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Anatomy & Physiology
Lesson 10
THE RESPIRATORY SYSTEM
The body’s cells are constantly using O2 in the
metabolic processes that release energy from
nutrient molecules and produce ATP. These
reactions also produce CO2, which must be
eliminated quickly, since too much CO2
produces acid that is toxic to cells.
 The cardiovascular system and the
respiratory system cooperate equally in the
process of supplying O2 to the cells of the body
and eliminating CO2 waste.

FUNCTIONS OF THE
RESPIRATORY SYSTEM
 Provides
for gas exchange—intake of O2
for delivery to body cells and elimination of
CO2 produced by body cells.
 Contains receptors for the sense of smell,
filters inspired air, produces sounds, and
helps eliminate waste.
RESPIRATION

Respiration is the process during which gases
are exchanged between the atmosphere, blood,
and body cells. It takes place in three basic
steps:



Pulmonary ventilation (or breathing)—The
inspiration (inflow) and expiration (outflow) of air
between the atmosphere and the lungs.
External (pulmonary) respiration—The exchange of
gases between the air spaces of the lungs and blood
in the pulmonary capillaries. The blood gains O2 and
loses CO2.
Internal (tissue) respiration—The exchange of
gases between blood and tissue cells. The blood
loses O2 and gains CO2. Within the cells, the
metabolic reactions that consume O2 and release
CO2 during ATP production are called cellular
respiration.
STRUCTURES OF THE
RESPIRATORY SYSTEM


The respiratory system consists of the nose, pharynx
(throat), larynx (voice box), trachea (windpipe), bronchi,
and lungs.
Structurally, the respiratory system consists of two parts:



The upper respiratory system—nose, pharynx, and associated
structures.
The lower respiratory system—larynx, trachea, bronchi, and
lungs.
Functionally, the respiratory system also consists of two
parts:


The conducting portion—a series of interconnecting cavities
and tubes that conduct air into the lungs (nose, pharynx, larynx,
trachea, bronchi, bronchioles, and terminal bronchioles).
The respiratory portion—the portions of the respiratory system
where the exchange of gases occurs (respiratory bronchioles,
alveolar ducts, alveolar sacs, and alveoli).
STRUCTURES OF THE
RESPIRATORY SYSTEM
NOSE

The nose has an external portion and an internal
portion.


The external portion is composed of a bone and hyaline cartilage
framework covered with muscle and skin and lined by mucous
membrane. The openings to the nose are called external nares
or nostrils.
The interior structures of the nose are specialized for three
functions:
• Incoming air is warmed, moistened, and filtered.
• Olfactory stimuli are received.
• Large, hollow resonating chambers modify speech sounds.


The space inside the internal nose is the nasal cavity, which is
divided into right and left halves by the bone and cartilage nasal
septum.
Three delicate boney projections called conchae subdivide each
side of the nasal cavity into the superior, middle, and inferior
meatuses, which act as turbines for warming, moisturizing, and
filtering air as it enters the body.
PHARYNX
 The
pharynx (throat) is a funnel-shaped
tube that measures about 13 cm (5 in)
long and extends from the internal nares
to the level of the cricoid cartilage.
 The pharynx functions as a passageway
for air and food, provides a resonating
chamber for speech sounds, and houses
the tonsils.
 The pharynx is further subdivided into the
nasopharynx, oropharynx, and
laryngopharynx.
NASOPHARYNX

The nasopharynx lies posterior to the nasal
cavity and extends to the level of the soft palate.
 It has five openings in its walls: the two internal
nares, the two openings into the Eustachian
canals, and the opening into the oropharynx.
 It is lined with pseudostratified ciliated columnar
epithelium, which moves dust-laden mucus
received from the nasal cavity downwards to be
eliminated by swallowing or spitting.
 It also exchanges small amounts of air with the
auditory (Eustachian) tubes, to equalize air
pressure between the pharynx and middle ear.
OROPHARYNX
 The
oropharynx extends from the soft
palate to the level of the hyoid bone and
opens into the mouth.
 Since it forms a common passageway for
food, drink, and air, it is both respiratory
and digestive in function.
 It is lined with nonkeratinized stratified
squamous epithelium to protect it from
trauma from abrasive food particles.
 It contains the two palatine and two lingual
tonsils.
LARYNGOPHARYNX
 The
laryngopharynx or hypopharynx
begins at the level of the hyoid bone and
connects the esophagus with the larynx.
 The laryngopharynx is also both
respiratory and digestive in function and is
lined with nonkeratinized stratified
squamous epithelium.
LARYNX

The larynx is a short passageway, composed of nine
pieces of cartilage, that connects the laryngopharynx to
the trachea.
 At its superior end, the larynx is protected by a flap of
epithelial-covered elastic cartilage called the epiglottis.
During swallowing, the larynx moves upward so the
epiglottis closes over its opening, diverting liquids and
foods into the esophagus and away from the airways.
 The glottis is in the superior portion of the larynx. It
consists of a pair of mucous membrane folds called the
vocal folds or true vocal cords and the space between
them called the rima glottidis. Deep to the vocal folds
are bands of elastic ligaments stretched between pieces
of rigid cartilage and controlled by skeletal muscles of
the larynx. As these ligaments are stretched tight and air
passes by the membrane, sound is produced. The vocal
folds, in conjunction with other structures of the head
and neck, allow for the production of speech.
THE UPPER RESPIRATORY
SYSTEM
TRACHEA


The trachea is a tubular air passageway that is about 12
cm (5 in) long by 2 ½ cm (1 in) in diameter. It is located
anterior to the esophagus and extends from the larynx to
the carina, where it divides into right and left primary
bronchi.
The trachea is composed of four layers. From deep to
superficial, they are:





A mucosa
Submucosa
Hyaline cartilage
Adventitia (areolar CT)
The epithelium of the mucosa provides the same
protection against dust as the membrane lining the nasal
cavity and the larynx.
 The cartilage consists of 16-20 “C” shaped pieces
arranged one above the other. The open side of the “C”
is adjacent to the esophagus, allowing flexibility for
swallowing. The cartilage rings protect the front and
sides of the trachea and keep it from collapsing while
breathing.
PRIMARY BRONCHI
the superior border of the 5th thoracic
vertebra, the trachea divides into a right
primary bronchus which goes into the
right lung and a left primary bronchus
which goes into the left lung.
 The right primary bronchus is more
vertical, shorter, and wider than the left, so
is more likely to trap an aspirated object.
 Like the trachea, the primary bronchi
contain incomplete cartilage rings and are
lined with pseudostratified ciliated
columnar epithelium.
 At
THE BRONCHIAL TREE

Upon entering the lungs, the primary bronchi branch to
form the smaller secondary (lobar) bronchi—one for
each lobe of the lung (three on the right, two on the left).
 The secondary bronchi again branch to form tertiary
(segmental) bronchi, which branch again to become
bronchioles. The bronchioles continue to further divide
until they eventually become the smallest terminal
bronchioles. All of these branching bronchial structures
are commonly referred to as the bronchial tree.
 As the branching of the bronchial tree progresses,
several structural changes can be noted:



The epithelium changes from pseudostratified ciliated columnar
epithelium to nonciliated simple cuboidal epithelium.
The incomplete rings of cartilage are gradually replaced by
plates of cartilage, which eventually disappear all together.
As the cartilage decreases, smooth muscle increases, encircling
the lumen in spiral bands. The smooth muscle can be affected
by both the ANS and various chemicals.
LUNGS

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The lungs, separated by the mediastinum, occupy most
of the thoracic cavity.
Each lung is surrounded by a double layer of serous
membrane called the pleural membrane. The
superficial layer, the parietal pleura, lines the wall of the
thoracic cavity. The deep layer, the visceral pleura,
covers the lungs. Between the parietal and visceral
pleurae is the pleural cavity, which contains lubricating
fluid.
Each lung is divided into lobes by deep fissures. The
right lung has three lobes, while the left lung only has
two. Each lobe has its own secondary bronchus.
The secondary bronchi branch into ten tertiary bronchi in
each lung. Each tertiary bronchus serves a specific
segment of lung tissue called a bronchopulmonary
segment.
LUNGS

Each bronchopulmonary segment of a lung contains
many small compartments called lobules.
 Each lobule is wrapped in elastic CT, contains a
lymphatic vessel, a venule, and a branch from a terminal
bronchiole.
 Terminal bronchioles subdivide into microscopic
respiratory bronchioles.
 Respiratory bronchioles again subdivide into alveolar
ducts.
 Around the circumference of each alveolar duct are
many alveoli and alveolar sacs.
 An alveolus is a tiny sac of simple squamous epithelium
supported by a thin elastic basement membrane. It is
through the walls of the alveoli that gas exchange occurs
with capillaries from the circulatory system.
LOBULE OF THE LUNG
PULMONARY VENTILATION

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Pulmonary ventilation is the process in which gases
are exchanged between the atmosphere and the alveoli
of the lungs.
Pulmonary ventilation consists of inspiration and
expiration and occurs as the result of a pressure
gradient.
Inspiration occurs when alveolar pressure falls below
atmospheric pressure. When the diaphragm and
external intercostal muscles contract, the size of the
thorax increases, causing the lungs to expand. The
expansion of the lungs decreases alveolar pressure,
moving air along the pressure gradient from the
atmosphere into the lungs.
Expiration occurs when alveolar pressure is higher than
atmospheric pressure. As the diaphragm and external
intercostal muscles relax, the chest wall and lungs recoil,
causing lung volume to decrease. This increases the
pressure within the alveoli, forcing air to move from the
lungs into the atmosphere.
CONTROL OF RESPIRATION

The respiratory center of the brain is composed of
neurons in the medullary rhythmicity center of the
medulla oblongata and the pneumotaxic and apneustic
areas of the pons.





The inspiratory area of the medullary rhythmicity area
establishes the basic rhythm of breathing.
The expiratory area of the medullary rhythmicity area only
causes contractions of the external intercostal and abdominal
muscles during forceful expiration.
The pneumotaxic area sends inhibitory impulses to the
inspiratory area, limiting inspiration and establishing expiration.
The apneustic area sends stimulatory impulses to the
inspiratory area to prolong inspiration when the pneumotaxic
area is inactive.
Respirations may also be modified by a number of other
factors, including: cortical influences, the inflation reflex,
chemical stimuli (O2, CO2, and H+ levels), neural
changes due to movement, blood pressure changes, the
limbic system, temperature, pain, and irritation to the
airways.
THE DIGESTIVE SYSTEM

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Digestion is the breaking down of larger food particles
into molecules small enough to enter body cells.
Absorption is the passage of these smaller molecules
into blood and lymph.
The organs that work together to perform digestion and
absorption are called the digestive system.
The organs of the digestive system can be broken down
into two main groups:


The gastrointestinal (GI) tract or alimentary canal is a
continuous tube that extends from the mouth to the anus. It
consists of the mouth, pharynx, esophagus, stomach, small
intestine, and large intestine.
The accessory structures form the second group of organs.
They are the teeth, tongue, salivary glands, liver, gallbladder,
and pancreas.
THE DIGESTIVE SYSTEM
FUNCTIONS OF THE DIGESTIVE
SYSTEM

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Ingestion—Taking food into the mouth.
Secretion—Liberation of water, acid, buffers,
and enzymes into the GI tract.
Mixing and propulsion—Churning and
passage of food through the GI tract.
Digestion—Mechanical and chemical
breakdown of food.
Absorption—Passage of food from the GI tract
into the blood and lymph.
Defecation—The elimination of indigestible
substances from the GI tract.
LAYERS OF THE GI TRACT

The wall of the GI tract, from the stomach to the
anal canal, has the same basic arrangement of
tissues:




The innermost mucosa—a mucous membrane.
The submucosa—highly vascularized areolar CT that
binds the mucosa to the muscularis.
The muscularis—a double layer of smooth muscle
fibers that is involuntary in action and propels food
through the GI tract. (Muscles in the mouth, pharynx,
superior part of the esophagus, and external anal
sphincter are skeletal and under voluntary control).
The serosa—the superficial serous membrane layer
that surrounds the organs suspended in the
abdominopelvic cavity and forms part of the
peritoneum.
THE PERITONEUM

The peritoneum is the largest serous
membrane of the body. The parietal
peritoneum lines the wall of the abdominopelvic
cavity and the visceral peritoneum covers
some of the organs in the cavity. Between the
parietal and visceral layers of the peritoneum is
the serous fluid-filled peritoneal cavity.
 Some organs (such as the kidneys and
pancreas) lay posterior to the abdominal wall, so
are said to be retroperitoneal.
 The pericardeum contains large folds that weave
between the viscera, bind the organs together,
and contain blood, lymph, and nerve supplies for
the abdominal organs.
THE PERITONEUM

The peritoneum has several extensions that
perform different functions:

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The mesentary binds the small intestine to the
posterior abdominal wall, while still allowing for
significant movement.
The mesocolon binds the large intestine to the
posterior abdominal wall, while still allowing for
movement during digestion.
The falciform ligament attaches the liver to the
anterior abodominal wall and diaphragm.
The lesser omentum suspends the stomach and
duodenum from the liver.
The greater omentum drapes over and protects the
transverse colon and small intestines.
MOUTH
 The
mouth or oral (buccal) cavity is
formed by the cheeks, hard and soft
palates, and tongue.
 Digestion begins in the mouth, where the
teeth mechanically break food into smaller
pieces and salivary enzymes begin to
chemically break down food.
TONGUE

The tongue, with its associated muscles, forms
the floor of the mouth. It is composed of skeletal
muscle covered with mucous membrane.
 The dorsum (top surface) and lateral borders of
the tongue are covered with projections called
papillae, some of which contain taste buds.
 The tongue has serous ducts at its posterior
border and glands on its dorsum that secrete
lingual lipase, an enzyme which begins
digesting triglycerides into fatty acids and
monoglycerides.
SALIVARY GLANDS


The salivary glands secrete saliva, a fluid which
moistens the mucous membranes of the mouth and
pharynx, cleanses the mouth and teeth, and lubricates,
dissolves, and begins the chemical breakdown of food.
Although there are many small accessory glands located
throughout the mouth, most saliva is produced by three
main pairs of salivary glands:



The parotid glands are located inferior and anterior to the ears,
just deep to the skin. The parotid ducts empty into the mouth
near the upper molars.
The submandibular glands are located beneath the base of the
tongue in the posterior floor of the mouth. The submandibular
ducts enter the mouth on either side of the lingual frenulum.
The sublingual glands are located superior to the
submandibular glands. Their ducts open onto the floor of the
mouth, on either side of the lingual frenulum.
SALIVA

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Chemically, saliva is 99 ½% water and ½% solutes.
Solutes in saliva include: sodium, potassium, chloride,
bicarbonate, and phosphate ions, some dissolved gases,
urea, uric acid, serum albumin and globulin, mucus,
lysozyme (an enzyme that destroys bacteria), and
salivary amylase (an enzyme which begins the
digestion of carbohydrates).
Each type of salivary gland produces saliva with slightly
different characteristics:




The parotid glands secrete a watery saliva that is rich in salivary
amylase.
The submandibular glands secrete a thicker fluid with more
mucous, but still containing a fair amount of enzyme.
The sublingual glands secrete a much thicker mucous fluid with
only a small amount of amylase.
The various solutes in saliva serve to activate salivary
amylase, act as a buffer against acidic foods, help to
remove waste, and work to destroy bacteria.
TEETH

Teeth are used for mastication (chewing)—
mechanically grinding food and (with the tongue)
mixing it with saliva to form a soft, flexible bolus
that is easily swallowed.
 Humans have two sets of teeth during their
lifetime:


The primary dentition consists of 20 deciduous
teeth, which begin to erupt at about 6 mos. of age
and are usually completely replaced by the time a
child is 12 or 13 years old.
The permanent dentition consists of 32 teeth that
usually appear between the ages of 6 and adulthood.
TEETH
 The




normal adult dentition consists of:
12 molars—posterior teeth used for crushing
and grinding food.
8 premolars or bicuspids—intermediate
teeth that assist the molars in crushing and
grinding.
4 canines or cuspids—pointed teeth that
tear and shred food.
8 incisors—sharp anterior teeth that cut into
or bite food.
PRIMARY & PERMANENT
DENTITIONS
TEETH

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Each tooth is composed of a crown (the part normally above the
gums or gingiva) and a root (the part normally below the gingiva
and embedded in alveolar bone).
At the center of a tooth is the pulp cavity, which contains the tooth’s
nerve and blood supply. It is the “living” part of the tooth.
The bulk of the tooth is composed of a hard, calcified CT layer called
dentin. Dentin is porous and, if exposed, allows for some
communication between the exterior of the tooth and the pulp cavity.
The crown of the tooth is covered with enamel, the hardest
substance in the human body. Enamel protects a tooth from the
wear of chewing and damage from acids.
The root of the tooth is covered with cementum, which is hard, but
softer than either enamel or dentin. The periodontal ligament
attaches the tooth to the alveolar bone, via the cementum. If root
becomes exposed, the cementum can easily be worn away, leaving
the underlying dentin unprotected.
TOOTH STRUCTURE
DENTAL CARIES

Dental caries is the disease condition of teeth
commonly referred to as “cavities.”
 Carious lesions result from bacterial infections of
the structures of teeth.
 Oral bacteria metabolize carbohydrates (sugars
and starches) to produce acid. When acid is
allowed to remain in contact with calcified tooth
structures, it begins to dissolve these structures.
 Although enamel is very hard, it will eventually
be destroyed by acid attack. Once enamel has
been breeched, unless the decay is removed, it
can spread quickly through the softer dentin until
it reaches the pulp cavity, causing acute pain,
inflammation, and death to the tooth.
ESOPHAGUS

The esophagus is a collapsible muscular tube located
posterior to the trachea. It is about 25 cm (10 in) long,
passes through the mediastinum and diaphragm, and
connects the laryngopharynx to the stomach.
 The esophagus secretes mucus, which provides
lubrication for the smooth passage of a food bolus.
 Muscular contractions of the esophagus, called
peristalsis, force the food bolus downwards toward the
stomach.
 The upper esophageal sphincter is located at the
entrance to the esophagus. It relaxes and opens during
swallowing.
 The lower esophageal (gastroesophageal) sphincter
is a narrowing in the esophagus just superior to the
diaphragm. It also opens during swallowing to allow
food to enter the stomach. If it does not close
adequately, it allows stomach acid to enter the
esophagus, resulting in painful heartburn and
indegestion.
STOMACH
The stomach is a “J” shaped enlargement of the
GI tract inferior to the diaphragm. It connects
the esophagus to the duodenum of the small
intestine.
 The stomach has four main areas:

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The cardia surrounds the superior opening to the
stomach.
The fundus is the rounded portion superior and to the
left of the cardia.
The body is the large central portion of the stomach.
The pylorus is the region of the stomach that
connects to the duodenum.
The pyloric sphincter separates the stomach
from the duodenum.
 When the stomach is relaxed, the mucosa forms
large folds called rugae.
STOMACH

The stomach participates in both mechanical
and chemical digestion.
 After food enters the stomach, gentle, rippling
mixing waves pass over the stomach. These
waves mechanically macerate food, mix it with
secretions of the gastric glands, and reduce it to
a thin liquid called chyme.
 As food moves through the stomach, the mixing
waves become more vigorous. When food
approaches the pyloric sphincter, small amounts
of chyme are forced through the sphincter into
the duodenum with each mixing wave. The rest
of the food is forced back into the body of the
stomach, where it is subjected to further mixing.



STOMACH
When foods entering the stomach are mixed with acidic
gastric juice, the enzymes lingual lipase and salivary
amylase are inactivated (this may take an hour or more).
Mucous cells in the stomach secrete a thick alkaline
mucus layer that coats the mucosal layer of the stomach
with a 1-3 mm barrier. This protects the stomach lining
from destruction by either hydrochloric acid or pepsin.
Mucosal cells also secrete a variety of other products to
aid in digestion:

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Pepsinogen is converted to pepsin—breaks certain peptide
bonds in proteins.
Gastric lipase—splits short-chain triglycerides into fatty acids
and monoglycerides.
Hydrochloric acid—kills microbes in foods, denatures (unfolds)
proteins, converts pepsinogen into pepsin, inhibits secretion of
gastrin, stimulates secretion of the hormones that promote bile
flow and pancreatic juice.
Intrinsic factor—necessary to absorb vitamin B12.
Gastrin—stimulates secretion of HCl acid and pepsinogen,
contracts lower esophageal sphincter, increases stomach
motility, relaxes pyloric sphincter.
PANCREAS

The pancreas is a retroperitoneal gland, about 12-15 cm
(5-6 in) long by 2 ½ cm (1 in) thick, and is connected to
the duodenum by two ducts.
 Clusters of glandular epithelial cells called acini
constitute the exocrine portion of the pancreas. (The
endocrine portion was discussed earlier with the
endocrine system.)
 Cells within the acini secrete a mixture of fluid and
digestive enzymes called pancreatic juice.
 Pancreatic juice contains:
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Pancreatic amylase—a carbohydrate-digesting enzyme.
Trypsin, chymotrypsin, carboxypeptidase, and elastase—
protein-digesting enzymes.
Pancreatic lipase—the principal triglyceride-digesting enzyme
in an adult.
Ribonuclease and deoxyribonuclease—nucleic acid-digesting
enzymes.
LIVER

The liver is the second largest organ of the body (after
the skin) and the heaviest gland, weighing about 1.4 kg
(3 lb). It is located on the right side of the
abdominopelvic cavity, inferior to the diaphragm.
 The liver has two primary lobes—the large right lobe
and the smaller left lobe.
 Each lobe of the liver is divided into many functional
units called lobules. A lobule consists of specialized
epithelial cells called hepatocytes arranged around a
central vein.
 The liver has endothelial-lined spaces called sinusoids
through which blood passes, instead of veins.
 The sinusoids are also partially lined with stellate
reticulo-endothelial (Kupffer’s) cells, which are
phagocytes that destroy worn-out WBCs and RBCs,
bacteria, and other foreign matter in the blood draining
from the GI tract.
BILE
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Each day, hepatocytes secrete about 800-1000 ml (1 qt) of bile,
which passes through a network of bile ducts, eventually exiting the
liver through the common hepatic duct.
The common hepatic duct joins with the cystic duct from the
gallbladder to form the common bile duct.
Bile is a yellow, brownish, or olive-green liquid with a pH of 7.6-8.6.
It is composed mostly of water and bile acids, bile salts, cholesterol,
a phospholipid called lecithin, bile pigments, and several ions.
Bile is partially an excretory product and partially a digestive
excretion.
Bile salts help with emulsification—the breakdown of large lipid
globules into a suspension of droplets, which helps pancreatic lipase
digest lipids more rapidly. Bile salts and lecithin also make
cholesterol soluble.
The primary pigment in bile is bilirubin.
LIVER

The liver performs many vital functions. Among these
are:
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Carbohydrate metabolism—The liver is very important in
maintaining blood glucose levels. When the blood glucose level
is low, the liver can break down glycogen into glucose, convert
certain amino acids and lactic acid to glucose, or convert other
sugars into glucose. When the blood glucose level is high, the
liver converts glucose into glycogen and triglycerides for storage.
Lipid metabolism—The liver stores some tryglycerides and
breaks down others. It synthesizes lipoproteins, which transport
fatty acids, triglycerides, and cholesterol to and from body cells.
Hepatocytes synthesize cholesterol and use cholesterol to make
bile salts.
Protein metabolism—Without the liver’s participation in protein
metabolism, death would occur in a few days. Hepatocytes
synthesize most plasma proteins (alpha and beta globulins,
albumin, prothrombin, and fibrinogen). Liver enzymes can
convert one amino acid to another (transamination). It also
removes the amino group from amino acids so they can be used
for ATP production or converted to carbohydrates or fats. The
liver converts toxic ammonia into less toxic urea for excretion in
urine.
LIVER

More functions of the liver:
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Removal of drugs and hormones—The liver can detoxify
some substances (alcohol), excrete drugs (penicillin,
erythromycin, sulfonamides) into bile, and chemically alter or
excrete thyroid and steroid hormones (estrogens, aldosterone).
Excretion of bilirubin—Bilirubin, derived from the heme of
worn-out blood cells, is absorbed by the liver and secreted into
bile. Most bilirubin in bile is metabolized by intestinal bacteria
and eliminated in feces.
Synthesis of bile salts—Bile salts are used in the small
intestine for the emulsification and absorption of lipids,
cholesterol, phospholipids, and lipoproteins.
Storage—The liver stores glycogen, vitamins (A, B12, D, E, K)
and mineral (iron, copper) for use elsewhere in the body, when
needed.
Phagocytosis—The stellate reticuloendothelial (Kupffer’s) cells
of the liver break down worn-out RBCs, WBCs, and some
bacteria.
Activation of vitamin D—The liver participates with the skin
and kidneys in activating vitamin D.
GALLBLADDER
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
The gallbladder is a pear-shaped sac about 7-10 cm (34 in) long, located in a depression on the posterior
surface of the liver.
The wall of the gallbladder is composed of only three
layers (it lacks a submucosa).

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The mucosa is composed of simple columnar epithelium
arranged in rugae similar to those of the stomach.
The muscular coat is composed of smooth muscle fibers that
eject the contents of the gallbladder (bile) into the cystic duct
when they contract.
The visceral peritoneum is the outer layer.
The gallbladder functions to store and concentrate (up to
tenfold) bile until it is needed by the small intestine. In
the concentration process, water and ions are absorbed
by the mucosa.
SMALL INTESTINE
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The small intestine is a long tube where the major
events of digestion and absorption occur. (90% of
absorption occurs in the small intestine, while 10%
occurs in the stomach and large intestine.)
It extends from the pyloric sphincter of the stomach, coils
through the abdominal cavity, and eventually terminates
at the large intestine. It averages 2 ½ cm (1 in) in
diameter and 3 m (10 ft) in length.
The small intestine is divided into three segments:
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The duodenum is the shortest segment, retroperitoneal, starts
at the pyloric sphincter, and extends about 25 cm (10 in).
The jejunum is about 1 m (3 ft) long and extends from the
duodenum to the ileum.
The ileum is about 2 m (6 ft) long and joins the large intestine at
the ileocecal sphincter.
SMALL INTESTINE
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Several special features of the small intestine increase
its surface area to enhance absorption.
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Circular folds or plicae cirulares are permanent ridges in the
mucosa that are about 1 cm high. Some extend all the way
around the circumference of the intestine, while other only
extend part way. The circular folds begin in the duodenum and
end midway through the ileum. In addition to increasing surface
area, they also cause the chyme to spiral as it moves through
the small intestine.
Villi are small fingerlike projections of the mucosa. Each villus is
½-1 mm long and contains an arteriole, a venule, a capillary
network, and a lacteal (lymphatic capillary). Nutrients absorbed
by the epithelial cells of the villi enter blood or lymph through the
capillaries or lacteals.
Microscopic microvilli line the surfaces of absorptive cells of the
mucosa. These microvilli form a fuzzy surface next to the lumen
of the small intestine called the brush border. The brush border
increases surface area to allow for greater absorption and
contains several digestive enzymes.
SMALL INTESTINE
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The small intestine participates in both mechanical and
chemical digestion.
 Mechanical digestion movements are divided into two
types:
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Segmentation is the major movement of the small intestine. It is
a localized contraction occurring only in areas containing food. It
mixes chyme with the digestive juices and brings food particles
into contact with the mucosa for absorption. This movement
squeezes segments of chyme back and forth along a length of
small intestine about 12-16 times, thoroughly mixing its contents.
Peristalsis propels chyme forward through the intestinal tract.
Peristatic contraction in the small intestine are weak compared
with those in the esophagus or stomach, so chyme remains in
the small intestine for 3-5 hours.
Both segmentation and peristalsis are controlled by the
ANS.
SMALL INTESTINE
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Every day the small intestine produces 1-2 L of
intestinal juice, a yellow fluid consisting of
water and mucus, with a pH of 7.6.
 Together, pancreatic and intestinal juices provide
a vehicle for the absorption of substances from
chyme as they come in contact with the microvilli
of the small intestine.
 The absorptive epithelial cells also synthesize
several digestive enzymes called brush border
enzymes.
SMALL INTESTINE
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All chemical and mechanical digestion occurring
between the mouth and the small intestine has the
purpose of changing food into forms that can pass
through the epithelial cells lining the mucous membrane
into the underlying blood and lymphatic vessels. These
forms are monosaccharides (glucose, fructose, and
galactose) from carbohydrates, single amino acids,
dipeptides, and tripeptides from proteins, and
monoglycerides from triglycerides.
The passage of these nutrients from the GI tract into the
blood or lymph is called absorption.
The small intestine participates in digesting and
absorbing all of the different food categories
(carbohydrates, proteins, and lipids), as well as nucleic
acids, water, electrolytes, and vitamins.
LARGE INTESTINE
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The large intestine is about 1 ½ m (5 feet) long and 6 ½
cm (2 ½ in) in diameter. It extends from the ileum to the
anus.
The large intestine is divided into four principal regions:
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The cecum is a blind pouch that hangs inferior to the ileocecal
valve. The appendix or vermiform appendix is a twisted,
coiled tube, measuring about 8 cm (3 in) in length attached to the
cecum.
The colon extends from the cecum to the rectum and is divided
into ascending, transverse, descending, and sigmoid portions.
The rectum, the last 20 cm (8 in) of the GI tract, lies anterior to
the sacrum and coccyx. The final 2-3 cm (1 in) of the rectum is
called the anal canal. The external opening of the anal canal is
the anus, which is guarded by an internal sphincter of smooth
muscle (involuntary) and an external sphincter of skeletal muscle
(voluntary).
LARGE INTESTINE
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The wall of the large intestine does not contain
any villi or permanent circular mucosal folds
(unlike the small intestine).
 The epithelium consists of mostly absorptive and
goblet cells. The absorptive cells are primarily
involved in water absorption. The goblet cells
produce mucus that lubricates the colonic
contents in their passage.
 Unlike other parts of the GI tract, the
muscularis layer possesses three thickened
longitudinal muscle bands called taeniae coli,
which run the full length of the large intestine.
Tonic contractions of the taeniae coli gather the
colon into a series of pouches called haustra,
which give the colon a puckered appearance.
LARGE INTESTINE
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Mechanical digestion occurs in the large
intestine through several types of movements:
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Haustral churning is characteristic of the large
intestine. The haustra remain relaxed and distended
as they fill up. When the distention reaches a certain
point, the walls contract, squeezing the contents into
the next haustrum.
Peristalsis occurs at a slower rate than in other parts
of the GI tract (3-12 contractions per minute).
Mass peristalsis is a strong peristaltic wave that
begins at about the middle of the transverse colon
and quickly drives colonic contents into the rectum.
Food in the stomach initiates this gastrocolic reflex,
so mass peristalsis usually occurs 3-4 times per day,
during or immediately after a meal.
LARGE INTESTINE
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The final stage of digestion occurs in the colon
through bacterial activity. (No enzymes are
secreted by the glands of the large intestine.)
 Bacteria in the large intestine prepare chyme for
elimination. They ferment any remaining
carbohydrates (releasing hydrogen, carbon
dioxide, and methane gas), convert remaining
proteins into amino acids and break down amino
acids into simpler substances, and decompose
bilirubin into simpler pigments (giving feces their
brown color).
 Colonic bacteria also produce several vitamins,
including some B vitamins and vitamin K.
LARGE INTESTINE
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In addition to any excess water remaining in the
chyme, the large intestine absorbs electrolytes
(including sodium and chloride) and some
vitamins.
 By the time chyme has remained in the large
intestine for 3-10 hours, it has become solid or
semisolid and is known as feces.
 Feces consists of water, inorganic salts,
sloughed-off epithelial cells from the mucosa of
the GI tract, bacteria, products of bacterial
decomposition, and undigested food parts.
LARGE INTESTINE
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Mass peristalsis pushes fecal matter from the
sigmoid colon into the rectum. Distention of the
rectal wall stimulates stretch receptors, which
initiate the defecation reflex to empty the
rectum.
 Since the external anal sphincter is under
voluntary control, if it is voluntarily relaxed,
defecation through the anus occurs. If it is
voluntarily constricted, defecation can be
postponed. Voluntary contractions of the
diaphragm and abdominal muscles also aid
defecation by increasing the pressure inside the
abdomen.