Download Chapter 7 Body Systems

Chapter 25
Anatomy of the
Digestive System
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Overview of the Digestive
System
• Role of the digestive system
– Prepares food for absorption and use by all
the cells of the body
– Food material not absorbed becomes
feces that is eliminated
– Digestion depends on both endocrine and
exocrine secretions and the controlled
movement of ingested food materials
through the gastrointestinal (GI) tract
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•
Overview of the Digestive
System
Organization of the digestive system
– Organs of digestion
• form the GI tract that extends through the abdominopelvic cavity
• Ingested food material passing through the lumen of the GI tract is
outside the internal environment of the body
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Walls of GI Tract
• 4 layers of tissues:
–
–
–
–
Mucosa
Submucosa
Muscularis
Serosa
Deep
Superficial
• Modifications of layers—
layers
of the GI tract have various
modifications to enable it to
perform various functions
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Mouth
• Oral cavity (buccal cavity)
– Lips
• covered externally by skin and internally by mucous
membrane
• junction between skin and mucous membrane is highly
sensitive
• when lips are closed, line of contact is oral fissure
– Cheeks
• lateral boundaries of oral cavity, continuous with lips and
lined by mucous membrane
• formed in large part by buccinator muscle covered by
adipose tissue
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• Contain mucus-secreting glands
Mouth
• Oral cavity (buccal cavity)
– Hard Palate
• consists of four bones
• Essential for clear articulation
– Soft Palate
• made of muscle arranged in an arch
• Helps with swallowing, yawning, and ear popping
• Suspended from midpoint of posterior border of the arch is the
uvula
– Tongue— solid mass of skeletal muscle covered by a mucous
membrane
• Important for mastication (chewing) and deglutition
(swallowing)
• Has three parts: root, tip, and body
• Stratified squamous epithelium
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• Lingual frenulum anchors tongue to floor of mouth
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• Salivary glands
Mouth
– 3 pairs of compound tubuloalveolar
glands
– secrete 1 liter of saliva each day
– >5% of total salivary volume but
provide for hygiene and comfort of oral
tissues
– Parotid glands
• largest of the paired salivary glands
• produce watery saliva containing
enzymes
– Submandibular glands
• compound glands that contain enzyme
and mucus-producing elements
– Sublingual glands
• smallest of the salivary glands
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•
Organs of mastication (chewing)
Teeth
• Crown
– exposed portion of a tooth, covered by
enamel
– ideally suited to withstand abrasion during
mastication
• Neck
– narrow portion that joins the crown to the root
– surrounded by the gingivae (gums)
• Outer shell contains two additional tissues: dentin
and cementum
– Dentin makes up the most of the tooth shellharder than bone!
– At crown, covered by enamel, and at neck
and root, covered by cementum
– Pulp cavity
» located in dentin
» contains connective tissue, blood, and
lymphatic vessels and sensory nerves
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Teeth
• Types of teeth
• Deciduous teeth (fall off)
– 20 baby teeth
• Permanent teeth
– 32 teeth, replace the deciduous teeth by pushing them out
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Pharynx & Esophagus
• Pharynx
– Tube through which a
bolus passes when
moved from the mouth
to the esophagus by the
process of deglutition
(swallowing)
• Esophagus
– Tube that extends from
the pharynx to the
stomach
– First segment of
digestive tube
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Stomach
• Size and position of the stomach
– Size varies according to factors such as
gender and amount of distention (enlarged)
• When no food is in stomach, it is about the size
of a large sausage
• In adults, capacity ranges from 1.0 to 1.5 liters
– Stomach location: upper abdominal cavity
under liver and diaphragm
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Stomach
• Divisions of the stomach
– Fundus— above opening of
esophagus into stomach
(roller coasters)
– Body— central portion of
stomach
– Antrum— lowest part of
stomach
– Pylorus— narrowing where
the stomach joins the small
intestine
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Stomach
• Sphincter muscles— circular fibers arranged so
that there is an opening in the center when relaxed
and no opening when contracted
– Lower esophageal sphincter (LES) or cardiac
sphincter controls opening of esophagus into
stomach
• If LES does not remain properly closed after
intake of food, stomach acid can spill into
esophagus causing damage
– This is acid reflux, or GERD (gastroesophageal
reflux disease)
– Pyloric sphincter controls outlet of pyloric portion
of stomach into duodenum
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• Stomach wall
– Gastric mucosa
Stomach
• Epithelial lining has rugae marked by gastric pits
• Gastric glands
– found below level of the pits
– secrete most of gastric juice
• Chief cells
– secretory cells found in gastric glands
– secrete the enzymes of gastric juice
• Parietal cells
– secretory cells found in gastric glands
– secrete hydrochloric acid
– Helps with vitamin B12 absorption
• Endocrine cells— secrete gastrin and ghrelin (grel′in)
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Stomach
– Stomach wall
– Gastric muscularis
• thick layer of muscle with three distinct sublayers
• this allows stomach to contract strongly at many angles
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Stomach
• Functions of the stomach
– Reservoir for food until it is partially digested
and moved further along GI tract
– Secretes gastric juice to aid in digestion of food
– Breaks food into small particles and mixes them
with gastric juice
– Limited absorption
– Produces gastrin and ghrelin
– Helps protect body from pathogenic bacteria
swallowed with food
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Small Intestine
• Size and position of the small intestine
– tube approximately 2.5 cm in
diameter and 6 m in length
– fills most of abdominal cavity
• Divisions of the small intestine
– Duodenum
• uppermost division
• Approximately 25 cm long
• shaped roughly like the letter C
– Jejunum— approximately 2.5 m long
– Ileum— approximately 3.5 m long
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Small Intestine
• Wall of the small intestine
– Intestinal lining has villi
– Villi— important modifications of mucosal layer
• Each villus contains an arteriole, venule, and lymphatic
vessel
• Covered by a brush border
made up of 1,700 ultrafine
microvilli per cell
• Villi and microvilli increase
surface area of small
intestine hundreds of times
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Large Intestine
• Also called colon
• Size of the large intestine
– average diameter, 6 cm
– length, approximately 1.5 to 1.8 m
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Large Intestine
• Divisions of the large intestine
– Cecum— first 5 to 8 cm of large intestine, blind pouch located in lower
right quadrant of abdomen
– Colon
• Ascending colon— vertical position on right side of abdomen
– valve prevents material passing from large intestine back into ileum
• Transverse colon passes horizontally across abdomen, above small
intestine
• Descending colon— vertical position on left side of abdomen
• Sigmoid colon joins descending colon to rectum
• Rectum— last 7 or 8 inches of intestinal tube
– terminal inch is anal canal with opening called the anus
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Large Intestine
• Wall of the large intestine
– Intestinal mucous glands produce lubricating
mucus that coats feces as they are formed
– Uneven distribution of fibers in the muscle
coat
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Vermiform Appendix
• Accessory organ of digestive system
• 8 to 10 cm in length
• communicates with cecum
• Appendicitis
– If becomes blocked  increased pressure
– If not treated, the appendix will break and leak infection into
the body
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• FYI: maintains a “safe house” for the beneficial bacteria
Peritoneum
• Large, continuous sheet of serous membrane
• Membrane covering the surfaces of organs
– allows movement of each and helps prevent
strangulation of the GI tract
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Liver
•
•
•
•
largest gland in body
weighs approximately 1.5 kg
lies under diaphragm
occupies most of right hypochondrium and
part of epigastrium
• Liver lobes and lobules
– Left lobe— forms about one sixth of liver
– Right lobe— forms about five sixths of liver
• divides into right lobe proper, caudate lobe,
and quadrate lobe
– Hepatic lobules— anatomical units of liver
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Liver
• Bile ducts
– Small bile ducts form
right and left hepatic
ducts
– Right and left hepatic
ducts join  common
hepatic duct
– Hepatic duct merges
with cystic duct 
common bile duct 
duodenum
1, Common bile duct; 2,
common hepatic duct; 3, cystic
duct; 4, gallbladder; 5, left
hepatic duct; 6, liver shadow
with tributaries of hepatic
ducts; 7, right hepatic duct.
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Liver
• Functions of the liver
– Detoxification by liver cells
– Bile secretion by liver
• Bile salts are formed in liver from cholesterol
and are the most essential part of bile
• Liver cells secrete approximately 1 pint of bile
per day
– Metabolism of proteins, fats, and
carbohydrates
– Storage of substances such as iron and fat
soluble vitamins
– Production of important plasma proteins
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Gallbladder
• Size and location of the gallbladder
– pear-shaped sac from 7 to 10 cm long and 3 cm
wide at its broadest point
– holds 30 to 50 ml of bile
– lies on undersurface of liver
• Structure of gallbladder
– serous, muscular, and mucous layers compose
the gallbladder wall; mucosal lining has rugae
• Functions of gallbladder:
– Storage of bile
– Concentration of bile fivefold to tenfold
– Ejection of the concentrated bile into duodenum
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Pancreas
• Size and location of the pancreas
– grayish pink–colored gland
– 12 to 15 cm long
– weighs approximately 60 g
– from duodenum and behind stomach to spleen
• Structure of the pancreas
– composed of endocrine and exocrine glandular tissue
– Exocrine portion (98%)
• Secretes digestive enzymes
• tiny ducts unite to form main pancreatic duct, which
empties into duodenum
– Endocrine portion (2%)
• embedded between exocrine units called pancreatic
islets
• made up of alpha cells and beta cells
• pass secretions into capillaries
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Pancreas
• Functions of the pancreas
– Secretes digestive enzymes into
duodenum
– Beta cells secrete insulin
– Alpha cells secrete glucagon
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Cycle of Life: Digestive System
• Changes in digestive function and structure
are age-related
– Result in diseases or pathological conditions
– May occur in any segment of intestinal tract
– Changes involve accessory organs: teeth, salivary
glands, liver, gallbladder, and pancreas
• Infants— immature intestinal mucosa
– intact proteins can pass through epithelial cells
lining the tract and trigger allergic response
• Lactose intolerance affects infants who lack
the enzyme lactase
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Cycle of Life: Digestive System
• Young age
– Mumps common in children
• swelling of the salivary glands
– appendicitis more common in
adolescents and then decreases with
advancing age
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Cycle of Life: Digestive System
• Middle age
– ulcers and gallbladder disease common
• Old age
– decreased digestive fluids
– slowing of peristalsis
– reduced physical activity lead to constipation
and diverticulosis
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