Digestive System Flashcards
What are the 5 stages of digestion?
– Ingestion: selective intake of food
– Digestion: mechanical and chemical breakdown of food
into a form usable by the body
– Absorption: uptake of nutrient molecules into the
epithelial cells of the digestive tract and then into the blood
and lymph
– Compaction: absorbing water and consolidating the
indigestible residue into feces
– Defecation: elimination of feces
What is mechanical digestion and what are some examples?
—the physical breakdown of
food into smaller particles
– Cutting and grinding action of the teeth
– Churning action of stomach and small intestines
– Exposes more food surface to digestive enzymes
What is chemical digestion and what are some examples?
—a series of hydrolysis reactions
that breaks dietary macromolecules into their
monomers (residues)
– Carried out by digestive enzymes produced by salivary
glands, stomach, pancreas, and small intestine
– Results
* Polysaccharides into monosaccharides
* Proteins into amino acids
* Fats into monoglycerides and fatty acids
* Nucleic acids into nucleotides
What nutrients are present in a usable form?
– Vitamins, amino acids, minerals, cholesterol, and water
What organs make up the digestive tract? GI tract?
– Mouth, pharynx,
esophagus, stomach, small
intestine, and large intestine
– Gastrointestinal (GI) tract
is the stomach and
intestines
What are the accessory organs?
– Teeth, tongue, salivary
glands, liver, gallbladder,
and pancreas
What is the enteric nervous system?
—nervous network in
esophagus, stomach, and intestines that regulates
digestive tract motility, secretion, and blood flow
What is the mesentary?
—connective tissue sheets that
suspend stomach and intestines from abdominal wall
What is the parietal peritoneum?
—a serous membrane that
lines the wall of the abdominal cavity
What is the dorsal mesentary?
a translucent two-layered
membrane extending to the digestive tract
What are the lesser and greater omentum?
- Lesser omentum—a ventral mesentery that
extends from the lesser curvature of the stomach
to the liver - Greater omentum—hangs from the greater
curvature of the stomach (its left inferior margin)
What is the mesocolon?
—extension of the mesentery that anchors
the colon to the abdominal wall
What is intraperitoneal? Retro?
- Intraperitoneal—when an organ is enclosed by
mesentery on both sides
– Considered within the peritoneal cavity
– Stomach, liver, and parts of small and large intestine - Retroperitoneal—when an organ lies against the
posterior body wall and is covered by peritoneum on its
anterior side only
– Considered to be outside the peritoneal cavity
– Duodenum, pancreas, and parts of the large intestine
What are the functions of the mouth?
– Ingestion (food intake)
– Taste and other sensory responses to food
– Chewing and chemical digestion
– Swallowing, speech, and respiration
What are the different types of teeth
8 incisors, 4 canines, 8 premolars, 12 molars
What is the function of saliva?
– Moistens mouth
– Begins starch and fat digestion
– Cleanses teeth
– Inhibits bacterial growth
– Dissolves molecules so they can stimulate the taste
buds
– Moistens food and binds it together into bolus to aid in
swallowing
What are the solutes in saliva?
– Salivary amylase: enzyme that begins starch digestion in
the mouth
– Lingual lipase: enzyme that is activated by stomach acid
and digests fat after food is swallowed
– Mucus: binds and lubricates a mass of food and aids in
swallowing
– Lysozyme: enzyme that kills bacteria
– Immunoglobulin A (IgA): an antibody that inhibits
bacterial growth
– Electrolytes: Na+, K+, Cl−, phosphate, and bicarbonate
What are the intrinsic salivary glands?
– Lingual glands: in the tongue; produce lingual lipase
– Labial glands: inside of the lips
– Palatine glands: roof of mouth
– Buccal glands: inside of the cheek
What are the extrinsic salivary glands?
– Parotid: located beneath the skin anterior to the earlobe
* Mumps is an inflammation and swelling of the parotid
gland caused by a virus
– Submandibular gland: located halfway along the body
of the mandible
* Its duct empties at the side of the lingual frenulum, near
the lower central incisors
– Sublingual gland: located in the floor of the mouth
What is a bolus?
—mass swallowed as a result of saliva
binding food particles into a soft, slippery, easily
swallowed mass
What are the pharyngeal constrictors?
– Has superficial layer of circular skeletal muscles that form
pharyngeal constrictors (superior, middle, and inferior)
that force food downward during swallowing
* When not swallowing, the inferior constrictor (upper
esophageal shincter) remains contracted to exclude air from the
esophagus
* Disappears at the time of death when the muscles relax, so it is a
physiological sphincter, not an anatomical structure
What are the features of the esophagus?
– Extends from pharynx to cardiac orifice of stomach
passing through esophageal hiatus in diaphragm
– Lower esophageal sphincter: food pauses here because
of constriction
* Prevents stomach contents from regurgitating into the
esophagus
* Protects esophageal mucosa from erosive stomach acid
* Heartburn—burning sensation produced by acid reflux into
the esophagus
What is the capacity of the stomach?
50 ml when empty, 1 to 1.5 l after meal, 4 l when very full
What is the main function of the stomach?
to mechanically digest and liquify, begin digestion of protein and fat
What are the regions of the stomach?
- Cardiac region (cardia)—small area within about 3 cm of
the cardiac orifice - Fundic region (fundus)—dome-shaped portion superior to
esophageal attachment - Body (corpus)—makes up the greatest part of stomach
- Pyloric region—narrower pouch at the inferior end
– Subdivided into the funnel-like antrum
– Narrower pyloric canal that terminates at pylorus
– Pylorus: narrow passage to duodenum
– Pyloric (gastroduodenal) sphincter—regulates the
passage of chyme into the duodenum
What nervous signals does the stomach recieve?
– Parasympathetic fibers from vagus
– Sympathetic fibers from celiac ganglia
What is the microscopic anatomy of the stomach?
- Stomach has a simple columnar epithelium
covers mucosa
– Apical regions of its surface cells are filled with mucin
– Mucin swells with water and becomes mucus after it is
secreted - Mucosa and submucosa are flat when stomach is
full, but form longitudinal wrinkles called gastric
rugae when empty - Muscularis externa has three layers (instead of
the two seen elsewhere)
– Outer longitudinal, middle circular, and inner oblique
layers
What are gastric pits? What glands are found there?
—depressions in gastric mucosa
* Cardiac glands in cardiac region
* Pyloric glands in pyloric regions
* Gastric glands in the rest of the stomach
What are the cells in the stomach?
mucous, regenerative, parietal, chief, enteroendocrine
What is gastric juice?
water, HCl, pepsin
What does HCl do?
- HCl activates pepsin and lingual lipase
- Breaks up connective tissues and plant cell walls
– Helps liquefy food to form chyme - Converts ingested ferric ions (Fe3+) to ferrous
ions (Fe2+)
– Fe2+ absorbed and used for hemoglobin synthesis - Contributes to nonspecific disease resistance
by destroying most ingested pathogens
What is the only necessary function of the stomach?
secretion of intrinsic factor to make B12 to synthesize Hb
What influences gastric motility?
- Swallowing center of medulla oblongata
signals stomach to relax - Soon stomach shows a rhythm of peristaltic
contractions controlled by pacemaker cells in
longitudinal layer of muscularis extern
Where does absorption occur?
small intestine
How is the stomach protected?
– Mucous coat: thick, highly alkaline mucus resists action
of acid and enzymes
– Tight junctions between epithelial cells prevent gastric
juice from seeping between them and digesting deeper
tissue
– Epithelial cell replacement: cells live only 3 to 6 days
* Sloughed off into the chyme and digested with food
* Replaced rapidly by cell division in gastric pits
What are most ulcers caused by?
H pylori
What does the liver do?
secretes bile to contribute to digestion
What are the 4 lobes of the liver
- Four lobes—right, left, quadrate, and caudate
– Falciform ligament separates left and right lobes - Sheet of mesentery that suspends the liver from the
diaphragm
– Round ligament (ligamentum teres)—fibrous remnant
of umbilical vein - Carries blood from umbilical cord to liver of the fetus
What is the porta hepatis?
—irregular opening between
quadrate and caudate lobes
– Point of entry for hepatic portal vein and proper
hepatic artery
– Point of exit for the bile passages
– All travel in lesser omentumWha
What is the hepatic triad?
two vessels
and a bile ductule
– Other vessel: branch of hepatic portal vein
– One vessel: branch of hepatic artery proper
– Both vessels supply blood to sinusoids which receive a
mixture of nutrient-laden venous blood from the
intestines, and freshly oxygenated arterial blood from
the celiac trunk
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What are hepatocytes?
– After a meal, hepatocytes absorb from the blood:
glucose, amino acids, iron, vitamins, and other nutrients
for metabolism or storage
– Between meals, hepatocytes break down stored
glycogen and release glucose into the blood
– Remove and degrade: hormones, toxins, bile pigments,
and drugs
– Secrete into the blood: albumin, lipoproteins, clotting
factors, angiotensinogen, and other products
What are bile canaliculi?
- Bile canaliculi—narrow channels into which the
liver secretes bile
– Bile passes into bile ductules of the triads
– Ultimately into the right and left hepatic ducts
– Common hepatic duct: formed from convergence of
right and left hepatic ducts on inferior side of the liver
– Cystic duct coming from gallbladder joins common
hepatic duct
– Bile duct: formed from union of cystic and common
hepatic ducts - Descends through lesser omentum toward the duodenum
What happens where the bile duct joins the pancreas?
Microscopic Anatomy of the Liver
(Continued)
– Near duodenum, bile duct joins duct of pancreas
– Forms expanded chamber: hepatopancreatic ampulla
* Terminates in a fold of tissue—major duodenal papilla on
duodenal wall
– Major duodenal papilla contains muscular
hepatopancreatic sphincter (sphincter of Oddi)
* Regulates passage of bile and pancreatic juice into
duodenum
* Between meals, sphincter closes and prevents release of
bile into the intestines
What is the gallbladder?
—a pear-shaped sac on underside of
liver
– Serves to store and concentrate bile by absorbing
water and electrolytes
– About 10 cm long
– Internally lined by highly folded mucosa with simple
columnar epithelium
– Head (fundus) usually projects slightly beyond inferior
margin of liver
– Neck (cervix) leads into the cystic duct
What is bile?
—yellow-green fluid containing minerals,
cholesterol, neutral fats, phospholipids, bile
pigments, and bile acids
– Bilirubin: principal pigment derived from the
decomposition of hemoglobin
– Bacteria in large intestine metabolize bilirubin to
urobilinogen
* Responsible for the brown color of feces
– Bile acids (bile salts): steroids synthesized from
cholesterol
* Bile acids and lecithin, a phospholipid, aid in fat digestion and
absorption
How much bile is reabsorbed?
– 80% of bile acids are reabsorbed in the ileum and
returned to the liver
* Hepatocytes absorb and resecrete them
* Enterohepatic circulation—route of secretion, reabsorption,
and resecretion of bile acids two or more times during digestion
of an average meal
– 20% of the bile acids are excreted in the feces
* Body’s only way of eliminating excess cholesterol
* Liver synthesizes new bile acids from cholesterol to replace
those lost in feces
What are the 2 portions of the pancreas?
- Endocrine portion—pancreatic islets that secrete insulin
and glucagon
– Concentrated in the tail of the gland - Exocrine portion—99% of pancreas that secretes 1,200 to
1,500 mL of pancreatic juice per day
– Secretory acini release their secretion into small ducts that
converge on the main pancreatic duct
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What are the ducts of the pancreas?
– Pancreatic duct runs lengthwise through middle of the
gland
* Joins the bile duct at the hepatopancreatic ampulla
* Hepatopancreatic sphincter controls release of both bile
and pancreatic juice into the duodenum
– Accessory pancreatic duct: smaller duct that
branches from the main pancreatic duct
* Opens independently into the duodenum
* Bypasses the sphincter and allows pancreatic juice to
be released into duodenum even when bile is not
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What is pancreatic juice?
– Pancreatic juice: alkaline mixture of water, enzymes,
zymogens, sodium bicarbonate, and other electrolytes
* Acini secrete the enzymes and zymogens
* Ducts secrete bicarbonate
– Bicarbonate buffers HCl arriving from the stomach
What are the pancreatic zymogens?
– Trypsinogen
* Secreted into intestinal lumen
* Converted to trypsin by enterokinase that is secreted by
mucosa of small intestine
* Trypsin is autocatalytic—converts trypsinogen into still
more trypsin
– Chymotrypsinogen: converted to chymotrypsin by
trypsin
– Procarboxypeptidase: converted to carboxypeptidase
by trypsin
What are the other pancreatic enzymes?
– Pancreatic amylase: digests starch
– Pancreatic lipase: digests fat
– Ribonuclease and deoxyribonuclease: digest RNA
and DNA respectively
What are the 3 stimuli responsible for the release of pancreatic juice and bile?
Ach, cholesystokinin, and secretin
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– Acetylcholine (ACh): from vagus and enteric nerves
* Stimulates acini to secrete enzymes during cephalic phase
of gastric control even before food is swallowed
– Enzymes remain in acini and ducts until chyme enters the
duodenum
– Cholecystokinin (CCK): secreted by mucosa of
duodenum in response to arrival of fats in small intestine
* Stimulates pancreatic acini to secrete enzymes
* Strongly stimulates gallbladder
* Induces contractions of gallbladder and relaxation of
hepatopancreatic sphincter to discharge bile into duodenum
– Secretin: released from duodenum in response to acidic
chyme arriving from the stomach
* Stimulates ducts of both liver and pancreas to secrete more
sodium bicarbonate
* Raises pH to the level required for activity of the pancreatic
and intestinal digestive enzymes
What are the regions of the small intestine?
– Duodenum: first 25 cm (10 in.)
* Begins at pyloric valve
– Major and minor duodenal papilla distal to pyloric valve
– Receives major and minor pancreatic ducts respectively
* Arches around head of the pancreas
* Ends at a sharp bend called the duodenojejunal flexure
* Most is retroperitoneal
– Jejunum: first 40% of small intestine beyond duodenum
* Has large, tall, closely spaced circular folds
* Its wall is relatively thick and muscular
* Especially rich blood supply which gives it a red color
– Ileum: forms last 60% of the postduodenal small intestine
* Thinner, less muscular, less vascular, and paler pink color
* Peyer patches—prominent lymphatic nodules in clusters on
the side opposite the mesenteric attachment
– Visible to naked eye; become larger near large intestine
* Ileocecal junction—end of the small intestine
– Where the ileum joins the cecum of the large intestine
* Ileocecal valve—a sphincter formed by the thickened
muscularis of the ileum
– Protrudes into the cecum
– Both jejunum and ileum are intraperitoneal and covered
with serosa
What helps with small intestine absorption?
– Lumen lined with simple columnar epithelium
– Muscularis externa is noted for a thick inner circular layer
and a thinner outer longitudinal layer
– Large internal surface area - great length and three
types of internal folds or projections
* Circular folds (plicae circulares)—increase surface area
by a factor of 2 to 3
* Villi—increase surface area by a factor of 10
* Microvilli—increase the surface area by a factor of 20
* Circular folds (plicae circulares)—largest folds of
intestinal wall
What are the features of villi?
– Villus covered with two types of epithelial cells
* Absorptive cells (enterocytes)
* Goblet cells—secrete mucus
– Epithelia joined by tight junctions that prevent digestive
enzymes from seeping between them
– Core of villus filled with areolar tissue of lamina propria
* Contains arteriole, capillaries, venule, and lymphatic
capillary called a lacteal
What are the features of microvilli?
—form a fuzzy brush border on apical
surface of each absorptive cell
– About 1 μm high
– Increases absorptive surface area
* Brush border enzymes—contained in plasma
membrane of microvilli
– Carry out some of the final stages of enzymatic digestion
– Not released into the lumen
– Contact digestion: chyme must contact the brush border
for digestion to occur
– Intestinal churning of chyme ensures contact with the
mucosa
What do the contractions of small intestine do?
– To mix chyme with intestinal juice, bile, and
pancreatic juice
* To neutralize acid
* Digest nutrients more effectively
– To churn chyme and bring it in contact with the
mucosa for contact digestion and nutrient absorption
– To move residue toward large intestine
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What does intestinal motility start with?
segmentation set by pacemaker cells
What is the purpose of segmentation
mix and churn
What does peristalsis do?
- Peristalsis moves contents of small intestine toward
colon - Peristaltic wave begins in duodenum, travels 10 to 70 cm
and dies out
What does the ileocecal valve do?
usually closed
– Food in stomach triggers gastroileal reflex that enhances
segmentation in the ileum and relaxes the valve
– As cecum fills with residue, pressure pinches the valve shut
* Prevents reflux of cecal contents into the ileum
Walk through carbohydrate digestion
Mouth: starch gets broken up by salivary amylase into oligosaccharide and maltose, sucrose and lactose continue down
Stomach: none
Small intestine (lumen): pancreatic amylase creates more maltose
Small intestine (epithelium): sucrase and lactase break down sugars into fructose and galactose, maltase breaks down maltose into glucose
Goes into blood capillary
Walk through protein digestion
Mouth: none
Stomach: pepsin breaks down into peptides
Small intestine (lumen): trypsin, chymotrypsin, and carboxypeptidase create dipeptides
Small intestine (epithelium): dipeptidase and aminopeptidase create amino acids
goes into blood capilary
Walk through fat digestion
mouth: none
stomach: lingual lipase activated
small intestine (lumen): the lipases create monoglycerides and free fatty acids
small intestine (epithelium): monoglyerice + ffa = triglyceride, + cholesterol etc lead to chylomicrons
goes into lacteal
what digests nucleic acids?
nucleases, nucleosidases, and phosphatases
What happens to vitamins?
absorb most
Where is most water absorbed?
small intestine
What makes diarrhea happen?
too little water absorbed
What makes constipation happen?
too much water absorbed
What is the large intestine anatomy?
– Begins as cecum inferior to ileocecal valve
– Appendix attached to lower end of cecum
* Densely populated with lymphocytes—a significant source
of immune cells
– Ascending colon, right colic (hepatic) flexure,
transverse colon, left colic (splenic) flexure, and
descending colon frame the small intestine
– Sigmoid colon is S-shaped portion leading down into
pelvic cavity
– Rectum: portion ending at anal canal
* Has three curves and three infoldings, called the
transverse rectal folds (rectal valves)
– Anal canal: final 3 cm of the large intestine
* Passes through levator ani muscle and pelvic floor,
terminates at the anus
* Anal columns and sinuses—exude mucus and
lubricant into anal canal during defecation
* Large hemorrhoidal veins for superficial plexus in
anal columns and around orifice
* Hemorrhoids—permanently distended veins that
protrude into anal canal or bulge outside the anus
– Muscularis externa of colon is unusual
* Taenia coli—longitudinal fibers concentrated in
three thickened, ribbon-like strips
* Haustra—pouches in the colon caused by the
muscle tone of the taeniae coli
* Internal anal sphincter—smooth muscle of
muscularis externa
* External anal sphincter—skeletal muscle of pelvic
diaphragm
– Omental appendages—club-like, fatty pouches of
peritoneum adhering to the colon; unknown function
Where is feces found?
transverse colon
What moves feces?
Haustral contractions, gastrocolic and duodenocolic reflexes
