The digestive system-3 Flashcards
J-shaped chamber lying between oesophagus and small intestine
Stomach
Three parts of the stomach
-Fundus
(lies above gastroesophageal sphincter)
-Body (middle part of stomach)
-Antrum
______ muscle layer in the ______ is ______ than lower part of stomach.
Why is this difference important?
Smooth
fundus
thinner
Difference is important for gastric motility
Two mucosal linings:
- Oxyntic mucosa
- Pyloric gland area
Difference between the two mucosal linings of the stomach is based on:
glandular secretion
terminal portion of stomach
Pyloric sphincter
Function of the pyloric sphincter
Serves as barrier between stomach and upper part of small intestine
Three main functions of stomach
- Stores ingested food until it can be emptied into small intestine.
- Secretes hydrochloric acid (HCl) and enzymes that begin protein digestion
- Mixing movements convert pulverized food (bolus) to chyme
The stomach stores ingested food until it can be emptied into small intestine
- Rate optimal for digestion and absorption
- Release of food into duodenum must not exceed small intestine capacity
In the stomach, mixing movements convert pulverised food (bolus) to chyme
- Thick liquid mixture of pulverized food mixed with gastric secretions
- Pulverized food must be converted to chyme before being emptied into duodenum
Gastric (stomach) motility is complex and has four aspects:
- Filling
- Storage
- Mixing
- Emptying
involves receptive relaxation
Gastric filling
Explain gastric filling:
-involves receptive relaxation
-When empty, stomach has a volume of 50ml, after a meal, it can expand to 1L (1000ml).
> Accomplished through
numerous stomach folds
-During meal, folds become smaller and flatten as stomach fills (relaxes) with food (bolus)
>Response known as receptive relaxation mediated by vagus nerve >Enhances stomach’s ability to accommodate the extra volume of food with little rise in stomach pressure >Triggered by act of eating
Explain gastric storage
- Pacemaker cells
(interstitial cells of Cajal)
in fundus generate slow-wave like potentials through length of stomach toward pyloric sphincter - Rhythm of spontaneous depolarizations = basic electrical rhythm (BER) occur continuously and can be accompanied by stomach contractions
- Depending on level of excitability, peristaltic waves can be initiated
> Weak in fundus and body,
stronger in antrum - Food mainly stored in body and mixing occurs in antrum
Pacemaker cells
interstitial cells of Cajal)
Takes place in the antrum
gastric mixing
Explain gastric mixing
- takes place in the antrum
- Strong, intense peristaltic contractions mix food (bolus) with gastric secretions into chyme
- Each peristaltic wave propels chyme towards pyloric sphincter
4. Pyloric sphincter is not entirely closed
> Opening large enough to
allow water and other fluids
to pass
> Particles larger than 2mm
in diameter cannot pass
- As peristaltic wave reaches pyloric sphincter, it closes
- Larger particles are propelled
back to body of stomach
- When next wave arrives,
larger particles propelled
towards sphincter
RETROPULSION –> shearing and grinding of particles to be small enough to be emptied into duodenum
shearing and grinding of particles to be small enough to be emptied into the duodenum
Retropulsion
controlled by duodenum
gastric emptying
driving force for gastric emptying
Antral peristatic contractions
Chyme that escapes during each contraction (during gastric emptying) depends on
strength of antral peristalsis
Intensity of antral peristalsis (rate of emptying) varies depending on
the signals received from stomach and duodenum
Signals (factors) influence stomach’s ________ by _______ or ________ of smooth muscle.
The greater excitability:
–>
The greater the strength of antral peristalsis
–>
excitability
depolarisation
hyperpolarization
–>the more frequent the BER will generate action potential
–> the faster the rate of emptying
Explain gastric emptying:
- A peristaltic contraction originates in the upper fundus and sweeps down towards the pyloric sphincter
- The contraction becomes more vigorous as it reaches the thick-muscled antrum.
- The strong antral peristaltic contraction propels the chyme forward.
- A small portion of chyme is pushed through the partially open sphincter into the duodenum. The stronger the antral contraction, the more chyme is emptied with each contractile wave.
- when the peristaltic contraction reaches the pyloric sphincter, the sphincter is tightly closed and no further emptying takes place.
- When chyme that was being propelled forward hits the closed sphincter, it is tossed back into the antrum.
Mixing chyme is accomplished as chyme is propelled forward and tossed back into the antrum with each peristaltic contraction
Factors in stomach influencing rate of gastric emptying
- Main gastric factor influencing
strength of contraction =
amount of chyme in stomach - Stomach distention (enlargement) triggers gastric motility
- degree of fluidity of chyme
amount of chyme in stomach
Stomach empties at rate proportional to volume of chyme
Stomach empties at rate proportional to volume of chyme
Occurs via stretching of smooth muscle but ALSO through intrinsic nerve plexus, vagus nerve stimulation and secretion of gastrin (hormone)
degree of fluidity of chyme
Stomach contents MUST be converted into finely divided into thick liquid form before being emptied > The sooner the appropriate degree of fluidity is reached, the more rapidly the contents can be released
Factors in duodenum influencing rate of gastric emptying
- Duodenum must be ready to receive chyme
> If not ready, strength of
antral peristaltic waves is
reduced - Main factors in duodenum important for gastric emptying include:
a. Fats
b. acid
c. hypertonicity
d. distention
> Presence of one or more in
duodenum, triggers neural OR
hormonal response, slowing
down rate of emptying - Blood carries hormones to stomach (released from small intestine) to reduce gastric emptying
Neural response mediated by
intrinsic (short) and autonomic (long) nerve plexus
nerves plexus for gastric emptying
ENTEROGASTRIC REFLEX
Hormonal response involves release of several hormones – collectively known as
ENTEROGASTRONES
Blood carries hormones to the stomach (released from the small intestine) to reduce gastric emptying.
Two important hormones include:
secretin and cholecystokinin (CCK)
released from S-cells
Secretin
released from I-cells
cholecystokinin (CCK)
S- and I-cells from
duodenum & jejum mucosa
Fat (lipids)
- Fat digestion and absorption takes place only within lumen of small intestine
- When fat is already in duodenum, further gastric emptying of additional fatty stomach contents is prevented
Acid
Unneutralized acid in duodenum inhibits further emptying of acidic gastric contents until neutralization can be accomplished
Hypertonicity
Gastric emptying is reflexly inhibited when osmolarity of duodenal contents starts to rise
Distention
Too much chyme in duodenum inhibits emptying of even more gastric contents
Additional factors that influence gastric motility
Emotions:
- Sadness and fear – tend to decrease motility
- Anger and aggression – tend to increase motility
Intense pain
– tends to inhibit motility
Additional factors that influence gastric motility
Emotions:
- Sadness and fear – tend to decrease motility
- Anger and aggression – tend to increase motility
Intense pain
– tends to inhibit motility
______ of gastric juices secreted daily.
Cells responsible _____ the stomach:
______ lines the fundus and body.
______ line antrum
2L
line
oxyntic mucosa
pyloric gland area
\_\_\_\_\_\_ of the stomach contains \_\_\_\_\_\_ (pockets) formed by \_\_\_\_\_\_\_\_\_\_\_\_ The first part referred to as \_\_\_\_\_\_\_\_\_\_ The base is referred to as \_\_\_\_\_\_\_\_\_ Secretory cells in folds: > > >
Lumen deep pits numerous folds gastric pits gastric glands Exocrine Endocrine Paracrine
What secretes HCl
Parietal cells
Functions of HCl
Activates pepsinogen to active enzyme pepsin and provides acid medium for optimal pepsin activity
- Aids in breakdown of connective tissue and muscle fibers
- Denatures protein
- Along with salivary lysozyme, kills most of the microorganisms ingested with food
kills most of the microorganisms ingested with food
HCl and salivary lysozyme
Rate of gastric secretion is influenced by:
a. Factors arising before food (bolus) reaches stomach
b. Factors resulting from food (bolus/chyme) in the stomach
c. Factors in the duodenum after food (bolus/chyme) has left the stomach
Control of gastric secretions involves three phases:
Cephalic
Gastric
Intestinal
Cephalic phase
Refers to increased secretion of HCl and pepsinogen that occurs in response to stimuli acting in the head before food reaches stomach
Gastric phase
- Begins when food actually reaches the stomach
- Presence of protein increases gastric secretions
Intestinal phase
- Inhibitory phase
Helps shut off flow of gastric juices as chyme begins to empty into small intestine
Gastric secretions gradually ______ as food empties from stomach into __________
decrease
small intestine
protects the stomach lining from gastric secretions
Gastric mucosal barrier
Carbohydrate digestion continues in the _____ of the stomach, protein digestion begins in _______ of the stomach
body
the antrum
Stomach absorbs:
alcohol and aspirin
BUT NOT food
Accessory digestive organs
- Accessory digestive organs
- Elongated gland located behind and below the stomach
Islets of Langerhans
Endocrine function of accessory digestive organs
- Found throughout the pancreas
- Secrete insulin and glucagon
Exocrine function of accessory digestive organs
Secretes pancreatic juice consisting of:
- Pancreatic enzymes actively secreted by acinar cells that form the acini
- Aqueous alkaline solution actively secreted by duct cells that line pancreatic ducts
Exocrine secretion of the pancreas is regulated by:
- secretin
- CCK
Digest protein
Proteolytic enzymes
Proteolytic enzymes: (pancreatic and biliary secretions)
Trypsinogen-converted to active form-trypsin
Chymotrypsinogen- converted to active form- chymotrysin
Procarboxypeptidase- converted to active form carboxypeptidase
Converts polysaccharides into disaccharides
Pancreatic amylase
Only enzyme secreted throughout entire digestive system that can digest fat
Pancreatic lipase
explain the control of pancreatic aqueous NaHCO3-
- Acid in the duodenal lumen is detected.
- Increased release of secretin from duodenal mucosa
- Secretin carried by blood to pancreatic duct cells.
- Pancreatic duct cells then stimulated to secrete aqueous NaHCO3 solution into duodenal lumen
- Acid is now neutralized
Explain the control of pancreatic digestive enzyme secreation
- Fat and protein products in the duodenal lumen
- Increased release of CCK from duodenal mucosa
- CCK is carried in blood to pancreatic acinar cells
- Which then stimulates the secretion of pancreatic digestive enzymes into duodenal lumen
- The fat and protein products are now digested
Largest and most important metabolic organ,
Major biochemical factory
Secretes bile salts (NB for digestive system)
Liver
Liver functions not related to digestion:
- Metabolic processing of the major categories of nutrients
- Detoxifying or degrading body wastes and hormones, drugs, and other foreign compounds,
- Synthesizes plasma proteins
- Stores glycogen, fats, Fe, Cu, and many vits
- Activates vit D
- Removes bacteria and worn-out red blood cells
- Excretes cholesterol and bilirubin
Stored and concentrated in the gallbladder
Bile
Bile is actively secreted by the _______ and it is actively _________to the __________ between meals. And after a meal, bile enters the ___________
liver
diverted
Gallbladder
duodenum
Bile consists of:
- Bile salts
- Cholesterol
- Lecithin
- Bilirubin
Bile salts:
- Derivatives of ____________
- Convert large ________ into ________
- After participation in fat digestion and absorption, most are _________
- Cholesterol
- fat molecules; liquid emulsion
- Reabsorbed into the blood
GI hormones
- Gastrin
- Secretin
- CCK
- GIP
- Release is stimulated by presence of protein in stomach
- Secretion is inhibited by accumulation of acid in the stomach
Gastrin
Gastrin function
- Acts in several ways to increase secretion of HCL and pepsinogen.
- Enhaces gastric motility
- stimulates ileal motility
- Relaxes ileocecal sphincter
- Induces mass movements in colon
- Helps maintain well-developed, funtionally viable, digestive tract lining
Presence of acid in duodenum stimulates release
Secretin
funtions of secretin
- Inhibits gastric emptying in order to prevent further acid from entering duodenum until acid already present is neutralized.
- Inhibits gastric secretion to reduce amount of acid being produced.
- Stimulates pancreatic duct cells to produce large volume of aqueous NaHCO3 secretion
- Stimulates liver to secrete NaCO3 rich bile which assists in neutralization process.
- Along with CCK, is trophic to exocrine pancreas
CCK funtions
- Inhibits gastric motility and secretion
- Stimulates pancreatic acinar cells to increase secretion of pancreatic enzymes
- Causes contraction of gallbladder and relaxation of sphincter Oddi
- Along with secretin, is trophic to exocrine pancreas
- Implicated in long-term adaptive changes in proportion of pancreatic enzymes in response to prolonged diet changes
- Important regulator of food intake.
- Glucose dependent insulinotrophic peptide
- Stimulates insulin release by pancreas
GIP
