Download I - Hastings High School

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Phases of Digestion
Cephalic Phase – stimulates and preps stomach for upcoming events; lasts a few minutes; triggered by
aroma, sight, taste, or thought
Gastric Phase – when food reaches stomach, hormones and neurons initiate this phase
1. last 3-4 hours typically (carbs fast, proteins slower, fats slowest)
2. stimulated by distention and peptides (partially digested proteins)
3. causes secretion of gastrin which increases the release of gastric juice
Intestinal phase – triggered by partially digested food entering the duodenum
1. intestinal gastrin secretion – stimulates gastric glands to continue secretion and stomach to
become mobile (move food in small intestine)
2. as more filling occurs – enterogastric reflex occurs
 enterogastric reflex in general stimulates the pancreas, liver, and gall bladder while
inhibiting the stomach
 triggers release of secretin and cholecystokinin in duodenum
Secretin – inhibits gastric phase, increases pancreatic juice output, and increases bile
output by liver (both are used to break down food)
Cholecystokinin (CCK) – increases pancreatic juice output, stimulates gallbladder
excretion to expel bile, and opens the Sphincter of Oddi to allow entry of bile and
pancreatic juice into duodenum


gastric secretions decline (stomach starts to shut down)
pyloric sphincter tightens – stops acid and more chyme from entering the small intestine
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, diameter is about 1” (less
than ½ the diameter of large intestine), encircled by large intestine
1. Absorbs at least 90% of the food 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
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small intestine
3. ileum – 6 ft.; joins the large intestine at the ileocecal valve
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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
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)
http://www.cvm.okstate.edu/instruction/mm_curr/histolog
y/HistologyReference/hrd1.h38.jpg
7. 2 movements in small intestine:
a. Segmentation: major movement that sloshes chyme back & forth
b. Peristalsis: much weaker than in stomach
B. Liver
http://www.mdanderson.org/images/liverQA1.gif
1. Liver – large, blood rich, largest gland in body; has
four lobes
 Right
 Left
 Caudate: inferior and posterior
 Quadrate: inferior and anterior
2. Liver’s main digestive function – produces bile for
export to duodenum
(Bile is a fat emulsifier – breaks up fat – bile is a
yellow-green mixture)
3. Structure of the Liver
a. Contains roughly 100,000 liver lobules
b. Each lobule consists of hepatocytes (hepato- means liver and cyt- means cell)
i. Lined up in sheets or plates of cells that are only 1 layer thick
ii. Space between each plate is called a sinusoid
c. The corners of each lobule contains a hepatic triad containing
i. A branch of the hepatic portal vein
ii. A branch of the hepatic artery
iii. A bile duct (flows the opposite direction)
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http://www.uwgi.org/gut/images/07_fig03.gif
4. Blood flow within Liver
a. Liver is supplied with oxygenated blood coming from the heart by way of the descending
aorta. The hepatic artery - branches off from the descending aorta and then further divides
within the liver
b. Liver is also supplied with deoxygenated blood coming from the veins of the digestive system
by way of the hepatic portal vein. Therefore, all nutrients pass through the liver before going
out to the body!
c. Blood from both a branch of the hepatic portal vein and a branch of the hepatic artery flow into
the sinusoids at the lobular level. The mixed blood travels through the sinusoids and empties
into the central vein (in the center of each lobule) which eventually leads to the hepatic vein
to the inferior vena cava and then back to the heart.
5. Bile Production in Liver
a. Within the lobules there are tiny canals called bile canaliculi that work their way through the
lobules. These narrow channels carry a fluid called bile (made by the hepatocytes) to bile
ducts that drain the bile away from the liver.
b. Bile contains:
i.
ii.
iii.
iv.
Water & ions (to help neutralize chyme)
bilirubin (waste product from hemoglobin – brownish color)
cholesterol (helps make bile salts)
bile salts which are a group of lipids (to emulsify fats in small intestine so that
lipase can work on breaking them down)
c. The major ducts leaving the liver include:
i. Common hepatic duct (empties into the common bile duct)
ii. Cystic duct from the gall bladder (empties into the common bile duct)
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iii. Common bile duct (empties into the small intestine through the Sphincter of
Oddi)
6. Other Important Functions of the Liver
a. Metabolic (nutrient) regulation
i. Extracts nutrients and toxins entering through the digestive system
ii. Monitors and adjusts the organic nutrients traveling within the blood
1. Absorbs and stores glucose as glycogen if blood sugar is high
2. Breaks down glycogen and releases glucose if blood sugar is low
3. Also regulates levels of amino acids and fatty acids
iii. Fat-soluble vitamins (A, D, E, K) are absorbed and stored until needed
b. Hematological (blood) regulation
i. Removes old red blood cells, debris, or pathogens from blood. Phagocytic
Kupffer cells (immune system) are located within the sinusoids.
ii. Synthesize proteins found in the blood
iii. Stores and releases iron
http://www.biologymad.com/Kidneys/liverlobule.jpg
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C. Gallbladder is thin muscular sac snuggled in liver that stores bile
D. 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 –
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a. ascending – up right side of
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b. transverse – across
abdominal cavity
c. descending – down left side
of posterior abdominal wall
d. sigmoid - enters pelvis at
sigmoid colon which joins
Appendix
the rectum
 three lateral curves –
internal transverse folds
are called rectal valves
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e. rectum – last part of the sigmoid
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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
1. Chyme from small intestine becomes solid in 3 to 10 hours
2. Haustral churning: haustra relax while filling up, then contract and move contents into
next haustra
3. Peristalsis much slower in large intestine
4. Mass peristalsis: starts in middle of transverse colon after a meal; triggered by contents
in stomach; strong peristaltic wave
5. Bacteria live in the large intestine; break down undigested food; produce vitamin K and
some B vitamins.
6. 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.
7. 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