Lecture 8 : Gastrointestinal System II : Stomach, Liver and Digestive Enzymes Flashcards
The stomach is located in the :
upper left quadrant of the abdominopelvic cavity
Greater omentum =
sheet of mesentery suspended from inferior border of stomach
Immune function
Lesser omentum =
sheet of mesentery between liver and stomach
The stomach is a J-shaped temporary “storage tank” that aids in ___________
chemical and mechanical digestion, propulsion of food
Can hold around 1 gallon of food
Rugae
longitudinal folds in wall that allow for expansion
Stomach anatomy
Thick muscularis externa with 3 layers
Rugae
Pyloric sphincter
Pyloric sphincter =
controls entry of chyme into small intestine
the stomach converts the bolus of food to ____ by mechanical (churning) and chemical (acid and enzymes) breakdown
chyme
4 Layered Wall:
(of stomach)
Mucosa = contains simple columnar epithelium (mucus cells) with gastric pits & gastric glands
Submucosa = contains submucosal plexus in connective tissue
Muscularis externa contains (3) smooth muscle layers:
oblique, circular, longitudinal
Serosa = covered by visceral peritoneum
4 Cell Types Contribute to Gastric Juice:
(found in the surface epithelium and “neck” of gastric gland)
Mucous cells
Parietal cells
Chief cells
Enteroendocrine cells
Mucous cells
Secretion of mucous & Bicarbonate (HCO3-)
protective barrier
Parietal cells
Secretion of intrinsic factor
> Allows Vitamin B12 absorption in small intestine (required for erythropoiesis)
Produce hydrochloric acid (HCl): secrete H+ and Cl- to create acidic environment: pH ~2
>Denatures proteins, breaks down cell walls of plant foods
> Kill bacteria & microorganisms
> Activation of pepsinogen to pepsin
Chief cells
Major function: Secrete pepsinogen – inactive form of protease pepsin (digests proteins)
Minor function: Secrete gastric lipase (digest fats)
Enteroendocrine cells
(“gut endocrine”) secrete hormones into blood
Sense food in the lumen
Secrete chemical messengers into interstitial fluid where they can act as paracrine (short distance) or hormone signals
G cells secrete ____ into bloodstream; stimulate HCl production and motility
Others secrete ______
gastrin
histamine, serotonin
Parietal cells secrete________ions separately, which combine in the lumen to form hydrochloric acid
H+ and Cl-
____ and ____ stimulate parietal cells to produce and release acid
Gastrin
histamine
The Mucous-Bicarbonate Barrier protects the stomach from ___
itself
_______ secrete mucus & bicarbonate (HCO3-)
Mucous cells
Three (3) Factors that create the barrier:
Thick coating of bicarbonate rich mucous from the mucous cells
Epithelial cells joined by tight junctions
High turnover of epithelial (mucous) cells
________ can disrupt mucosal barrier and be absorbed in stomach
Alcohol and aspirin
Disruption of the mucous barrier leads to ______
gastric ulcers
Gastric ulcer
can damage organ wall and lead to perforation, peritonitis, hemorrhage
exacerbated by acidic conditions & stress, NSAIDs
most ulcers caused by H. pylori bacteria
Helicobacter pylori
~ 50% of us have it, but causes ulcers in only ~10-20% of people who have it
drill into mucus layer
Neuronal & hormonal mechanisms regulate gastric activity in 3 phases
cephalic phase
gastric phase
intestinal phase
Gastric (stomach) activity includes _________
gastric secretions and gastric motility
cephalic phase
1a) sight and thought of food
1b) stimulation of taste and smell receptors
takes place in cerebral cortex -> hypothalamus and medulla oblongata -> vagus nerve -> stomach
gastric phase
2a) stomach distention activates stretch receptors -> long and short reflexes -> stomach
2b) food chemicals and rising pH activate chemoreceptors -> G cells -> Gastrin release to blood -> stomach
intestinal phase
3a) presence of partially digested foods in duodenum or distension of the the duodenum when stomach begins to empty -> intestinal gastrin release to blood -> brief effect -> stomach
______ includes propulsion and mechanical breakdown
Gastric motility
Filling of stomach:
relaxation mediated by vagus nerve and stress-relaxation response of smooth muscle
__________ (interstitial cells of Cajal) set a constant (~3/min) rate of stomach contraction
Stomach enteric pacemaker cells
Factors that enhance gastric secretions also enhance_______ (i.e. stretch of stomach wall and presence of food activate long and short reflexes & gastric secretions)
strength of contractions
Fatty, hypertonic, acidic chyme in duodenum ______ and slow emptying
decrease strength of contraction
Slow entry of chyme into small intestine is controlled by _______; prevents SI from being overwhelmed.
enterogastric reflex and GI hormones
propulsion =
Peristaltic waves move from the fundus toward the pylorus
grinding =
the most vigorous peristalsis and mixing action occur close to the pylorus
the pyloric end of the stomach acts as a pump that delivers small amounts of chyme into the duodenum
retropulsion =
the peristaltic wave closes the pyloric valve, forcing most of the contents of the pylorous backward into the stomach
The stomach delivers the chyme to the ____
small intestine
The _____ is where the most of digestion and absorption occurs
small intestine
Some digestion of proteins & breakdown of the food occurs in the ____, but the majority of digestion continues in the first part of the small intestine.
stomach
Most of the substances required for digestion come from accessory organs:
liver, gall bladder, pancreas
Liver =
Upper right quadrant
4 anatomical lobes
Gall bladder attached to posterior inferior portion
Porta hepatis =
“Door” of the liver
Important structures enter and leave the liver here
Liver
Primary digestive function =
produces bile to emulsify fats
The liver processes nutrient-rich venous blood from digestive organs (via hepatic portal vein):
helps maintain blood glucose homeostasis
involved in fat metabolism, storage and transport of lipids
synthesizes cholesterol and lipoproteins
stores some vitamins
produces plasma proteins
The liver cleans and Detoxifies blood:
converts ammonia to urea
metabolizes alcohol, drugs, medications, hormones
processes bilirubin
secretes bile pigments
Liver lobule =
Structural and functional unit
Hepatocytes
(liver cells)
Produce and secrete bile
Process nutrients (store glucose, use amino acids to make plasma proteins)
Store fat soluble vitamins
Detoxification of blood
Portal triad : 3 structures
Branch of hepatic artery (oxygen-rich arterial blood)
Branch of (hepatic) portal vein (oxygen-low, nutrient-rich blood)
Bile duct
Sinusoids =
“leaky” capillaries between rows of hepatocytes; contain macrophages
_____ carries blood away from lobule toward inferior vena cava
Central vein
_________ bring blood to the liver
Portal vein and Hepatic artery
Blood from digestive organs of the abdominal cavity (especially intestines) drains into ______
portal venous system
Portal vein brings ______ to liver for processing by hepatocytes
nutrient-rich, (but low oxygen) venous blood
Blood leaving liver through hepatic vein joins ______ feeding toward heart
inferior vena cava
Hepatocyte activity
1) Arterial and portal blood in sinusoids
2) Hepatocytes extract nutrients
Arterial and portal blood in sinusoids =
Arterial blood brings oxygen
Portal venous blood brings newly absorbed nutrients from intestines
Hepatocytes extract nutrients =
Store glucose as glycogen
Build amino acids into proteins and add back into sinusoids
Produce bile and secrete into bile canaliculi which flows into bile duct
Bile is produced in the liver and stored in the ____
gallbladder
Bile produced by liver hepatocytes:
Green, alkaline solution
Contains :
Bile salts (derived from cholesterol)
Bilirubin (waste product of heme breakdown from red blood cells)
Phospholipids
Cholesterol
Bile flows out of liver though ___
bile duct toward small intestine
Excess bile backs up in bile duct and through cystic duct into gall bladder =
Bile is released under influence of hormones from enteroendocrine cells in the small intestine
Gallbladder stores and _____
concentrates bile (removes water)
In small intestine, bile will be used to emulsify fats =
Aids in digestion and absorption of fats in small intestine
Digestive enzymes and bicarbonate are produced by the _____ portion of the pancreas
exocrine
Exocrine pancreas:
Pancreatic acini and ducts form exocrine portion of pancreas and secrete digestive enzymes and pancreatic juice
Endocrine pancreas:
Pancreatic islets secrete hormones (insulin & glucagon) that enter the blood and regulate blood glucose
The pancreas produces _____
pancreatic juice
= digestive enzymes and bicarbonate
Acini
clusters of secretory acinar cells
Acini secretes digestive enzymes into ducts:
Proteases (digest proteins)
Amylase (digest starch)
Lipases (digest fats)
Nucleases (digest nucleic acids)
Most digestive enzymes are released in their inactive forms _____ and will be activated in small intestine
(zymogens)
Ducts =
Provide transport for acinar cell secretions
Duct cells secrete bicarbonate ion buffer and water to neutralize acidic chyme
Bile and Pancreatic juice empty into the ____ via a common duct system
duodenum
Control of ______, _______and ______ by CCK and secretin hormones released from small intestine enteroendocrine cells
gall bladder contraction
digestive enzyme release
hepatopancreatic sphincter
The small intestine secretes the digestive hormones _____
CCK & Secretin
Small intestine enteroendocrine cells secrete hormones into _____
portal blood
Cholecystokinin (CCK)=
release is stimulated byproteins and fats in chyme
Secretin =
release isstimulated by acidic chyme
CCK & Secretin effects on the Pancreas =
Cholecystokinin (CCK) causes acinar cells to secrete enzymes into pancreatic ducts
Secretin causes pancreatic duct cells to secrete bicarbonate buffer into duct
CCK effects on the Gall Bladder =
Cholecystokinin (CCK) causes contraction of smooth muscle in gall bladder wall
Secretin =
is a weak stimulus for bile production and release from liver
Secretin effects on bile production by the Liver =
Recycled bile salts in the portal blood strongly stimulate bile production and secretion
CCK effects on the Hepatopancreatic sphincter =
causes relation of the smooth muscle in the sphincter, allowing bile and pancreatic juice to enter the small intestine
____ form from precipitates of bile solutes as the gallbladder concentrates bile
Gallstones
Gallstones symptoms:
pain in upper right or epigastric quadrants, or referred to shoulder
Problematic when caught in cystic duct or common bile duct = can block opening of pancreatic duct and cause pancreatitis
Inactive pancreatic digestive enzymes (zymogens) are activated in the _____
small intestine
___ is an enzyme attached to the apical surface of small intestine epithelial cells (brush border enzyme)
Enteropeptidase
Enteropeptidase activates trypsinogen to form ____
trypsin
Trypsin can then activate the other zymogens
Chyme mixes with bile, activated enzymes and buffer in the ____
duodenum
