The Stomach Flashcards
Why is very little absorbed in the stomach?
Thick layer of mucosa
Stomach has unique oblique muscle layer. What is purpose of this?
Enables vigorous movement to break found down and mix with stomach secretion
What are the 4 main anatomical divisions of the stomach?
- Cardia 2. Fundus 3. Body 4. Pylorus
Where is the cardia?
Surrounds the superior opening of the stomach at the T11 level
Where is the fundus?
The rounded, often gas filled portion superior to and left of the cardia
Where is the body?
The large central portion inferior to the fundus
Were is the pylorus?
This area connects the stomach to the duodenum
What is the pylorus divided into?
- Pyloric antrum 2. Pyloric canal 3. Pyloric sphincter
What do the medial and lateral borders of the stomach form?
Are curved, forming the lesser (medial surface) and greater (lateral surface) curvatures
What surface forms the lesser curvature?
Medial surface
What surface forms the greater curvature?
Lateral surface
Where does the greater curvature arise? Where does it end?
At the cardiac notch, arching backwards and passing inferiorly to the left. It curves to the right as it continues medially to reach the pyloric antrum.
What supplies the greater curvature?
The short gastric arteries and the right and left gastro-omental arteries supply branches to the greater curvature
Which curvature is longer?
Greater curvature
Describe shape of greater curvature
Long, convex
Describe shape of lesser curvature
Shorter, concave
What is the most inferior part of the lesser curvature called? What does it indicate?
The angular notch Indicates the junction of the body and pyloric region
What does the lesser curvature give attachment to?
Hepatogastric ligament
What is the lesser curvature supplied by?
The left gastric artery and right gastric branch of the hepatic artery
What is found superior to the stomach?
Oesophagus and left dome of diaphragm
What is found anterior to the stomach?
Diaphragm, greater omentum, anterior abdominal wall, left lobe of liver, gall bladder
What is found posterior to the stomach?
Lesser sac, pancreas, left kidney, left adrenal gland, spleen, splenic artery, transverse mesocolon
How many sphincters of the stomach are there? Where are they located? What is their function?
2 - located at each orifice. They control the passage of material entering and exiting the stomach. 1. Inferior oesophageal sphincter 2. Pyloric Sphincter
Where does the oesophagus pass through the diaphragm? What level is the inferior oesophageal sphincter (LES)?
Through the oesophageal hiatus at T10 Descends a short distance to the inferior oesophageal sphincter at the T11 level
What does the inferior oesophageal sphincter mark?
The transition point between the oesophagus and stomach
Where is the superior oesophageal sphincter located?
In the pharynx
What is function of inferior oesophageal sphincter?
It allows food to pass through the cardiac orifice and into the stomach and is not under voluntary control.
What does the LES and cardia (upper) region secrete?
- Mucus - HCO3-
What is the motility role of the LES and cardia region?
Prevention of reflux Entry of food Regulation of belching
Where is the pyloric sphincter?
Between the pylorus and the first part of the duodenum
What is function of pyloric sphincter?
It controls of the exit of chyme (food and gastric acid mixture) from the stomach
How does the pyloric sphincter differ from the inferior oesophageal sphincter?
This is an anatomical sphincter. It contains smooth muscle, which constricts to limit the discharge of stomach contents through the orifice.
When does emptying of the stomach occur?
Occurs intermittently when intragastric pressure overcomes the resistance of the pylorus
Why is the pylorus normally contracted?
So that the orifice is small and food can stay in the stomach for a suitable period.
What is effect of gastric peristalsis?
Pushes the chyme through the pyloric canal into the duodenum for further digestion
What does the fundus and body region (middle) secrete?
H+ Intrinsic actor Mucus HCO-3 Pepsinogens Lipase
What is the motility function of the fundus and body region?
Reservoir Tonic force during emptying
What does the antrum and pylorus (inferior) region secrete?
- Mucus - HCO3-
What is the motility function of the antrum and pylorus?
Mixing Grinding Sieving Regulation of emptying
What region of the stomach is responsible for accepting and holding food?
Fundus and body –> relaxation and accommodation
What region of the stomach is responsible for churning and grinding food?
Antrum
What region of the stomach is responsible for emptying?
Antrum and pylorus
What does accommodation of the stomach depend on?
- Vago-vagal reflex - Stretch sensors signalling via the vagus
Describe the vago-vagus reflex of accommodation of the stomach?
Descending signals (via the vagus) act on the enteric nervous system Results in relaxation and dilation of the fundus and body of the stomach
What does accommodation of the stomach allow?
Up to about 1 litre of food to be stored in the stomach with very little rise in pressure.
How can the antral pump adapt to the type of food in the stomach?
The relative stillness of the fundus and body: occasionally peristaltic waves squeeze off another “bite”, and pass it to the antral pump The way the antral pump can adapt to the type of food in the stomach: liquid is allowed to pass through the pyloric sphincter, while thicker material is churned and ground until it is smooth enough to be passed to the duodenum.
What size particles cannot pass the pyloric sphincter?
Particles larger than 1-2mm
How does duodenum control gastric emptying?
- Senses delivery of acid, amino acids and lipids - Secretes hormones (CCK, secretin, GIP) which decrease gastric motility and emptying - Feedback to enteric nervous system
What are the phases of gastric secretion?
- Cephalic phase 2. Gastric phase 3. Intestinal phase
Describe cephalic phase
Sight/thought of food –> cerebral cortex –> hypothalamus and medulla oblongata –> vagus nerve –> stimulates stomach secretory activity (conditioned reflex) Loss of appetite (e.g. depression) –> cerebral cortex –> lack of stimulation to parasympathetic centre –> inhibits stomach secretory activity
How does stomach distension (gastric phase) lead to increased stomach secretions?
Stomach distensions activates stretch receptors –> vagovagal reflexes (and local reflexes) –> medulla –> vagus nerve –> stimulates stomach secretion
How do food chemical and rising pH (gastric phase) lead to stomach secretion?
Cause G cells to secrete gastrin into blood –> stimulates stomach secretion
How does excessive acidity affect gastric secretion?
Causes gastrin secretion to decline
How does emotional distress inhibit stomach secretion?
Sympathetic nervous system activation –> overrides parasympathetic control
What is effect of low pH and partially digested foods in duodenum as stomach empties?
Intestinal gastrin released into blood –> stimulates stomach secretory activity
What is effect of distension of duodenum, presence of fatty, acidic, hypertonic chyme and/or irritants?
Inhibits secretory activity by reflex and also release of intestinal hormones
What does the gastric pit consist of?
Lamina propia Mucous cells
What do mucous cells secrete?
HCO3-
What does the gastric gland consist of?
Parietal cells, G cells, Chief cells (aka peptic cells)
What do parietal cells secrete?
Secrete HCl and intrinsic factor into fundus and body
What are parietal cells also called?
Oxyntic cells
What do G cells secrete?
Gastrin (into blood stream)
Where are G cells mainly found?
In antral/pyloric glands
What do Chief cells secrete?
Pepsinogen
What is gastrin?
Peptide hormone released from G cells of stomach and duodenum into bloodstream
What are the 2 forms of gastrin?
- G17 (17 amino acids) - G34 (34 amino acids - the C-terminal 17 are the same as G17)
Where is G17 secreted from?
Main form secreted from antrum
Where is G34 secreted from?
Main form secreted from duodenum
What is gastrin homologous with? What is the gastrin receptor also called?
CCK-PZ (C-term 5 amino acids are the same) and can share receptors –> gastrin receptor also called CCK-B receptor
What is gastrin release stimulated by?
- Lumenal proteins / amino acids - Parasympathetic input (cephalic, and in response to gastric stretch), mediated by gastrin releasing peptide from interneurons
What is gastrin release inhibited by?
Lumenal [H+] - negative feedback
What are the main actions of gastrin on the stomach?
- Stimulate acid secretion - Promote mucosal growth
What is somatostatin? What are the 2 forms?
A peptide hormone: 14 and 28 amino acid forms
Which form of somatostatin is mostly found in gut?
28aa
Where is somatostatin released from?
D cells of stomach, duodenum and pancreas
How does somatostatin act?
in a paracrine fashion (i.e. into interstitium, to act on adjacent cells), and maybe also in an endocrine fashion.
What is somatostatin release stimulated by?
Lumenal [H+]
What is somatostatin release inhibited by?
ACh
What do somatostatin act on?
- G cells to inhibit the release of gastrin - Also inhibits the release of CCK and secretin
What are the 2 states of parietal cells?
Resting and active
Describe resting state of parietal cells
Tubolovesicles The proton pumps (H,K-ATPase) are mainly in a complex intracellular membrane compartment consisting of many small vesicles having spherical, elongated or flattened shapes and commonly called “tubulovesicles.” These membranes lack a potassium (K+) permeability, blocking activity of the H,K-ATPase.
Describe active state of parietal cells
Canaliculus Upon stimulation, many of the tubulovesicles fuse with the canalicular membrane, moving the proton pumps to a position in which they can actively exchange H+ for K+. This stimulation involves a massive movement of membrane to the canaliculi, distending them, elongating their microvilli, and greatly expanding the apical surface area.
What stimulates/inhibits acid secretion?
Gastrin and ACh stimulate Somatostatin inhibits Through both direct and indirect pathways
What is the indirect effect on acid secretion mediated by?
Histamine
What protects the surface epithelium of the stomach?
Mucus and bicarbonate secretion
What is intrinsic factor?
55 kDa glycoprotein
What is intrinsic factor released from?
Parietal cells into the stomach
What is effect of intrinsic factor?
Binds to cobalamin (Vit B12), but not until the small intestine. Essential for cobalamin uptake in the ileum.
What is the effect of gastrin mediated by?
CCK-a receptors
What are pepsins? Where are they secreted from?
Family of proteases, secreted from the chief and mucus cells
What are pepsins secreted in response to?
ACh, [H+], minor effects of secretin, CCK and gastrin (latter 2 via CCK-A receptor)
What are pepsins secreted as?
Prohormones (pepsinogens)
What are pepsinogens cleaved by?
- Cleave spontaneously at low pH (
What do endoproteases belong to?
Pepsins (family of proteases)
How are endoproteases cut?
Cut within the peptide chain, rather than going for the terminal amino acid
What do endoproteases preferentially target?
Preferentially target amino acids with aromatic or large side chains, but fairly broad specificity
What is the pH optimum of endoproteases?
Around 2-3
When are pepsins denatured?
Above pH 5-7 (i.e. once they get to the small bowel)
What is function of gastric lipase?
Initial digestion of triglycerides: Cleave the outer fatty acids off triglycerides, leaving diacyl glycerol
What is optimum pH of gastric lipase?
About 4 - Stable in the stomach, but denatured by pancreatic protease
What does gastric lipase work together with?
Lingual lipase
What is ptyalin a-amylase involved in?
Initial digestion of polysaccharides (starch) into maltose and dextrin
What is optimum pH of ptyalin a-amylase?
pH Optimum is about 7: denatured at pH 4
What reflex is involved in vomiting?
Reflex closure of the glottis and soft palate and opening of lower and upper oesophageal sphincters
What is vomiting caused by?
- Centrally controlled –> Area postrema = chemoreceptor trigger zone - Vagal afferents in response to irritants in or around the bowel - Psychogenic - Pain, revulsion - Motion sickness/labyrinthine disorders - Drugs or toxins with a direct effect - Pregnancy
What are the consequences of vomiting?
- Salivation, sweating, hyperventilation - Loss of normal gut motility (Retrograde peristalsis) - Retching (Involuntary contractions of the diaphragm and abdominal wall muscles) - Displacement of the cardia into the thorax
What is the vagovagal reflex?
GI tract reflex circuits where afferent and efferent fibers of the vagus nerve coordinate responses to gut stimuli.
What does the vagovagal reflex control?
The vagovagal reflex controls contraction of the gastrointestinal muscle layers in response to distension of the tract by food This reflex also allows for the accommodation of large amounts of food in the gastrointestinal tracts.
What is the vagus nerve composed of?
Both sensory afferents and parasympathetic efferents
Where does the vagus nerve carry signs to and from (regarding the vagovagal reflex)?
From stretch receptors, osmoreceptors, and chemoreceptors to dorsal vagal complex where the signal may be further transmitted to autonomic centers in the medulla. Efferent fibres of the vagus then carry signals to the gastrointestinal tract up to two-thirds of the transverse colon.
What does the gastric mucosa secrete?
Gastric juice
What is chyme?
Gastric juice converts the gastric contents to a semiliquid mass called chyme
How are the stomach walls protected from gastric acid?
The membrane on the surface of the epithelial cells bordering the lumen of the stomach is rich in lipoproteins which are resistant to acid
What is gastric juice secreted in response to?
Vagal stimulation (either directly by electrical impulses or indirectly by stimuli received through the senses) E.g. by sight, thought, presence of peptides, distension, amino acids
How is the stomach structurally different to other parts of the digestive system?
Extra layer of oblique smooth muscle
Describe the stomach in the empty phase
Submucosa are thrown up into distinct folds called rugae
What type of cells line the stomach?
Columnar epithelium dedicated to secretion
What is the lining of the stomach covered by?
The openings of gastric pits which extend into the mucosa as straight and branched tubules, forming gastric glands.
What are the 4 major types of secretory epithelial cells covering the surface of the stomach and extending down into gastric pits and glands?
- Mucous cells 2. Parietal cells 3. Chief cells 4. G cells
What do mucous cells secrete?
Secrete an alkaline mucus that protects the epithelium against shear stress and acid
What do parietal cells secrete?
Secrete hydrochloric acid
What do chief cells secrete?
Secrete pepsin, a proteolytic enzyme
What do G cells secrete?
Gastrin (hormone)
What is gastrin released in response to?
In response to vagal and gastrin-releasing peptide GRP stimulation secondary to ingestion of peptides, amino acids, gastric distention, and an elevated stomach pH.
What is the vagovagal reflex?
- GI reflex circuits where afferent and efferent fibers of the vagus nerve coordinate responses to gut stimuli via the dorsal vagal complex in the brain.
- The vagovagal reflex controls contraction of the GI muscle layers in response to distension of the tract by food. This reflex also allows for the accommodation of large amounts of food.
- The vagus nerve, composed of both sensory afferents and parasympathetic efferents, carries signals from stretch receptors, osmoreceptors, and chemoreceptors to dorsal vagal complex.
- Efferent fibers of the vagus then carry signals to the gastrointestinal tract up to two-thirds of the transverse colon
