BHS 116.2- Phyisology II
Notetaker: Stephanie Cullen
Date: 1/16/13
Page: 1
Objective: Describe the functions of the stomach.
Function of the Stomach
- Storage of large quantities of food until they can be processed by
the intestines
o MAIN FUNCTION
o Not digestion!
- When food enters the stomach, a vago-vagal reflex reduces the tone
of the muscular stomach wall
o Efferent and afferent vagal fibers that trigger relaxation so
the stomach can accept the food
o Receptive relaxation
o Max volume: 1-1.5 L
Motor Function of the Stomach
- Mixing the food w/ gastric secretions until it forms chyme
o Need to break it down to an extent until it is liquid
- When the stomach contains food, slow waves initiate weak peristaltic constrictor waves “mixing
waves”
- As these waves progress from the body of the stomach towards the antrum, they become more
intense, providing powerful action potential-driven constrictor rings mixing the stomach content
and pushing it toward the pyloric sphincter (separates the stomach from the SI)
o Upper part: fundus
o Bulk: body
o Last part w/ thicker muscular layer: antrum
- At the pyloric sphincter, most of the chyme is propelled back into the mixing waves since it is
closed (tonically constricted)
o Food is pushed up then washed back to get the mixing
- Emptying of the stomach is promoted by strong peristaltic contractions in the stomach antrum
toward the pyloric sphincter
o Only a small amount leaks out into the small intestine each time
 Once it gets into a liquid state
o Allows for a slow emptying of chyme into the small intestine at an appropriate rate for
digestion and absorption
 Allows that little bit to be processed and assimilated before the next batch comes
through
 Wouldn’t be able to absorb a good portion of the
nutrients w/o this process
- The pyloric pump
o Contractions 6x stronger in the anturm than mixing
contractions
o Forces several (5-10) ml of chyme through pyloric
sphincter into duodenum
VIDEO
Objective: Describe the regulation of stomach emptying (stomach, duodenal, and nervous factors).
Regulation of Stomach Emptying
- Primary factor stimulating stomach emptying
o Increased food volume
 As the food volume increases, the need to empty the stomach increases
BHS 116.2- Phyisology II
Notetaker: Stephanie Cullen
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Date: 1/16/13
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Other factors
 Fluidity of the chyme is also an influence
 More liquefied chyme = easier to empty the stomach contents
 Hormonal control: The antral mucosal cells of the stomach secrete gastrin which
enhances the activity of the pyloric pump
 Increases strength of pyloric/antral contractions
Primary factor inhibiting stomach emptying
o Food entering the duodenum
 Triggers multiple nervous reflexes to trigger inhibition of movement of more
food
 Enteric nervous system (short): Enterogastric Reflex
o Entero = small intestine
o Gastric = stomach
o Reflex from the small intestine to the stomach to inhibit stomach
motility
 Sympathetic nervous system (long)
o Starts w/ afferent in the SI
o Goes to sympathetic chain
o Goes to stomach
Main duodenal factors are fat (most important), acid (pH), hypertonicity (salt), and distension
(stretching of the duodenum)
o As fats enter the duodenum, they bind receptors on the epithelial cells
o In turn, the epithelial cells secrete hormones into the blood which travel to the stomach
 Inhibit the pyloric pump
 Strengthen the pyloric sphincter
Cholecystokinin (CCK)
o Secreted by the mucosa of the duodenum and jejunum in response to FAs in the chyme
o Blocks the effects of gastrin on stomach motility
 Inhibits gastric motility
Secretin
o Secreted by the mucosa of the duodenum in response to gastric acid release from the
stomach
o Inhibits gastric motility
Gastric inhibitory peptide (GIP)
o Secreted from the upper small intestine primarily in response to FAs in the chyme
o Stimulates insulin secretion
 Primary effect
o Can inhibit gastric motility: More prominent function if the other hormones fail
W/in the stomach: volume of chyme is more important
Once chyme enters the SI, geared toward inhibiting gastric motility
BHS 116.2- Phyisology II
Notetaker: Stephanie Cullen
Date: 1/16/13
Page: 3
Objective: Describe movements through the SI and colon and the various controls (hormones and
reflexes).
Movements of the Small Intestine
- Mixing contractions (segmentation contractions)
o When chyme distends the small intestine, the wall is stretched
 Elicits concentric contractions spaced at intervals lasting a fraction of a minute
 Mix w/ enzymes to breakdown contents of chyme into nutrients
o Divide intestine into a “chain of sausages” Max frequency of these contractions is
determined by the frequency of the slow waves
o Help propel chyme through the small intestine
 To a smaller degree since the main function is mixing
 Net movement of chyme is very slow (1cm/min)
o Usually takes 3-5 hours for chyme to reach the ileocecal valve
(junction of the small intestine and large intestine) which is through
the entire SI
- Propulsive contractions
o Chyme is primarily propelled through the small intestine by peristaltic waves
 Migrating motility complex (occurs between meals)
 After stomach has processed chyme and it has been released into SI
 These waves can occur anywhere in the SI
 Weak and usually die out in 3-5 cm
 Pushes food a little further
 Get contractions for a little while then its pushed along again by another
migrating motility complex
- Peristalsis is greatly increased after a meal due to neuronal and hormonal control
o Gastroenteric reflex is responsible for this effect
 Initiated by distension of stomach and triggers the SI
 Speeds along food into the SI because more food has just entered the
stomach
 Conducted primarily by myenteric plexus of the enteric nervous system
o Hormonal control
 Gastrin, CCK, insulin, and serotonin all enhance SI motility
 Gastrin = stomach hormone to move food into SI
 CCK = released when fat enters duodenum to further trigger movement
 Insulin = stimulates motility of SI b/c the faster its moving, the less being
absorbed into the blood
o Know there is enough glucose in the blood when insulin is
present
o Insulin wants to move the food along so no more is absorbed
 Secretin and glucagon inhibit SI motility
 Secretin = released in response to acid
o Need to neutralize the acid (using HCO3-) as to not damage the
epithelium of the GI tract w/ the low pH chyme
 Glucagon = released w/ low blood sugar
o Slows down movement to absorb more glucose into the blood
BHS 116.2- Phyisology II
Notetaker: Stephanie Cullen
Date: 1/16/13
Page: 4
Ileocecal Junction
- Ileocecal valve
o Primarily prevents backflow of fecal content from colon
into SI
- Ileocecal sphincter
o Remains mildly constricted
o Constriction is regulated by the pressure in the cecum
o Slows the emptying of the SI into the cecum
Movements of the Colon
- Normally very sluggish because the functions of the colon (absorption and storage) do not require
intense movements
o Does a lot of the absorption of water in the chyme
- Mixing movements are similar to segmented movement of
SI but are more powerful
o Can sometimes constrict colon to occlusion
o Causes unstimulated portion of the muscle to
bulge out (haustrations)
o Provide a minor amount of movement
o Responsible for much of the movement in the
cecum and ascending colon
 Takes 8-15 hours to get through this
section of colon
- Propulsive movements (mass movements)
o Responsible for movement beginning w/ the transverse colon (and in descending colon)
o Occur only 1-3 x per day
 Many times after a meal
- Mass movement
o Modified form of peristalsis
o Constriction of rings in response to a distended colon that move along the colon
o Constriction for 30 sec followed by relaxation of 2-3 min then another very strong
constriction
o Whole series of mass movements may occur for 10-30 min and then may stop for half a
day
o Initiation after meals is facilitated by gastrocolic and duodenocolic reflexes
 Results from distension of the stomach and duodenum
 Triggers movement from the stomach to large intestine or small intestine
to large intestine
 Initiated by the autonomic nervous system
 Shouldn’t feel it
 Involuntary
o When they force material into the rectum, the desire for defecation is felt
 Voluntary
o When it becomes convenient for defecation, a deep breath can elicit the defecation reflex:
Valsalva maneuver
 Builds up abdominal pressure
 Important role: Valsalva Retinopathy- Vision loss after asthma attack
 Caused by weight lifting (blood pressure is too high and retinal blood
vessels can burst)
BHS 116.2- Phyisology II
Notetaker: Stephanie Cullen
Date: 1/16/13
Page: 5
Objective: Describe the various secretions of the GI tract as well as their functions and regulation
(salivary, esophageal, stomach).
Secretion of Saliva
- Saliva is produced by the parotid, submandibular, sublingual, and
many buccal glands
o Acini (cells): produces the serous and mucus parts of saliva
o Duct: carries saliva to the mouth
o Cells of the duct absorb Na and Cl from serous fluid causing
saliva to be devoid of these
 Why we are sensitive to the taste of salt
- Serous (watery) secretion containing ptyalin (α-amylase, salivary
amylase) that breakdown starch and lysozyme that breaks down
bacterial cell walls
- Mucus (sticky) secretion containing mucin for lubricating and protection
- pH between 6-7 (neutral)
- 0.5 ml/min is secreted during waking hours
o Produce 1 L of saliva/day
o All secretions from the GI tract: 7 L/day
 Most are reabsorbed so not losing that fluid
Function of Saliva in Oral Hygiene
- Wash bacteria away from teeth to prevent cavities
- Wash food stuff away from teeth
- Begin to digest food (polysaccharids like starch)
o Amylase
- Destroy bacteria w/ proteolytic enzymes
o Lysozyme
- Destroy bacteria w/ antibodies
o IgA
o Also helps to destroy bacteria in the food or in the mouth
Regulation of Salivation
- Salivation is controlled mainly by parasympathetic signals from the
salivatory nuclei in the brain stem that are stimulated by both taste
and touch
o Physical pressure of food on the tongue stimulates salivation
- The appetite area of the central nervous system (anterior
hypothalamus) can stimulate salivation in response to the smell or
taste of a favorite food
- Reflexes from the stomach and intestines can stimulate salivation
(long reflex)
Esophageal Secretion
- Mucus glands secrete mucus that acts to lubricate and protect the epithelial walls
o Lubrication eases the movement of the bolus of food into the stomach
Gastric Secretion
- Surface mucus cells (epithelia) along the entire lining
o Alkaline mucus secretion
o Protects against digestive enzymes and acid that damage of the epithelium
BHS 116.2- Phyisology II
Notetaker: Stephanie Cullen
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Date: 1/16/13
Page: 6
Gastric glands located in the body and fundus of the stomach secrete:
o Pepsinogen (chief and mucous cells)
 Inactive form
 Breaks down proteins
o Hydrochloric acid (parietal cells)
o Intrinsic factor (parietal cells)
 Important for the absorption of vitamin B12 in the intestine into the bloodstream
 Pernicious anemia results from lack of vitamin B12
o Mucus (mucous neck cells)
 Have to protect the gland
o Histamine (ECL (enterochromaffin) cells)
 Both endocrine and paracrine function
 Acts distally and locally on its neighboring cells
Pyloric glands located in the antrum of the stomach secrete:
o Pepsinogen (chief cells)
o Mucus (mucous neck cells)
o Gastrin (G cells)
o Somatostatin (D cells)
 Pancreas, hypothalamus (GHIH), and now
stomach
 Stimulated by acid
 Inhibits the actions by the G cells (parietal, chief cells)
Pepsinogen is converted to the active proteolytic enzyme (pepsin) in the
presence of acid (HCl) and other pepsin proteins
o Pepsin is a protease that breaks down proteins
o Pepsin can further convert other pepsinogen molecules into pepsin
Regulation of Gastric Secretion
- Gastrin (hormonal)
o Released from G cells in response to protein in the stomach
o Stimulates chief cells to secrete pepsinogen
o Stimulates parietal cells to secrete HCl
 HCl helps convert pepsinogen into pepsin and prevent
bacterial infection due to the low pH
o Stimulates enterchromaffin cells to secrete histamine
 Histamine stimulates parietal cells to secrete more HCl (paracrine function of
histamine)
- Acetylcholine (nervous)
o Released from vagal nerve endings
o Stimulates chief cells to secrete pepsinogen
o Stimulates enterochromaffin cells to secrete histamine
 Histamine stimulates parietal cells to secrete HCl
o Stimulates G cells to secrete gastrin
 Stimulates chief cells to secrete pepsinogen
 Stimulates parietal cells to secrete HCl
o Overactive vagus stimulation in the stomach produces excess HCl and pepsin
 Ulcers occur when tissues and cells in the stomach are broken down when there
is no food in the stomach
- Acid in the stomach and ACh released from vagus nerves cause the release of pepsinogen
- Feedback inhibition of both HCl and pepsinogen secretion by excessive acidity that prevents the
stomach pH from getting too low (< 3)
BHS 116.2- Phyisology II
Notetaker: Stephanie Cullen
Date: 1/16/13
Page: 7
Objective: Describe the phases of gastric secretion.
Phases of Gastric Secretion
- Cephalic phase
o Occurs before food enters the stomach
o Due to sight, smell, thought, and taste
o Neurogenic signals from the cerebral cortex and the appetite center transmit through the
vagus nerves (via ACh) resulting in production of 20-30% of the gastric juices secreted
during a meal
- Gastric phase
o Occurs once food enters the stomach eliciting the secretion of 70% of the gastric juices
secreted during a meal
 Due to both nervous reflexes (ACh) and the production of gastrin from the
pyloric glands
- Intestinal phase
o Occurs as food enters the SI
o Inhibits the production of gastrin and the production of the remaining gastric juices
 CCK
Clicker Question: Which is not a function of the stomach?
a. Mixing
b. Storage
c. Digestion
d. Absorption of nutrients
- only aspirin and alcohol are absorbed in the stomach