Exam 4 Study Guide
GASTROINTESTINAL PHYSIOLOGY
Anatomy of the GI system – NOT ON STUDY GUIDE
•Digestive system – enteric nervous system (extrinsic and intrinsic)
o
Autonomic (involuntary) and hormonal control [except chewing, swallowing, defecation]
▪Vagus nerve
o Ingested substances trigger hormones that stimulate or inhibit
▪Muscular contractions – GI motility; timely secretion of substances that aid in digestion
o Located near the areas that they innervate and control
• Mouth
o Reservoir for chewing and mixing of food with saliva
o Taste buds (chemoreceptors) and olfactory nerves are stimulated – initiates salivation and
secretion of gastric juices in the stomach
• Salivary glands
o Submandibular, sublingual, parotid (largest; secretes saliva)
o Saliva is composed of water with mucus, sodium, chloride, bicarbonate, potassium and salivary
a-amylase (carb digestion) and is controlled by ANS
▪Cholinergic parasympathetic fibers stimulate salivary glands
• Ex: anticholinergic meds inhibit salivation
▪Sympathetic beta-adrenergic stimulation decreases salivary secretion – none, little, or little
with rich protein content
▪A pH of 7.4 which neutralizes bacterial acids aiding to prevent injection
▪Contains IgA – prevents orally ingested microorganisms
• Esophagus
o Conducts substances from oropharynx to stomach
o Upper esophageal sphincter prevents entry of air into the esophagus during respiration
o Lower esophageal sphincter (cardiac sphincter) prevents regurgitation from the stomach
Increase tone by cholinergic vagal stimulation and the digestive hormone gastrin
, Decrease tone/relax by non-adrenergic, non-cholinergic vagal impulse and the hormones
progesterone, secretin, and glucagon
o Vagus nerve allows relaxation of esophagus during swallowing
o Swallowing: complex event mediated by the swallowing center in reticular formation of brainstem
Phases: oropharyngeal (voluntary) and esophageal (involuntary)
Must coordinate with respiratory center
• Respiration is inhibited – epiglottis slides downward to prevent food from entering
larynx and trachea
o Peristalsis:
Primary: immediately follows oropharyngeal phase of swallowing
Secondary: bolus of food becomes stuck – wave of contraction and relaxation occurs that is
independent of voluntary swallowing
• Stomach
o Hollow muscular organ – stores food, secretes digestive juices, mixes food with juices – propels
chyme into duodenum
o Muscle layers – outer (longitudinal), middle (circular), inner (oblique)
o Sphincters – lower esophageal (chyme enters from esophagus into stomach), pyloric (chyme exits
stomach into duodenum of small intestine
o Functional portions – upper (fundus), middle (body), lower (antrum)
o Blood supply via branch of celiac artery; drainage via splenic vein and tributaries
o Few substances absorbed in stomach – can absorb alcohol, aspirin, NSAIDS
o Innervated by sympathetic and parasympathetic
o Gastric motility
Swallowing causes fundus to relax
Gastrin and cholecystokinin enhance relaxation of stomach
Gastrin and motilin and low blood glucose enhance peristalsis
Sympathetic activity, secretin, cholecystokinin inhibit peristalsis
Vagus nerve (parasympathetic) stimulates gastric secretion and motility
Gastric mixing enhanced by retropulsion
Gastric emptying
Increased by larger volumes of food
Delayed by solids, fats, and non-isotonic solutions
- Fat – cholecystokinin – inhibits intake, gastric motility, decreases gastric emptying
so that fats do not exceed rate of bile and enzyme secretion
,Basics of gastric secretion; what cells secrete what
-Composition of gastric juice depends on volume and flow rate
-Potassium levels higher in gastric secretions than in plasma – important in vomiting, suctioning, etc.
-Rate of secretion varies with time of day – lower in AM, higher in PM
-Products: mucus (protective), HCl acid, enzymes (pepsinogen), hormones (gastrin), intrinsic factor
(vit B12), gastroferrin (iron absorption) - empty into gastric pits
o Gastric hydrochloric acid
▪Secreted by parietal cells of the stomach
▪Dissolves food fibers
▪Acts as bactericidal agent
▪Converts pepsinogen to pepsin
▪Stimulated by: acetylcholine via vagus nerve, caffeine, calcium, gastrin, histamine ▪Inhibited
by: prostaglandins (mucus), gastric inhibitory peptide, somatostatin, secretin
o Pepsin
▪Breaks down protein-forming polypeptides in the stomach
▪Strongest stimulation: acetylcholine via vagal stimulation
▪Inactivation: alkaline environment of duodenum – needs HCl to be converted from pepsinogen
o Mucus
Prostaglandins and nitric oxide: protect the mucosal barrier by stimulation of mucous and
bicarbonate to inhibit acids
Protect against aspirin, NSAIDS, H. Pylori, ethanol, regurgitated bile, and ischemia to prevent
inflammation and ulceration
o Intrinsic factor (IF)
Required for absorption of vitamin B12 in the ileum – combines with vit B12 in the stomach
Gastritis and failure of absorption mechanisms of vit B12 can lead to pernicious anemia
o Gastric glands: primary secretory units
▪Parietal cells: HCl acid and intrinsic factor, gastroferrin
▪Chief cells: pepsinogen – pepsin
▪G cells: gastrin (hormone)
▪Enterochromaffin-like cells: histamine
▪D cells: somatostatin
• Phases of gastric secretion:
o Celiac phase (mouth)
▪Begins with smelling, seeing, tasting, chewing, swallowing
▪Mediated by vagus nerve
▪Acid, pepsinogen, gastrin secreted
▪Insulin secreted by pancreas to hyperglycemia and is strong stimulus to gastric secretion
, o Gastric phase (stomach)
▪Begins with arrival of food to stomach
▪2 major stimuli to secretory effect – distension of stomach and presence of digested protein
Vagus nerve stimulated by distention – contributes to secretion stimulus
Mediated by acetylcholine and can be blocked by atropine
o Intestinal phase
▪Begins with movement of chyme from stomach to duodenum
▪Decelerated gastric secretion – when chyme enters alkaline environment of duodenum –
inhibitory vagal reflexes decrease gastric motility
▪Secretin and cholecystokinin stimulate pancreatic secretions and inhibit gastric secretions
Basic function of small intestine, and large intestine
• Small intestine
o Three segments:
-Duodenum: begins at pylorus and ends where it joins the jejunum called the Treitz
ligament
-Jejunum
-Ileum: ilecocecal valve controls the flow of digested material from the ileum into the large
intestine and prevents reflux into the small intestine
o Peritoneum: serous membrane surrounding the organs of the abdomen and pelvic cavity
▪Visceral: lies over the organs
▪Parietal: lines the wall of the abdominal cavity
▪Peritonitis: inflammation of the peritoneum – occurs with perforation of the intestine or after
surgery
▪Peritoneal Cavity: space between the two layers of the peritoneum
o Muscle layers: outer longitudinal and inner circular
▪Mucosal folds (plica): within the small intestine slow the passage of food providing more time for
digestion and absorption
• More numerous in the jejunum and upper ileum
o Blood flow provided by superior mesenteric artery
o Myenteric plexus and mucosal plexus: site of intrinsic motor innervation mediation
-Parasympathetic: secretion, motility, pain sensation, intestinal reflexes
-Sympathetic: inhibits motility and produces vasoconstriction
o Villi: cover the mucosal folds and are the functional units of the intestine
▪Where absorption occurs – villi and microvilli increase surface area for absorption
▪Secretes enzymes necessary for digestion and absorbs nutrients
▪Composed of absorptive columnar cells (enterocytes) and mucus secreting goblet cells
of mucosal epithelium – contain microvilli
▪Water and electrolytes absorbed through intracellular spaces called tight junctions
▪Entire epithelial population of cells is replaced every 4-7 days
GASTROINTESTINAL PHYSIOLOGY
Anatomy of the GI system – NOT ON STUDY GUIDE
•Digestive system – enteric nervous system (extrinsic and intrinsic)
o
Autonomic (involuntary) and hormonal control [except chewing, swallowing, defecation]
▪Vagus nerve
o Ingested substances trigger hormones that stimulate or inhibit
▪Muscular contractions – GI motility; timely secretion of substances that aid in digestion
o Located near the areas that they innervate and control
• Mouth
o Reservoir for chewing and mixing of food with saliva
o Taste buds (chemoreceptors) and olfactory nerves are stimulated – initiates salivation and
secretion of gastric juices in the stomach
• Salivary glands
o Submandibular, sublingual, parotid (largest; secretes saliva)
o Saliva is composed of water with mucus, sodium, chloride, bicarbonate, potassium and salivary
a-amylase (carb digestion) and is controlled by ANS
▪Cholinergic parasympathetic fibers stimulate salivary glands
• Ex: anticholinergic meds inhibit salivation
▪Sympathetic beta-adrenergic stimulation decreases salivary secretion – none, little, or little
with rich protein content
▪A pH of 7.4 which neutralizes bacterial acids aiding to prevent injection
▪Contains IgA – prevents orally ingested microorganisms
• Esophagus
o Conducts substances from oropharynx to stomach
o Upper esophageal sphincter prevents entry of air into the esophagus during respiration
o Lower esophageal sphincter (cardiac sphincter) prevents regurgitation from the stomach
Increase tone by cholinergic vagal stimulation and the digestive hormone gastrin
, Decrease tone/relax by non-adrenergic, non-cholinergic vagal impulse and the hormones
progesterone, secretin, and glucagon
o Vagus nerve allows relaxation of esophagus during swallowing
o Swallowing: complex event mediated by the swallowing center in reticular formation of brainstem
Phases: oropharyngeal (voluntary) and esophageal (involuntary)
Must coordinate with respiratory center
• Respiration is inhibited – epiglottis slides downward to prevent food from entering
larynx and trachea
o Peristalsis:
Primary: immediately follows oropharyngeal phase of swallowing
Secondary: bolus of food becomes stuck – wave of contraction and relaxation occurs that is
independent of voluntary swallowing
• Stomach
o Hollow muscular organ – stores food, secretes digestive juices, mixes food with juices – propels
chyme into duodenum
o Muscle layers – outer (longitudinal), middle (circular), inner (oblique)
o Sphincters – lower esophageal (chyme enters from esophagus into stomach), pyloric (chyme exits
stomach into duodenum of small intestine
o Functional portions – upper (fundus), middle (body), lower (antrum)
o Blood supply via branch of celiac artery; drainage via splenic vein and tributaries
o Few substances absorbed in stomach – can absorb alcohol, aspirin, NSAIDS
o Innervated by sympathetic and parasympathetic
o Gastric motility
Swallowing causes fundus to relax
Gastrin and cholecystokinin enhance relaxation of stomach
Gastrin and motilin and low blood glucose enhance peristalsis
Sympathetic activity, secretin, cholecystokinin inhibit peristalsis
Vagus nerve (parasympathetic) stimulates gastric secretion and motility
Gastric mixing enhanced by retropulsion
Gastric emptying
Increased by larger volumes of food
Delayed by solids, fats, and non-isotonic solutions
- Fat – cholecystokinin – inhibits intake, gastric motility, decreases gastric emptying
so that fats do not exceed rate of bile and enzyme secretion
,Basics of gastric secretion; what cells secrete what
-Composition of gastric juice depends on volume and flow rate
-Potassium levels higher in gastric secretions than in plasma – important in vomiting, suctioning, etc.
-Rate of secretion varies with time of day – lower in AM, higher in PM
-Products: mucus (protective), HCl acid, enzymes (pepsinogen), hormones (gastrin), intrinsic factor
(vit B12), gastroferrin (iron absorption) - empty into gastric pits
o Gastric hydrochloric acid
▪Secreted by parietal cells of the stomach
▪Dissolves food fibers
▪Acts as bactericidal agent
▪Converts pepsinogen to pepsin
▪Stimulated by: acetylcholine via vagus nerve, caffeine, calcium, gastrin, histamine ▪Inhibited
by: prostaglandins (mucus), gastric inhibitory peptide, somatostatin, secretin
o Pepsin
▪Breaks down protein-forming polypeptides in the stomach
▪Strongest stimulation: acetylcholine via vagal stimulation
▪Inactivation: alkaline environment of duodenum – needs HCl to be converted from pepsinogen
o Mucus
Prostaglandins and nitric oxide: protect the mucosal barrier by stimulation of mucous and
bicarbonate to inhibit acids
Protect against aspirin, NSAIDS, H. Pylori, ethanol, regurgitated bile, and ischemia to prevent
inflammation and ulceration
o Intrinsic factor (IF)
Required for absorption of vitamin B12 in the ileum – combines with vit B12 in the stomach
Gastritis and failure of absorption mechanisms of vit B12 can lead to pernicious anemia
o Gastric glands: primary secretory units
▪Parietal cells: HCl acid and intrinsic factor, gastroferrin
▪Chief cells: pepsinogen – pepsin
▪G cells: gastrin (hormone)
▪Enterochromaffin-like cells: histamine
▪D cells: somatostatin
• Phases of gastric secretion:
o Celiac phase (mouth)
▪Begins with smelling, seeing, tasting, chewing, swallowing
▪Mediated by vagus nerve
▪Acid, pepsinogen, gastrin secreted
▪Insulin secreted by pancreas to hyperglycemia and is strong stimulus to gastric secretion
, o Gastric phase (stomach)
▪Begins with arrival of food to stomach
▪2 major stimuli to secretory effect – distension of stomach and presence of digested protein
Vagus nerve stimulated by distention – contributes to secretion stimulus
Mediated by acetylcholine and can be blocked by atropine
o Intestinal phase
▪Begins with movement of chyme from stomach to duodenum
▪Decelerated gastric secretion – when chyme enters alkaline environment of duodenum –
inhibitory vagal reflexes decrease gastric motility
▪Secretin and cholecystokinin stimulate pancreatic secretions and inhibit gastric secretions
Basic function of small intestine, and large intestine
• Small intestine
o Three segments:
-Duodenum: begins at pylorus and ends where it joins the jejunum called the Treitz
ligament
-Jejunum
-Ileum: ilecocecal valve controls the flow of digested material from the ileum into the large
intestine and prevents reflux into the small intestine
o Peritoneum: serous membrane surrounding the organs of the abdomen and pelvic cavity
▪Visceral: lies over the organs
▪Parietal: lines the wall of the abdominal cavity
▪Peritonitis: inflammation of the peritoneum – occurs with perforation of the intestine or after
surgery
▪Peritoneal Cavity: space between the two layers of the peritoneum
o Muscle layers: outer longitudinal and inner circular
▪Mucosal folds (plica): within the small intestine slow the passage of food providing more time for
digestion and absorption
• More numerous in the jejunum and upper ileum
o Blood flow provided by superior mesenteric artery
o Myenteric plexus and mucosal plexus: site of intrinsic motor innervation mediation
-Parasympathetic: secretion, motility, pain sensation, intestinal reflexes
-Sympathetic: inhibits motility and produces vasoconstriction
o Villi: cover the mucosal folds and are the functional units of the intestine
▪Where absorption occurs – villi and microvilli increase surface area for absorption
▪Secretes enzymes necessary for digestion and absorbs nutrients
▪Composed of absorptive columnar cells (enterocytes) and mucus secreting goblet cells
of mucosal epithelium – contain microvilli
▪Water and electrolytes absorbed through intracellular spaces called tight junctions
▪Entire epithelial population of cells is replaced every 4-7 days
