GI structure and function Flashcards
What are the four general functions of the digestive system?
Digestion, absorption, secretion and motility.
What are the three main salivary glands? Which supplied saliva from above?
Parotid, sublingual and submandibular. Parotid supplied from above.
What are the four layers of the alimentary canal?
Mucosa, submucosa, muscularis externa, serosa/adventitia.
What are the three layers of the mucosa?
Epithelium, lamina propria, muscularis mucosae.
What are the two types of epithelium found in the alimentary canal? Where is each type found?
Stratified squamous and simple columnar. Stratified squamous found in the mouth, oesophagus and anus. Simple columnar found in the peritoneal cavity.
Why is simple columnar epithelium referred to as “simple”?
All cells are attached to a basement membrane.
What are three functions of alimentary epithelium?
- Acts as a barrier separating the alimentary canal from the body.
- Synthesises and secretes digestive enzymes, hormones, mucous.
- Absorbs digestive products.
What is the alternate name for the submucosal plexus? How is it innervated?
Meissner’s plexus. Innervated from parasympathetic nervous system.
What are the two layers of the muscularis externa? Which is inner and which is outer? What are their functions?
Circular (inner, segmentation) and longitudinal (outer, peristalsis).
What is the alternate name for the myenteric plexus? How is it innervated?
Auerbach’s plexus. Innervated from parasympathetic and sympathetic nervous systems.
Where in the alimentary canal is the connective tissue layer known as “adventitia”?
Within the peritoneal cavity.
Where in the alimentary canal is the connective tissue layer known as “serosa”?
In the oesophagus, rectum and anus.
How are long reflexes carried to everywhere (not including the salivary glands)?
The vagus nerve carried the neurones of the parasympathetic system.
How are long reflexes carried to the salivary glands?
Via the facial (CN VII) and glossopharyngeal (CN IX) nerves.
What are the structures of the foregut?
Stomach, liver, pancreas, spleen, part of the duodenum.
What artery supplies the foregut? At what vertebral level does it branch from the abdominal aorta?
Coeliac artery/trunk, L1.
What are the structures of the midgut?
Part of the duodenum, jejunum, ileum, caecum, ascending colon, part of the transverse colon.
What artery supplies the midgut? At what vertebral level does it branch from the abdominal aorta?
Superior mesenteric artery, L2.
What are the structures of the hindgut?
Part of the transverse colon, descending colon, sigmoid colon, rectum.
What artery supplies the hindgut? At what vertebral level does it branch from the abdominal aorta?
Inferior mesenteric artery, L3.
What is chyme?
Chyme is the pulpy fluid that passes from the stomach to the duodenum. It contains juices and partially digested food.
From what structures does the superior mesenteric vein receive blood?
Small intestine, caecum, ascending colon, transverse colon.
From what structures does the splenic vein receive blood?
Pancreas, spleen.
From what structures does the gastric vein receive blood?
Stomach.
From what structures does the inferior mesenteric vein receive blood?
Transverse colon, descending colon, sigmoid colon, rectum.
What are the three monosaccharides recognised by the gut?
Glucose, galactose, fructose.
What bond joins two monosaccharides to form a disaccharide?
Glycosidic bond.
What disaccharide is made up of glucose and galactose? By which brush border enzyme is it broken down?
Lactose, lactase.
What disaccharide is made up of glucose and fructose? By which brush border enzyme is it broken down?
Sucrose, sucrase.
What disaccharide is made up of glucose and glucose? By which brush border enzyme is it broken down?
Maltose, maltase.
What are the two polysaccharides broken down by humans? How are the glucose monomers linked?
Starch and glycogen. Monomers are linked with alpha-1,4 glycosidic bonds.
What are the two components of starch? How do they differ structurally?
Alpha-amylose and amylopectin. Alpha-amylose is straight chain, amylopectin is highly branched.
Transport of glucose:
- What is the name of the symporter that transports both sodium and glucose into cells from the gut tube?
- What is the name of the transporter that glucose moves through from the cell into the blood?
- SGLT-1
2. GLUT-2
How does fructose move from the gut lumen to the blood?
Fructose moves down its concentration gradient to the cell and again to the blood, simply through specific channels.
Where do endopeptidases act on proteins?
In the middle, producing smaller fragments.
Where do exopeptidases act on proteins? What are the two sub-types?
At the end, producing individual amino acids. Carboxypeptidases and aminopeptidases.
What is the name of the symporter that transports amino acids from the lumen into the cell? How many amino acids is it able to transport?
SAAT-1.
20 (all).
How many specific amino acids are the transporters on the basolateral membrane able to move into the blood?
1 - specific for each amino acid, so 20 are required.
What is the name of the symporter than transfers dipeptides into cells? What ion is moved in at the same time as the dipeptide?
PepT1.
Hydrogen ion.
Which enzyme digests triacylglyceride?
Lipase.
Name two emulsification agents. In which medium are they secreted?
Bile salts and phospholipids. Secreted in bile.
Emulsified triacylglyceride forms amphipathic molecules. What is an amphipathic molecule?
A molecule with both a polar (hydrophilic) and a non-polar (hydrophobic) section.
Where in the small intestine are lipids absorbed?
Throughout the small intestine.
Where are glucose and amino acids absorbed?
Jejunum.
What are miscelles comprised of? What is their purpose?
Bile salts, monoglycerides, fatty acids, phospholipids.
Enhance emulsification and digestion.
Once within the cell, where do monoglycerides and fatty acids go?
Smooth endoplasmic reticulum.
Once TAG has moved from the sER to the Golgi, where is it exocytosed to? What does TAG become?
The ECF, through the serosal membrane.
TAG becomes chylomicrons.
Where are chylomicrons absorbed?
Lacteals - lymphatic vessels of the small intestine that absorb digested fat.
What are vitamins?
Vitamins are organic compounds that are essential for normal growth and nutrition. They are required in the diet because they cannot be synthesised by the body.
What are the fat-soluble vitamins? Which molecule do they have the same absorptive pathway as?
Vitamins A, D, E and K.
What are the water soluble vitamins? By which two ways can they be absorbed?
The B group, vitamin C, folic acid.
Absorbed by passive diffusion or carrier-mediated transport.
What does vitamin B12 bind to in the stomach?
Intrinsic factor.
Where is the specific transport protein that absorbs the vitamin B12-intrinsic factor complex?
Distal ileum.
What condition does vitamin B12 deficiency cause?
Pernicious anaemia.
What is the name of the transporter that absorbs iron into duodenal enterocytes?
DMT1.
What is the protein that acts as a “cage” for iron, storing and releasing it?
Ferritin.
When unbound iron enters the blood, what is the name of the transport glycoprotein that prevents the formation of free radicals?
Transferrin.
Where is gastrin produced? What cells produce gastrin?
Antrum, G-cells.
What is the extra layer of muscle found in the muscularis externa of the stomach? What motion does it allow for?
Oblique, twisting motion.
What is the opening of the gastric gland called? What cells are present?
Surface mucous cells.
What cells are at the top of the gastric gland? What is their function and why is this necessary?
Mucous neck cells. They are immature surface mucous cells, constantly migrating to the surface because the lifespan of a surface cell is so short.
Parietal cells are found in the gastric gland. What do they secrete?
Hydrochloric acid and intrinsic factor.
Chief cells are found in the gastric gland. What do they secrete?
Pepsinogen.
To form hydrogen ions for gastric acidic, carbon dioxide must enter the cell. It does so through diffusion. What happens then?
CO2 combines with H2O to form carbonic acid. Carbonic acid breaks down to form bicarbonate and a hydrogen ion.
How is a hydrogen ion transferred from an epithelial cell into the stomach? What is the difference in pH between the cell and the stomach?
Hydrogen-potassium pump.
Stomach pH = 2, cell pH = 7.4.
How does chloride enter the stomach from the epithelial cell?
Chloride exchanged with bicarbonate at the basolateral membrane. Moves into lumen through a channel.
How does gastrin stimulate gastric acid secretion?
Binds to a gastrin receptor in stomach. Intracellular calcium rises. Calcium binds to PKC, switches on sodium-potassium pump.
Acetylcholine stimulates the release of gastrin. Why is it released? (2 reasons)
- In response to sight, smell or taste of food (all of which cause increased vagal tone)
- Produced by ENS in response to distension of stomach.
What muscarinic receptor does acetylcholine bind to to stimulate intracellular calcium and gastric acid secretion?
M3.
What cell releases histamine? What stimulates the cell?
ECL cells - enterochromaffin-like. Gastrin and acetylcholine.
Histamine binds to which receptor on parietal cells? What is notable about this receptor?
H2 receptor.
The stomach is the only place in the body where H2 receptors are found - all others are H1.
Why is gastric acid produced before food is actually eaten?
To ensure that any food falling into the stomach is immediately sterilised.
In terms of gastric secretion, what does the end of the cephalic phase cause?
A decrease in vagal tone/parasympathetic activity, so less gastric secretion.
What change in stomach pH causes production of gastrin to fall?
A fall in pH, because acid is no longer being used up for digestion.
What is the name of the reflex caused by acid entering the duodenum? Through what nerve is the reflex transmitted? What two things does the reflex do?
Enterogastric reflex.
Splanchnic nerve (sympathetic reflex).
1. Inhibits gastrin production, 2. Releases secretion, which reduces production of gastrin.
The presence of fat in the duodenum causes the release of what peptide?
Gastric inhibitory peptide.
Enterogastrones work to stop the build up of acid in the duodenum. What causes them to be released from the duodenal mucosa?
Acid, hypertonic solutions, fatty acids, monoglycerides.
Give two reasons why acid build-up in the duodenum is not good.
- Acid damages the duodenum
2. Pancreatic enzymes have a narrow pH window, so the pH cannot fall too much.
At what pH is pepsinogen converted to pepsin?
3
Why does an ileostomy require lifelong vitamin B12 injections?
Vitamin B12 is absorbed in the ileum.
How are pacemaker cells in the longitudinal layer connected?
Gap junctions.
In terms of peristalsis, gastrin and acetylcholine increase a) the magnitude of action potentials, or b) the frequency of action potentials?
b
In what layer of the duodenum are Brunner’s glands found?
Submucosa.
How are Brunner’s glands stimulated?
By the parasympathetic and enteric nervous systems, and by secretin.
What anatomical structure is the head of the pancreas located in?
The curvature of the duodenum.
What are the three parts of the pancreas?
Head, body and tail.
What cell mainly comprises the endocrine pancreas? What hormones does the cell secrete?
Islets of Langerhans.
Insulin, glucagon and somatostatin.
Acinar cells are the main cell type of the exocrine pancreas. What do they produce?
Digestive zymogens.
Acinar cells form lobules. What are the lobules connected with?
Intercalated ducts.
Describe the progression of how intercalated ducts end up as part of the main pancreatic duct.
Intercalated duct –> intralobular duct –> interlobular duct –> main pancreatic duct.
What are the two ways that digestive enzymes can get from the pancreas to the duodenum?
The sphincter of Oddi and the accessory pancreatic duct.
What parts of the stomach does a selective vagotomy de-innervate? Why is the preserved part preserved?
De-innervates fundus and body to lessen secretion. Doesn’t de-innervate antrum to maintain motility.
Duct cells, found in the exocrine pancreas, secrete what? Why?
Bicarbonate, secreted along with pancreatic enzymes to ensure that they are not denatured by acid.
Trypsinogen is converted to trypsin by which enzyme?
Enterokinase
What enzyme is secreted in response to the presence of acid in the duodenum? What is its function?
Secretin, stimulates duct cells to produce bicarbonate.
Zymogen secretion is triggered by what enzyme? What is the enzyme released in response to?
Cholecystokinin. Released in response to fat/amino acid in the duodenum.
What is the hard palate?
The bony anterior part of the roof of the mouth/palate.
What are the main components of saliva?
Water, mucins, alpha-amylase, electrolytes, lysozymes.
Parasympathetic stimulation of the salivary glands comes from which two nerves?
Facial (CN VII) and glossopharyngeal (IX).
What type of epithelium is present in the oesophagus?
Non-keratinised stratified squamous.
What kind of muscle is present in the oesophagus?
Upper third - skeletal.
Lower two thirds - smooth.
How many sphincters does the oesophagus have? What are their names?
2 - upper and lower.
The salivary glands have sympathetic innervation. What receptors are targeted by noradrenaline to produce
a) mucous?
b) amylase?
a) alpha-1
b) beta-2.
What is bolus?
Chewed food.
After which phase of swallowing does it become difficult to reverse the process?
Oral phase.
What area of the brain controls the pharyngeal phase of swallowing?
Medullary swallowing centre.
What structure presents food from entering the nasopharynx?
Soft palate.
How long does it take peristalsis to sweep bolus from the oesophagus to the stomach?
10 seconds.
Due to rugae, the stomach can inflate from 50ml to what volume without increasing in pressure?
1500ml.
What ligament separates the left and right lobes of the liver?
Falciform ligament.
What is the name of the area on the inferior surface of the liver where blood vessels, lymphatics, ducts and nerves enter and leave?
The porta.
Two minor lobes lie superiorly and inferiorly to the porta of the liver. What are their names?
Caudate lobe - superior.
Quadrate lobe - inferior.
What two ducts combine to form the common bile duct?
Common hepatic duct and cystic duct.
What is the name of the duct that carries bile from the gall bladder to the hepatopancreatic ampulla?
Cystic duct.
What ligament connects the liver to the inferior surface of the diaphragm?
Coronary ligament.
How is the liver peritonised?
Intraperitoneal.
How is the stomach peritonised?
Intraperitoneal.
How is the transverse colon peritonised?
Intraperitoneal.
How are the jejunim and ileum peritonised?
Intraperitoneal.
How is the sigmoid colon peritonised?
Intraperitoneal.
How is the spleen peritonised?
Intraperitoneal.
How is the majority of the duodenum peritonised?
Retroperitoneal.
How is the ascending and descending colon peritonised?
Retroperitoneal.
How is the majority of the pancreas peritonised?
Retroperitoneal.
What are the nine regions of the abdomen?
Right and left hypochondriac, epigastric, right and left lumbar, umbilical, right and left iliac/inguinal, hypogastric/suprapubic.
What is the only area of the liver not covered by connective tissue/peritoneum? Where is it found?
Bare area of the liver, found on the diaphragmatic surface (surrounded by the coronary ligament).
What is the name of the hexagonal functional units of the liver?
Septa.
What are the three components of the portal triad?
Hepatic artery, hepatic portal vein, hepatic duct.
Where does blood from the liver drain to?
Hepatic vein, IVC.
Hepatocytes dump whatever they make into lumens that run between them. What are the lumens called?
Bile canaliculi.
Hepatocytes are arranged into hepatic cords. What is the name of the space between hepatic cords?
Hepatic sinusoids.
Name the 6 components of bile.
Bile acids, lecithin, cholesterol, bile pigments, toxic metals, bicarbonate.
What two amino acids can bile acids be conjugated with?
Glycine and taurine.
What hormone is required for the sphincter of Oddi to open?
Cholecystokinin.
Roughly how long is the duodenum?
25cm.
Which segment of the small intestine does the majority of nutrient absorption take place?
Jejunum.
What is the main site of water and salt absorption in the small intestine?
Ileum.
What cells in the small intestine secrete chloride into the intestinal lumen?
Crypt cells.
What is the name of the chloride channel in intestinal epithelial cells?
CFTR - cystic fibrosis transmembrane conductance regulator.
What is the purpose of segmentation?
Thorough mixing of contents with digestive enzymes, and bringing chyme into contact with its absorbing surfaces.
Describe how segmentation works.
Small part of bowel contracts while parts on either side relax. Chyme pushes into areas of relaxation. Process repeats ad infinitum. Slight bias towards food being pushed down tract.
Segmentation is generated by the depolarisation of pacemaker cells in which layer of muscularis mucosae muscle?
Circular.
In which two alimentary structures does segmentation occur?
Stomach and small bowel.
What is the name of the pattern of peristaltic activity starting in the gastric antrum and ending in the terminal ileum?
Migrating motility complex.
What are the two purposes of the migrating motility complex?
1) Move undigested food into the large bowel.
2) Limit bacterial colonisation of the small bowel.
What hormone is involved in the initiation of the migrating motility complex?
Motilitin.
What stimulates the gastroileal reflex?
Food entering the stomach.
Describe what the gastroileal reflex does.
Increases segmentation activity, opens ileocaecal valve, distends colon.
What are the tenia coli? How many are there?
Tenia coli are bands of longitudinal muscularis externa that run from the ascending colon to the sigmoid colon. There are three.
What is the the name of the pouches formed by the contraction of tenia coli?
Haustra.
What type of epithelium is found in the colon?
Simple columnar.
Why are there many goblet cells in crypts of Lieberkuhn in the colon?
To lubricate faecal movement.
How is the defecation reflex initiated?
Mechanoreceptors detect distension of the rectal wall.
What nerves carry the defecation reflex?
Pelvis splanchnic nerves.
What does the defecation reflex do?
Contracts rectum, relaxes internal anal sphincter, contracts external anal sphincter, increases colonic peristalsis.
How do enterotoxic bacteria cause diarrhoea? What is their mechanism of action?
Cause crypt cells to maximally secrete chloride and thus water into the colon.
Enterotoxin elevate levels of secondary messengers, such as cAMP and calcium.
