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.
It is collapsed when not involved in food propulsion.
- Lined by stratified squamous epithelium to
accommodate high friction.
- It 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.
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
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.
• An empty stomach has a volume of about 50 ml but
it can hold about 4 L of food and may extend nearly
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 .
• The convex lateral surface of the stomach is its
greater curvature, and its concave medial surface is
the lesser curvature.
• 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.
• Microscopic Anatomy of the stomach
-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 .There are millions of gastric glands
that produce the stomach secretion called gastric
juice. The glands contain four types of cells:
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
enzyme. The acidity 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 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
-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. It is 6–
7 m long in a cadaver, but is only about 2–4 m long
during life because of muscle tone.
-Within its twisted passageways, digestion is
completed and virtually all absorption occurs.
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.
-The bile duct and the main pancreatic duct
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 are suspended from the
posterior abdominal wall by the fan-shaped
mesentery .
• They 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
- 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.
-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 .
- Microvilli, tiny projections 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( nearly
double 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:
1-The saclike cecum which lies below the ileocecal
valve in the right iliac fossa.
2-The blind, wormlike vermiform appendix. The
appendix contains masses of lymphoid tissue, and
as part of MALT.
3- The colon has several distinct regions:
-the ascending colon
-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 abdominal wall as the descending colon.
-Inferiorly, it enters the pelvis, where it becomes the Sshaped sigmoid colon.
4- In the pelvis, 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.
5-The anal canal is about 3 cm long, it
opens to the body exterior at the anus.
The anal canal has two sphincters:
a- an involuntary internal anal sphincter
composed of smooth muscle and
b-a voluntary external anal sphincter
composed of skeletal muscle. The
sphincters, are ordinarily closed except
during defecation.
Accessory Digestive organs
• The 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 two-month intervals until about
24 months, when all 20 milk teeth have emerged.
- As the deep-lying permanent teeth enlarge
the mil; teeth 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 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 .
• 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.
• 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. 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 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
-water—97 to 99.5%..
- slightly acidic (pH 6.75 to 7.00).
- 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.
-Pancreatic islets (islets of Langerhans) are miniendocrine glands release insulin and glucagon,
hormones that play an important role in
carbohydrate metabolism .
• Composition of Pancreatic Juice
-Approximately 1200 to 1500 ml is produced daily.
-The high pH (about pH 8)of pancreatic fluid helps
neutralize acid chyme and provides the optimal
environment for activity of intestinal and pancreatic
enzymes.
-Trypsin, and chymotrypsin .
-Amylase
-lipases, and
-nucleases.
• 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 .Located under the diaphragm, the
liver lies almost entirely within the rib cage, which
provides some protection .
- The liver is said to have four primary lobes. The
largest of these is the right lobe.
-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
-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.
• The chief bile pigment is bilirubin a waste product of
the heme of hemoglobin formed during the breakdown
of worn-out erythrocytes
• The liver produces 500 to 1000 ml of bile daily, and
production increases when the GI tract contains fatty
chyme .
The gallbladder
is a thin-walled green muscular sac about 10 cm long
in a shallow fossa on the ventral surface of the liver .
- The gallbladder stores and concentrates bile 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 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.
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
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 ,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 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.
METABOLISM
The term metabolism encompasses all of the reactions
that take place in the body .It is divided into :
1-Anabolism:means synthesis or “formation of larger
molecules. Such reactions require energy, usually in the
form of ATP.
2- Catabolism means decomposition, the breaking of
bonds of larger molecules to form smaller ones.
-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
-all food carbohydrates are eventually transformed
to glucose
-Glucose enters the tissue cells by facilitated
diffusion, a process that is greatly enhanced by
insulin.Cellular respiration is the sourse of ATP
- Glucose is needed for the synthesis of the pentose
Sugars(ribose) found in DNA and RNA.
• 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.
Proteins Metabolism
Amino acids are the structural building blocks of the
body.
• Animal products provide high-quality complete
protein containing all (10) essential amino acids.
• Most plant products lack one or more of the
essential amino acids.
• Protein synthesis can and will occur if all essential
amino acids are present .Otherwise, amino acids
will be burned for energy.
• A dietary intake of 0.8 g of protein per kg of body
weight is recommended for most healthy adults.
- Excess amino acids; will be deaminated and
converted to simple carbohydrates 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.
- About 15% of blood cholesterol comes from the
diet. The other 85% is made from by the liver.
Cholesterol is never used as an energy source.
-The end products of fat digestion that are not
needed immediately for energy production 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.
• Cholesterol Transport Because fats are
insoluble in water, they are transported to and
from tissue cells bound to small lipid-protein
complexes, formed by the liver, called
lipoproteins.
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).
• 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.
• 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.
METABOLIC RATE
The metabolic rate is usually expressed as an
amount of heat production.
-The energy required for merely living (lying quietly
in bed) is the basal metabolic rate (BMR)
-Total Metabolic Rate(TMR):the amount of energy
actually expended during an average day (24
hours).
To estimate your own basal metabolic rate (BMR):
-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 .
Example: A 55 kg- woman:
1. Multiply kg weight by the BMR factor:
55 kg x 0.9 kcal/kg/hr = 49.5 kcal/hr
2. Multiply kcal/hr by 24:
49.5 kcal/hr x 24 = 1188 kcal/day (An
approximate BMR, about 1200 kcal/day).
Example: A 70 kg - man:
1. 70 kg x 1.0 kcal/kg/hr = 70 kcal/hr
3. 70 kcal/hr x 24 = 1680 kcal/day (An
approximate BMR, about 1700 kcal/day).
To approximate, the TMR 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% to100% of the BMR
to the BMR
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.
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 following
mechanisms help:
1-Constriction of cutaneous blood vessels. As a
result, blood is restricted to deep body areas and
largely bypasses the skin.
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. Enhanced thyroxine release. Because thyroid
hormone increases metabolic rate, body heat
production increases.
4. Some behavioral modifications :
-Putting on more or warmer clothing.
-Drinking hot fluids
-Increasing physical activity (jumping up and down,
clapping the hands).
Heat-Loss Mechanisms
Most heat loss occurs through the skin via radiation,
conduction, convection, and evaporation. Heat-loss
mechanisms include :
1. Dilation of cutaneous blood vessels.
2. Enhanced sweating.
3. Some behavioral modifications:
• Reducing activity (“laying low”)
• Seeking a cooler environment (a shady spot) or using a
device to increase convection (a fan or 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 skin
becomes hot and dry and 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).
• HOMEOSTATIC IMBALANCE
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.
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. Important congenital abnormalities of
the digestive tract include cleft palate/lip,
tracheoesophageal fistula, and cystic
fibrosis. All interfere with normal
nutrition.
2-The efficiency of all digestive system
processes declines in the elderly. Fecal
incontinence, and GI tract cancers such as
stomach and colon cancer appear with
increasing frequency in an aging
population.