Download The Digestive System

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The Digestive System
Digestive System Components
A. Alimentary canal (also called the Gastrointestinal “GI” Tract) – continuous, muscular
digestive tube that winds through the body (30 ft. long when dead & relaxed, shorter when alive
due to muscular tone.) The GI tract is considered outside the body.
1.
2.
3.
4.
5.
6.
B. Accessory Digestive Organs
1.
2.
3.
4.
5.
6.
Digestive Processes
A. Ingestion:
B. Propulsion:
a. swallowing: voluntarily initiated
b. peristalsis: involuntary - alternate waves of contraction & relaxation of muscles in the organ
walls (squeeze food along tract)
C. Mechanical digestion:
D. Chemical digestion:
E. Absorption:
F. Defecation:
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Supporting Structures
A. Peritoneum – serous membrane that lines the organs and the
abdominal cavity. This membrane secretes serous fluid for protection.
1. Visceral peritoneum –
2. Parietal peritoneum –
B. Mesentery/Mesocolon – a double layer of the peritoneum
that extends to the digestive organs, anchoring every organ to
the wall of the abdominal cavity.
A. Hold organs in place and stores fat (curtain over
intestines for protection)
B. Contains blood vessels, lymph vessels, and nerves.
Provides a place for these vessels and nerves, yet
prevents entanglement and allows them to pass
through to other areas.
C. Histology:
1. Mucosa:



secretes mucus, digestive enzymes, and
hormones
absorbs products of digestion into blood
protects against infectious disease
2. Submucosa - connective tissue w/ blood
vessels, lymph vessels, nerves, & elastic fibers that
allow the organ to return to normal shape
3. Muscularis – muscle layer, smooth muscle
 inner-circular (runs around the lumen)
 outer-longitudinal (runs lengthwise w/
lumen)
 inner circular layer also thickens to form
sphincters
4. Serosa:
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D. Innervation:
1. Enteric plexi – a “network” of branched nerves attached to the submucosa within the walls
of the GI tract. Includes the Submucosal plexus and the Myenteric plexus
http://www.vivo.colostate.edu/hbooks/pathphys/digestion/basics/ginerves.gif
2. The Enteric Nervous System is linked to the Central Nervous System
 the parasympathetic nervous system stimulates digestion “rest and digest”
 the sympathetic nervous system inhibits digestion “fight or flight”
Oral Cavity & Associated Organs
A. Mouth – mucosal lined cavity continuous w/ oropharynx (throat)
1. Hard and Soft Palate (roof of mouth)
1. hard –
2. soft –
3. uvula –
2. Tongue – mixes food w/ saliva = bolus
a. intrinsic muscles –
b. extrinsic muscles –
c. lingual frenulum –
d. filiform papillae =
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3. Salivary glands
a. Parotid –
b. Submandibular –
c. Sublingual –
d. Saliva characteristics:
 1000-1500 ml produced on average per day
 pH of 6.75 – 7 (slightly acidic)
 Cleans mouth, dissolves food so you can taste, moistens food = bolus
 Contains salivary amylase (from parotid gland) that starts the breakdown of starch
only
 Contains mucin (mixes w/ water → mucus), lysozyme (also in tears & cells, it’s a
chemical that breaks down cells, attacks & kills bacteria), and IgA
(antibody→fights infection)
 Sympathetic effect = decrease in production → dry mouth
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4. Teeth
a. deciduous teeth – 20 “baby teeth”
b. permanent teeth – 32 including
wisdom
c. anatomy:
 crown – exposed part;
covered by enamel
 root – part embedded in jaw
bone
 neck – connect crown &
root (just below gumline)
 gingival – gum
 enamel – hardest substance
in body, mineralized Ca+
salts – covers outer crown;
cells that make enamel
degenerate after tooth
erupts through gums;
 dentin – bone like
http://www.mydr.com.au/content/images/categories/dental/tooth_
material under enamel =
anatomy_vs2.gif
bulk of tooth (gets
damaged when bacteria gets through enamel)
 pulp/cavity – supplies nutrients & tooth sensation; connective tissue, blood
vessels, nerves
 periodontal ligament (membrane) – anchors tooth in jaw
Pharynx –passageway for food, fluid, and air (air only →unique to
nasopharynx)
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

nasopharynx – air only
oropharynx – behind oral cavity, food/fluids/air
laryngopharynx – near larynx (larynx connected to esophagus or trachea) , food/fluids/air
Esophagus – food enters when epiglottis (at top of larynx) closes off trachea to food
1. Esophagus- pierces diaphragm to enter abdomen
2. Joins stomach at gastroesophageal or cardiac sphincter (at top of stomach; improper
function → acid reflux)
3. Uses peristalsis to move food to stomach
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Stomach – mainly a storage area where chemical breakdown of protein begins
& food is converted to chyme
Functions:
1.
2.
3.
4.
Main structures: “J-shaped”
1. Cardiac Sphincter
2. Cardia
3. Fundus/Body
4. Pylorus
5. Pyloric Sphincter
6. Rugae:
3 layers of smooth muscle in muscularis instead of 2:
1.
2.
3.
Gastric glands: made of cells found in deep pits in the mucosa of
the stomach
1. mucous cells – release mucus
2. parietal cells – release HCL and intrinsic factor
 HCL – needed to activate pepsinogen
into pepsin (digests proteins) and kill
Collectively
bacteria
these
substances
 Intrinsic factor – required for vitamin
are called
B12 absorption in small intestine
3. chief cells – releases pepsinogen
gastric juice
4. enteroendocrine cells or G-cells – release
gastrin which stimulates stomach
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Phases of Digestion
Cephalic Phase –
Gastric Phase – when food reaches stomach, hormones and neurons initiate this phase
1.
2.
3.
Intestinal phase –
1.
2.

triggers release of secretin and cholecystokinin in duodenum
Secretin –
Cholecystokinin (CCK) –


gastric secretions decline (stomach starts to shut down)
pyloric sphincter tightens – stops acid and more chyme from entering the small intestine
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Small Intestine and Associated Structures
A. Small intestine – major digestive organ from pyloric sphincter to ileocecal valve
 longest part of the GI tract, 10 feet long in living person, less than ½ the diameter
(1”) of large intestine, encircled by large intestine
1. Absorbs at least 90% of the food and water for the
body.
2. 3 sections of the small intestine:
1. duodenum – “12 fingers” which is about 10
inches; initial part of small intestine; curves
around pancreas; receives the secretions of the
liver, gallbladder, and pancreas; common bile
and pancreatic ducts unite at the Sphincter of
Oddi (in the wall of the duodenum)
2. jejunum – “empty”; 3 ft.; middle portion of
small intestine
3. ileum – 6 ft.; joins the large intestine at the ileocecal valve
3. parasympathetic and sympathetic messages are relayed through mesenteric plexus
4. arterial and venous supply also through mesentery
5. Surface area in the small intestine is increased through the
following:
i. plicae circulares: deep, circular, permanent folds of
mucosa and submucosa
 1 cm tall
 folds force chyme through lumen – allowing
time for nutrient absorption due to more
surface area
http://content.answers.com/main/content/wp
/en-commons/thumb/3/36/220pxMultiple_Carcinoid_Tumors_of_the_Small
_Bowel_2.jpg
ii. villi: fingerlike projections of mucosa; give a velvety appearance
 1mm tall
 20 per mm2
 absorptive columnar cells
 core of each villus has dense capillary bed (so nutrients can be absorbed) and a
lacteal (lymph vessel)
http://www.udel.edu/biolo
gy/Wags/histopage/wagne
rart/anaglyphpage/villi.gif
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iii. microvilli: tiny projections of cell membranes of absorptive cells = brush border
 brush border enzymes complete digestion of carbs and proteins
 200 million per mm2
http://www.cvm.okstate.edu/instruction/mm_curr/histolog
y/HistologyReference/hrd1.h38.jpg
6. histology of wall – composed of numerous types of cells
a. absorptive cells – columnar cells
b. goblet cells – secretes mucus
c. enteroendocrine cells – secrete hormones (CCK and secretin)
d. intestinal crypts – glands in the pits of the mucosa that secrete intestinal juice
(water solution)
7. 2 movements in small intestine:
a. Segmentation: major movement that sloshes chyme back & forth
b. Peristalsis: much weaker than in stomach
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B. Liver and Gallbladder
1. Liver – large, blood rich, largest gland in
body; has four lobes
 Right
 Left
 Caudate: inferior and posterior
 Quadrate: inferior and anterior
2. Gallbladder is thin muscular sac snuggled
in liver that stores bile
3. Liver’s digestive function – produces bile
for export to duodenum
a. bile is a fat emulsifier – breaks up fat –
bile is a yellow-green mixture
http://www.mdanderson.org/images/liverQA1.gif
b. Bile consists of:

c. bile salts – recycled by enterohepatic circulation
i. reabsorbed into blood by ileum
ii. returned to liver via hepatic portal blood
http://www.uwgi.org/gut/images/07_fig03.gif
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http://www.biologymad.com/Kidneys/liverlobule.jpg
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C. Pancreas – nestled in the first curve of the small intestine
http://www.colorado.edu/kines/Class/IP
HY3430-200/image/pancreas.jpg
1. Acini – cluster of secretory cells around a duct that carries enzymes to small intestine; full of
rough ER
2. Islets of Langerhans – secretes insulin & glucagon to control blood sugar
3. Pancreatic Juice – enzymes and bicarbonates (neutralizes chyme for optimal environment for
enzyme activity)
a. trypsin, elastase, carboxypeptidase, and chymotrypsin work to finish breaking
proteins into amino acids
b. lipases, work on fatty acids
c. nucleases work on nitrogen bases (DNA) and simple sugars
d. maltase, lactase, sucrase work on simpler sugars (disaccharides)
e. enterokinase – brush border enzyme that activates trypsin
4. secretin and Cholecystokinin (CCK) (hormones that stimulate or inhibit various components
of the digestive system) Secretin is responsible for triggering the production of sodium
bicarbonate and other buffers to neutralize the acids coming from the stomach.
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4. Large Intestine
1. Removes water, completes absorption, and manufacture certain vitamins
2. Frames the small intestine; is less than half as long (5 ft.); 2.5” in diameter
3. Longitudinal Muscle reduced to three bands of smooth muscle = teniae coli
Its tone causes the wall to pucker into sacks called haustra
4. Cecum – first part of large intestine which follows the ileocecal valve; like a second stomach
5. Veriform Appendix – attached to the posterior-medial surface of cecum
6. Colon –
a. ascending – up right side of
cavity to level of right
kidney
b. transverse – across
abdominal cavity
c. descending – down left side
of posterior abdominal wall
d. sigmoid - enters pelvis at
sigmoid colon which joins
the rectum
 three lateral curves –
internal transverse folds
are called rectal valves
http://stb.msn.com/i/E8/4A9F45284ACCBB1060DBC2
e. rectum – last part of the sigmoid
25C86CB2.jpg
colon
f. anal canal – last segment; 3cm long; opens to exterior at anus
 internal anal sphincter – involuntary smooth muscle
 external anal sphincter – voluntary skeletal muscle
 contains many blood vessels in folds of the anus (inflammation of these causes
hemorrhoids or piles
7. Chyme from small intestine becomes solid in 3 to 10 hours
8. Haustral churning: haustra relax while filling up, then contract and move contents into next
haustra
9. Peristalsis much slower in large intestine
10. Mass peristalsis: starts in middle of transverse colon after a meal; triggered by contents in
stomach; strong peristaltic wave
11. Bacteria live in the large intestine; break down undigested food; produce vitamin K and some
B vitamins.
12. Sensation to defecate begins when the contents move into the rectum. If defecation does not
occur, the contents will move back into the sigmoid colon.
13. Feces: undigested food (fiber), bacteria and products of bacterial decomposition, decomposed
bilirubin (gives feces its color); inorganic salts, sloughed off cells of GI tract, and water