Download Chapter 16 - Digestive System

Chapter 16 Lecture Notes
page 1
alimentary canal (gut) + accessory organs
A. functions
1. digestion - breaking down food into molecules small enough to be transported
into the body
a. enzymes break chemical bonds to make molecules smaller - these are
secreted by the salivary glands, stomach glands, and pancreas
(other enzymes are components of the intestinal epithelial cell
membranes)
ENZYME
large molecule  small molecule
b. mechanical processing makes food particles smaller and provides a
larger surface area for the enzymes to work on
2. absorption - moving molecules (food, water, ions, vitamins) from the lumen of
the gut across the epithelial cells and into capillaries
3. excretion of undigested materials, dead cells and bacteria, and bilirubin
BIOL 2404
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Chapter 16 Lecture Notes
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B. general histological structure
1. mucosa (mucous membrane) - modified to increase surface area or allow
expansion
a. epithelium - protects, secretes, absorbs
b. lamina propria (c.t.) - contains capillaries and glands
c. muscularis mucosae - moves mucosal folds and villi
2. submucosa - c.t. containing larger blood vessels, lymph vessels, nerve
network
3. muscularis externa - layers of smooth muscle that mix and propel gut
contents; contains nerve network and pacesetter cells
a. peristalsis - wave of contraction that moves contents forwards
b. segmentation - alternating contraction and relaxation that mixes
contents
4. serosa (visceral peritoneum) covers stomach, small intestines, cecum and
colon
the other organs have a layer of c.t. called the adventitia that that attaches
them to surrounding structures
BIOL 2404
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Chapter 16 Lecture Notes
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C. mesenteries are double layers of peritoneum that suspend some abdominal organs
inside the abdominal cavity
mesenteries contain blood vessels, lymph vessels and nerves for these organs
greater omentum - greater curvature of stomach to transverse colon
D. mouth
1. oral or buccal cavity
vestibule = area between cheeks or lips and teeth
2. tongue - attached posteriorly
lingual frenulum attaches inferior surface of tongue to floor of mouth
used in chewing, speech and swallowing
3. salivary glands
saliva contains water, mucous, salivary amylase, antibodies, lysozyme
a. parotid - anterior to ear, ducts open at 2nd upper molar
b. sublingual - below tongue, ducts open lateral to lingual frenulum
c. submandibular - inside lower mandible, ducts open lateral to lingual
frenulum
BIOL 2404
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Chapter 16 Lecture Notes
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4. teeth - 20 deciduous, 32 permanent
5. chewing / mastication = mechanical processing
6. swallowing / deglutition
a. oral phase - tongue pushes food into pharynx
b. pharyngeal phase - pressure receptors in pharynx signal medulla
oblongata, which causes larynx to elevate and epiglottis to cover
glottis, pharyngeal muscles push food into esophagus
c. esophageal phase - peristalsis and relaxation of gastroesophageal
sphincter
E. esophagus - muscular tube that extends from pharynx to stomach; located posterior
to trachea
1. sphincters
a. upper – between pharynx and esophagus
b. (lower) gastroesophageal or cardiac – between esophagus and
stomach
2. esophageal hiatus - opening in diaphragm for esophagus (hiatal hernia)
F. stomach - storage organ (1-1.5 liters) located inferior to diaphragm in left upper
abdomen
1. divisions:
a. cardia
b. fundus
c. body
d. pylorus
pyloric sphincter controls movement of food into duodenum
BIOL 2404
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Chapter 16 Lecture Notes
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2. glands in the mucosa produce gastric juice
a. HCl - activates pepsinogen
b. pepsinogen - inactive protein-digesting enzyme
c. intrinsic factor - binds to vitamin B12
3. rugae are folds of the mucosa that allow expansion
4. control
a. ANS
sympathetic - inhibits stomach activity
parasympathetic - stimulates stomach activity
b. hormones
gastrin stimulates stomach activity
intestinal hormones inhibit stomach activity
c. enterogastric reflex (nerve signals from duodenum) inhibits stomach
activity
G. small intestine (5 hour residence time)
1. divisions
a. duodenum - 25 cm long - between stomach and jejunum
common bile duct and major pancreatic duct enter at duodenal
papilla
most digestion and absorption occurs here
duodenal glands secrete alkaline mucous
b. jejunum - 2.5 meters long - between duodenum and ileum
c. ileum - 3.5 meters long - between jejunum and large intestine
ileocecal valve prevents backflow from large intestine
2. histology
a. columnar epithelial cells have microvilli on apical surface - increases
surface area
BIOL 2404
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Chapter 16 Lecture Notes
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b. mucosa has finger-like projections called villi - increases surface area
villi contain large lymphatic capillaries called lacteals and also
vascular capillaries; absorbed materials enter these
capillaries
c. mucosa and submucosa form circular folds (plicae circulares) increases surface area
3. segmentation mixes contents
4. control
a. ANS
sympathetic - inhibits activity
parasympathetic - stimulates activity
b. hormones
H. pancreas - located posterior and inferior to stomach
1. exocrine pancreas consists of secretory units that make digestive enzymes
a. amylase breaks down large carbohydrates
b. protease breaks down proteins
c. lipase breaks down triglycerides
2. duct enters duodenum
3. secretion controlled by ANS and hormones
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Chapter 16 Lecture Notes
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I. liver - located in upper right abdominal cavity just below diaphragm
1. lobes
a.
b.
c.
d.
right
left
caudate
quadrate
2. lobes are divided into lobules
a. hepatocytes secrete bile into bile ducts
b. hepatic sinusoids carry blood from hepatic portal vein and hepatic
artery past hepatocytes, which absorb materials from the blood
c. blood from sinusoids enters central veins → hepatic veins → inferior
vena cava
3. functions
a. bile secretion
b. storage of glucose, lipids, vitamins, iron
c. detoxification of various molecules, inactivation of hormones
d. synthesis of plasma proteins (including clotting factors)
e. metabolic activities
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J. gall bladder - located inferior to right lobe of liver
1. function - storage, dehydration and release of bile
bile emulsifies fat, which increases the surface area for enzymatic activity
and speeds up lipid digestion
2. control: ANS and hormones
cholecystokinin (CCK) causes contraction of gall bladder
3. bile duct system
right hepatic duct
left hepatic duct
common hepatic duct
cystic duct
common bile duct
duodenum
K. large intestine - 1.5 meters long
1. divisions
a. cecum - inferior to ileocecal valve, vermiform appendix opens into
cecum and is located posterior to it
b. colon: ascending, transverse, descending, sigmoid
haustra - pouches in wall of colon
c. rectum
opens into anal canal
anus is inferior opening of gut
internal anal sphincter smooth muscle controlled by reflex
external anal sphincter is skeletal muscle and controlled voluntarily
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2. functions
a. absorption of water
b. absorption of vitamins made by resident bacteria: K, biotin, B 5
c. storage of feces
d. recycles bile salts
3. mass movements are peristaltic waves that move feces into the rectum
stimulated by distention of stomach (gastrocolic reflex) and duodenum
4. defecation
a. reflex initiated by stretch receptors in rectum
causes contraction of colon and rectum
b. voluntary relaxation of external anal sphincter
L. digestion and absorption (figure 16-18)
1. carbohydrates
salivary amylase
pancreatic amylase
intestinal epithelial enzymes
absorbed through intestinal epithelial cells into vascular capillaries
2. proteins
pepsin
pancreatic proteases
intestinal epithelial enzymes
absorbed through intestinal epithelial cells into vascular capillaries
BIOL 2404
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Chapter 16 Lecture Notes
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3. lipids
emulsification by bile (this is not chemical digestion)
pancreatic lipase
triglycerides are resynthesized inside the intestinal epithelial cells and
combined with proteins to make chylomicrons
absorbed through into lacteals
4. water and electrolytes
a. sodium and chloride are absorbed from the intestine
b. water follows by osmosis
c. iron absorption is carefully controlled to prevent toxicity
5. vitamins
a. water soluble
b. fat soluble (A, D, E, K) - absorbed along with lipids from food
c. vitamin B12 - must be bound to intrinsic factor
BIOL 2404
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