonly taken
to reduce body heat include :
• Reducing activity (“laying low”)
• Seeking a cooler environment (a shady spot) or
using a device to increase convection (a fan) or
cooling (an air conditioner)
• Wearing light-colored, loose clothing that reflects
radiant energy. (This is actually cooler than being
nude because bare skin absorbs most of the
radiant energy striking it.)
• HOMEOSTATIC IMBALANCE Under conditions of
overexposure to a hot and humid environment,
normal heat-loss processes become ineffective. The
hyperthermia (elevated body temperature) that
ensues depresses the hypothalamus. Increasing
temperatures increase the metabolic rate, which
increases heat production. The skin becomes hot
and dry and, as the temperature continues to spiral
upward, multiple organ damage becomes a distinct
possibility, including brain damage. This condition,
called heat stroke, can be fatal unless corrective
measures are initiated immediately (immersing the
body in cool water and administering fluids).
• Hypothermia (hi″po-ther′me-ah) is low body
temperature resulting from prolonged uncontrolled
exposure to cold. Vital signs (respiratory rate, blood
pressure, and heart rate) decrease as cellular
enzymes become sluggish. Drowsiness sets in and,
oddly, the person becomes comfortable even
though previously he or she felt extremely cold.
Shivering stops at a core temperature of 30–32°C
(87–90°F) when the body has exhausted its heatgenerating capabilities. Uncorrected, the situation
progresses to coma and finally death (by cardiac
arrest), when body temperatures approach 21°C
(70°F).
Developmental Aspects of the Digestive System
1. The glandular accessory organs (salivary glands,
liver, pancreas, and gallbladder) form from
outpocketings of the foregut endoderm. The
mucosa of the alimentary canal develops from the
endoderm, which folds to form a tube. The
remaining three tunics of the alimentary canal wall
are formed by mesoderm.
2. Important congenital abnormalities of the
digestive tract include cleft palate/lip,
tracheoesophageal fistula, and cystic fibrosis. All
interfere with normal nutrition.
• 3. Various inflammations affect the digestive
system throughout life. Appendicitis is common in
adolescents, gastroenteritis and food poisoning can
occur at any time (given the proper irritating
factors), ulcers and gallbladder problems increase in
middle age.
4. The efficiency of all digestive system processes
declines in the elderly, and periodontal disease is
common. Diverticulosis, fecal incontinence, and GI
tract cancers such as stomach and colon cancer
appear with increasing frequency in an aging
population.