Download Digestive System Notes - Full Version

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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 because its contents have not crossed a membrane and truly
“entered” the body.
http://ww
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1.
2.
3.
4.
5.
6.
oral cavity
pharynx
esophagus
stomach
small intestine
large intestine
B. Accessory Digestive Organs
1. salivary glands
2. tongue
3. teeth
4. pancreas
5. liver
6. gallbladder
Digestive Processes
http://trc.ucdavis.edu/biosci10v/bis10v/week10/digestive.gif
A. Ingestion: taking food into digestive tract (through mouth)
http://www.youtube.com/watch?v=Ri8bBhw9ms
B. Propulsion: moving food through alimentary canal
Q&NR=1
a. Swallowing/deglutition: voluntarily initiated
b. Peristalsis: involuntary - alternate waves of contraction & relaxation of muscles in the
organ walls (squeeze food in one direction along tract using two layers of muscle)
C. Mechanical digestion: (physical) chewing/mastication, tongue mixing food w/ saliva, churning
food in stomach, and segmentation (rhythmic local constrictions that can move the food back &
forth) within the small intestine → mixes food w/ digestive juices
D. Chemical digestion: catabolic steps by enzymes secreted into lumen (hollow open tube of
intestine); begins in mouth and is complete in the small intestine
E. Absorption (small intestine) – digested product (food that has been broken down into molecules)
from lumen passes through mucosal cells into blood or lymph
F. Defecation – elimination of indigestible substances (fecal matter: fiber, bacteria, mites,
protozoans, viruses). Feces also contains other substances mentioned later in this packet
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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 – covers external surface of organs
& is continuous w/ the parietal peritoneum
2. Parietal peritoneum – lines the wall of the abdominal
cavity
B. Mesentery – 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. Greater Omentum a double layer of the peritoneum that originates from the greater curvature
of the stomach and extends over the organs in the digestive system. Functions in fighting disease
by holding a great number of lymph nodes
D. Histology:
1. Mucosa (inner-most layer, moist epithelial
membrane)
 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, has an innercircular (runs around the lumen) and an
outer-longitudinal (runs lengthwise w/ lumen)
layer of smooth muscle; inner circular layer
also thickens to form sphincters
4. Serosa – the visceral peritoneum in GI
tract; layer that wraps the organs
http://www.lib.mcg.edu/edu/eshuphysio/program/section6/6ch1/6ch1img/page4.jpg
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E. Innervation:
1. Enteric plexi – “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 – anterior
2. soft – posterior; rises to close off nasopharynx when swallowing
3. uvula – flap attached to soft palate - involved in sleep apnea
2. Tongue – mixes food w/ saliva = bolus
a. intrinsic muscles – change shape of tongue
(rolls tongue in)
b. extrinsic muscles – alters position of tongue
(stick tongue out, touch roof of mouth)
c. lingual frenulum – holds tongue to floor of mouth,
limits movement (prevents us from swallowing
tongue)
d. filiform papillae = roughness – specks on tongue
NOT tastebuds
(cats have pronounced filiform papillae needed to lap & “grab” food)
http://kbb.uludag.edu.tr/image/oralkavite-ders/Resim-6.jpg
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3. Salivary glands
a. Parotid – anterior to ear, between masseter muscle and skin (large muscle in jaw)  Produces most of the salivary amylase for starch digestion.
 Ducts open near upper molars
b. Submandibular – medial aspect of mandible body
 Ducts open at base of lingual frenulum
c. Sublingual – under tongue laterally
 Ducts open on both sides of tongue, underneath
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
Contains the following substances:
o Salivary Amylase - starts the breakdown of starch
o Mucin - mixes with water to create mucus
o Lysozyme - a chemical that breaks down cells, attacks & kills bacteria
o Immunoglobulin A (IgA) - antibody that helps fights infection
Sympathetic effect - (fight/flight) decrease in production of saliva leading to a dry mouth
http://health.yahoo.com/media/mayoclinic/images/image_popup/ah6a192.jpg
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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 material under enamel =
bulk of tooth (gets damaged when bacteria
gets through enamel)
 pulp/cavity – supplies nutrients & tooth
http://www.mydr.com.au/content/images/categories/dental/tooth_
sensation; connective tissue, blood
anatomy_vs2.gif
vessels, nerves
 periodontal ligament (membrane) – anchors tooth in jaw
Pharynx –passageway for food, fluid, and air



nasopharynx – behind nasal cavity, air only
oropharynx – behind oral cavity, food/fluids/air
laryngopharynx – near larynx (larynx connected to esophagus or trachea) , food/fluids/air
http://www.youtube.com/watch?v=DB_E2BygPJk&feature=related glottis & epiglottis
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
http://www.youtube.com/watch?v=hm1nEPHxJ0E&feature=related
Epiglottis - The epiglottis guards the entrance of the trachea (wind pipe) when we swallow, directing
food and liquids down the esophagus. It is normally pointed upward while one is breathing, but while one
is swallowing, elevation of the hyoid bone draws the larynx upward; as a result, the epiglottis folds down
to a more horizontal position. In this manner it prevents food from going into the trachea and instead
directs it to the esophagus, which is posterior.
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Stomach – storage area where protein digestion begins
Functions:
1. Holding place for food so that it can
enter the duodenum gradually
2. Churns and mixes food – bolus now
becomes “chyme”
3. Chemical digestion of proteins begins
in the stomach
4. Absorbs water, aspirin, alcohol, and
electrolytes
Borborygmi
Main structures: “J-shaped”
= “stomach
1. Cardiac Sphincter growling”
2. Cardia
3. Fundus
4. Body
5. Pylorus
6. Pyloric Sphincter
7. Rugae: folds in mucosa that allow
for stretching when stomach is full
http://www.rivm.nl/interspeciesinfo/Images/stomach_tcm75-26442.gif
3 layers of smooth muscle in muscularis instead of 2:
1. **Inner oblique smooth muscle layer (diagonal)
found only in the stomach that allows the stomach to
churn food
2. Middle circular smooth muscle layer (around
stomach)
3. Outer longitudinal smooth muscle layer (lengthwise)
Gastric glands: made of cells found in deep pits in the mucosa
of the stomach
1. mucus made by mucous cells
2. HCL made by parietal cells
 needed to activate pepsinogen into
pepsin (digests proteins) and kill
Collectively
bacteria
these
3. Intrinsic factor made by parietal cells
substances,
not including
 required for vitamin B12 absorption in
the gastrin,
small intestine
are called
4. pepsinogen made by chief cells
gastric juice
 breaks down proteins into smaller
component
 must be activated by HCL first – called pepsin once activated
5. enteroendocrine cells or G-cells – releases gastrin (a hormone) into the blood which
stimulates the stomach to release gastric juice
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Phases of Digestion
Cephalic Phase – (prefix cepha = head/brain)
 stimulates and preps stomach for upcoming events – may last just a few minutes
 triggered by aroma, sight, taste, or thought
Gastric Phase – (prefix gastro = stomach)

hormones & neurons initiate this phase as food reaches stomach

last 3-4 hours typically (carbs fast, proteins slower, fats slowest)

stimulated by distention and peptides (partially digested proteins)

causes secretion of gastrin (a hormone) which increases the release of gastric juice
Intestinal phase – (prefix entero = intestine)

triggered by partially digested food entering the duodenum

intestinal gastrin secretion – stimulates gastric glands to continue secretion and stomach to
become more active in order to move food in small intestine

as more filling occurs – enterogastric reflex occurs


enterogastric reflex in general stimulates the pancreas, liver, and gall bladder while
inhibiting the stomach, allowing more time for chemical digestion to take place in
the small intestine
triggers release of secretin and cholecystokinin in duodenum

Secretin – inhibits gastric phase (slowing stomach mobility), increases pancreatic
juice output, and increases bile output by liver in order to increase chemical
digestion (and eventually absorption) of nutrients within the small intestine

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
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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, diameter is about 1” (less
than ½ the diameter of large intestine), encircled by large intestine
1. Absorbs at least 90% of the food/nutrients for the body.
2. 3 sections of the small intestine:
1. duodenum – 10 inches in length

Initial part of small intestine

Curves around pancreas

Receives the secretions of the liver, gallbladder, and pancreas that come
from the common bile and pancreatic ducts and unite at the Sphincter of Oddi
(in the wall of the duodenum)
2. jejunum – 3 ft in length

Middle portion of small intestine
3. ileum – 6 ft in length

Joins the large intestine at the ileocecal (between ileum and cecum) valve
3. Parasympathetic and sympathetic messages are relayed through mesenteric plexus (mesentery)
4. Arterial and venous supply also through mesentery – this blood brings oxygen to the small
intestine and carries nutrients from the small intestine (to the liver)
http://health.yahoo.com/media/healthwise/nr551448.jp
g
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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)
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.udel.edu/biolo
gy/Wags/histopage/wagne
rart/anaglyphpage/villi.gif
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6. histology of wall – composed of numerous types of cells
a. absorptive cells – columnar cells for absorption
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
http://www.cvm.okstate.edu/instruction/mm_curr/histolog
y/HistologyReference/hrd1.h38.jpg
Endocrine glands – (endo = in) secrete their products (hormones) into the blood or next to cells that they
interact with directly – examples are the enteroendocrine glands that secrete the hormones gastrin,
secretin, and cholecystokinin. The pancreas also produces some hormones that are secreted directly into
the blood.
Exocrine glands – (exo = out) secrete their products outside the body – examples are the sweat glands,
salivary glands, mammary glands, stomach, liver, and pancreas. The sweat glands secrete sweat to the
surface of the skin. The liver, pancreas, and salivary glands secrete their products into the lumen of the
digestive organs (which is considered outside the body).
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B. Liver
1. Liver – large, blood rich, largest gland in body; has
four lobes
 Right
 Left
 Caudate: inferior and posterior
 Quadrate: inferior and anterior
http://www.mdanderson.org/images/liverQA1.gif
2. Liver’s main digestive function –
 Produces bile for export to duodenum
 Bile is not an enzyme
 Bile is a fat emulsifier – breaks up fat –
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)
http://www.uwgi.org/gut/images/07_fig03.gif
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4. Blood flow within Liver
a. Liver is supplied with oxygenated blood coming from the heart. Oxygenated blood leaves the
heart by way of the aorta. The hepatic artery branches off from the descending aorta and then
further divides within the liver providing all liver cells with oxygen.
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 remaining cells of 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
and then back to the heart through the inferior vena cava (the largest vein in the body).
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 the breakdown of 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)
iii. Common bile duct (empties into the small intestine through the Sphincter of
Oddi)
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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 - a thin muscular sac snuggled in liver that stores bile produced by the liver
D. Pancreas – a gland nestled in the first curve of the small intestine that is responsible for making many
proenzymes used in digestion
Cells of the Pancreas
http://www.colorado.edu/kines/Class/IP
HY3430-200/image/pancreas.jpg
1. Acini (acinar cells) – cluster of secretory cells around a duct that carries enzymes to small intestine;
produce bicarbonate NaHCO3 (to neutralize HCL of stomach) and mucus; full of rough ER
(EXOCRINE)
2. Islets of Langerhans – area of the pancreas that contains cells that secrete insulin & glucagon to
control blood sugar
a. Glucagon is an important hormone produced by the pancreas. It is released when the
glucose level in the blood is low (hypoglycemia), causing the liver to convert stored
glycogen into glucose and release it into the bloodstream.
b. Insulin is a hormone with intensive effects on metabolism. It is released when the glucose
level in the blood is high. Insulin causes most of the body's cells to take up glucose from
the blood (including liver, muscle, and fat tissue cells), storing it as glycogen in the liver
and muscle, and stops use of fat as an energy source.
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3. Pancreatic Juice – proenzymes 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
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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. Haustra – (singular haustrum) of the colon are the small pouches caused by sacculation, which
give the colon its segmented appearance.
a. Haustral churning: haustra relax while filling up, then contract and move contents into next
haustra
4. Cecum – first part of large intestine which follows the ileocecal valve; can act as a storage area or
second stomach in some mammals
5. Appendix – attached to the posterior-medial surface of cecum
a. Does not have a function, but can become infected (appendicitis/appendectomy)
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6. Colon – divided into four sections
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
7. Rectum – last part of the sigmoid colon
a. the urge to defecate occurs when feces enter the rectum
b. if defecation does not occur, feces back into the sigmoid colon and the urge to defecate
subsides for a limited amount of time
8. Anal canal – last segment of large intestine; 3cm long; opens to exterior at anus
a. internal anal sphincter – involuntary smooth muscle
b. external anal sphincter – voluntary skeletal muscle
i. contains many blood vessels in folds of the anus (inflammation of these causes
hemorrhoids or piles
9. As water is removed by the large intestine chyme from small intestine becomes solid in 3-10 hours
a. When digested food moves too fast through the large intestine diarrhea results
b. Constipation can lead to very hard stools
10. Peristalsis much slower in large intestine
11. Mass peristalsis: starts in middle of transverse colon after a meal; triggered by contents in
stomach; strong peristaltic wave which leads to feces entering the rectum
12. Bacteria live in the large intestine in order to break down undigested food and produce vitamin K
and some B vitamins (each of these vitamins help in blood cell formation/function)
a. We have mutualistic symbiotic relationship with he bacteria in our digestive tract. We
provide them with nutrients and a place to live, and in return they help us digest food and
produce vitamins. A win-win for all.
b. If the bacteria within our digestive system escape and enter other parts of our body they
can be deadly (e. coli infections by oral/fecal route or burst appendix)
c. A common side effect of consuming antibiotics is diarrhea due to the inadvertent
destruction of the bacteria within the digestive tract
d. Following diarrhea feces can be different in texture, color, and smell until the bacteria
colony builds back up
13. Feces: undigested food (cellulose/fiber), bacteria and products of bacterial decomposition, viruses
and protists, decomposed bilirubin from the destruction of hemoglogin in red blood cells (gives
feces its color); inorganic salts, sloughed off cells of GI tract, and water