GIT Flashcards
Approx. how long is the alimentary tract?
~7m
What are the accessory glands of the GIT?
Salivary, pancreas, liver & appendix
What is peristalsis?
Rhythmic waves that push food through the alimentary canal
Describe the basic architecture of the GIT walls?
Mucosa layer: epithelial cells + lamina propria + muscularis mucosa
Submucosa layer: circular muscle + longitudinal muscle + nerve plexus (intrinsic & extrinsic)
What are the two intrinsic nerve plexus’s in the submucosa layer of the GIT wall?
submucosal plexus: responsible for increasing secretion of the cells in the epithelial lining
Myenteric plexus: responsible for contraction (peristalsis)
Describe the borders of the foregut, midgut and hindgut
foregut: esophagus to mid duodenum
midgut: mid duodenum to proximal ⅔’s transverse colon
hind gut: last ⅓ transverse colon to rectum
Name the vessels that supply the foregut, midgut and hindgut
foregut: celiac trunk
midgut: superior mesenteric artery
hindgut: inferior mesenteric artery
What is found in saliva?
bicarbonate + amylase (primary enzyme)
What is the pH of saliva and why?
7 because amylase requires an alkaline medium to act
Name the glands in/around the oral cavity
sublingual glands, submandinbular glands, parotid glands
What is the role of the pharynx in the digestive system?
directing food to the esophagus and air to the larynx
How long does it take food & water to reach the stomach from ingestion?
food: 7s
drink: 3-5s
What cranial nerves are involved in the taste pathways?
7, 9 & 10
What cranial nerves are involved in the swallowing reflex?
9 & 10
What is dysphagia?
inability to swallow
What is the enzyme that breaks down proteins?
pepsin
What is the stomachs capacity?
~2.5L
What are two important cells in the stomach and what are they responsible for secreting?
chief cells: secrete pepsin (enzyme for breaking down protein)
parietal cells: secrete intrinsic factor (which absorbs B12) & hydrochloric acid (stomach acid)
What is the pH of gastric acid and why?
pH 2, because pepsin needs an acidic medium to act
What is the purpose of mucus in the stomach?
to maintain a balance between hostile factors (gastric acid) and protective factors (mucus) to maintain the integrity of the stomach lining (imbalance can cause stomach ulcers/breakdown of stomach wall)
What is the purpose of prostaglandins in the stomach?
they are responsible for the amount of acid secreted in the stomach
Describe how hydrochloric acid is formed in the stomach
parietal cells secrete H+ and Cl- into the gastric lumen via 2 different pumps
H+ via a primary H+ K- ATPase active-transport pump at the parietal cells luminal border
This H+ is formed alongside HCO3- within the cell from H20 & CO2 in a reaction catalyzed by carbonic anhydrase
Cl- via secondary active transport driven by the HCO3- concentration gradient which is formed by a Cl-HCO3- antiporter in the basolateral membrane which pumps HCO3- down its gradient into the plasma and Cl- into the cell against its gradient
What are the primary activators of gastric acid secretion? What are the primary inhibitors?
activators: 10th cranial nerve (ACh), gastrin & histamine
inhibitors: secretin & somatostatin
How heavy is the liver?
~2kg
What are some functions of the liver?
bile production, storage of glycogen, detoxification, phagocytosis, synthesis of plasma proteins
What is the function of the gall bladder?
concentrate bile, secrete/resorb materials from bile (small ducts produce bile & refine to an extent then large ducts refine further)
What does the spleen do? Why is it the ‘odd’ spleen?
filter for blood removing old, malformed or damaged red blood cells
The “odd” spleen: 1-3-5-7-9-11. The spleen is 1 inch (thick) by 3 inches (breadth) by 5 inches (length), weighs 7 ounces, and lies between the 9th and 11th ribs.
What is the functional unit of the pancreas? What is the significance of the Isle of Langerhans?
Pancreatic acinar cell: they produce and secrete digestive enzymes
Isle of Langerhans: secrete insulin which increases the absorption of glucose 10-fold
What is the functional unit of the pancreas? What is the significance of the Isle of Langerhans?
Pancreatic acinar cells: they produce and secrete digestive enzymes
Isle of Langerhans: secrete insulin which increases the absorption of glucose 10-fold
How long is the small intestine? What are the 3 components?
~6m
duodenum, jejunum, ileum
What is absorbed in the three sections of the small intestine?
duodenum: no absorption here, chyme simply mixing with bile salts & pancreatic juices
jejunum: absorbs amino acids, iron, fats, carbs & calcium
ileum: absorbs B12 and bile salts for reuse
What is segmentation in the small intestine?
as one segment contracts, the next relaxes allowing material to be pushed through
What is MMC?
migrating motor complex: ‘cleaning’ mechanism which see weak repetitive peristaltic waves initiated by motilin that move a short distance in the small intestine before dying out
What are mixing peristaltic waves and where do they occur?
wringing motion for moving food from the oesophagus to the stomach
Describe the motility of the short intestine
as it contracts it both squeezes and shortens allowing material to move forward
What is the primary function of the large intestine?
reabsorption of water
What are the regions of the large intestine?
cecum, ascending colon, transverse colon, descending colon, sigmoid colon
What are the boundaries of the oral cavity?
vestibule (space in front of closed teeth), oral cavity proper, fauces (arch at the back of the mouth)
What is the purpose of the frenulum?
anchors the tounge, allows speech
What kind of secretions do the salivary glands produce and how much do each contribute to saliva?
parotid glands: secrete serous, contribute 25% to saliva
submandibular glands: secrete a mix of serous and mucus, contribute 70% to saliva
sublingual glands: secrete mucus, contribute 5% to saliva
What does bactericidal mean?
something that kills bacteria (as opposed to stopping it from multiplying as antibacterials do)
Salivary secretions: describe the simple reflex that brings on salivation as compared with a conditioned reflex
simple reflex: pressure receptors and chemoreceptors in the mouth pick up on food and trigger the salivary center in the medulla which triggers autonomic nerves to communicate with the salivary glands to produce more saliva
conditioned reflex: thinking, seeing and smelling food triggers in the cerebral cortex which triggers the salivary center in the medulla which triggers autonomic nerves to communicate with the salivary glands to produce more saliva
What are the three cranial nerves involved in the taste pathway? Where do they travel?
7th (facial), 9th (glosso-pharyngeal) & 10th (vagus)
via the solitary nucleus in the medulla oblongata to the thalamic nucleus to the gustatory cortex
What are the two important cells in the stomach and what do they do?
parietal: located in the fundus & body, secretes hydrochloric acid & intrinsic factor (important for the absorption of B-12)
chief: located in the fundus & body, secretes pepsin
What is the gastrocolic reflex?
The gastrocolic reflex is a physiological reflex that controls the motility of the lower gastrointestinal tract following a meal. As a result of the gastrocolic reflex, the colon has increased motility in response to the stretch of the stomach with the ingestion of food.
Describe the mechanism of HCl secretion
via parietal cells:
H+ secreted into lumen via H+/K+ ATPase pump (concentration gradient of K+ established by K+ leaving the cell via K+ channels)
Cl- secreted into lumen via secondary active transport
Where is iron absorbed in the small intestine?
jejunum
What is CCK (Cholecystokinin) & secretin?
CCK: a peptide hormone that stimulates the gallbladder to contract, causing bile to be secreted into the duodenum
secretin: a hormone that triggers secretion of bile due to stretching
How are carbohydrates absorbed?
Describe fat absorption
Begins with mechanical digestion (chewing)
Bile comes in at the duodenum which assists chemical digestion, creating a lipase emulsion.
Triglycerides and broken down into monoglycerides and fatty acids.
These move into the epithelial cells as micelles via passive discussion.
Micelles form triglycerides which are coated with proteins to form chylomicrons.
Chylomicrons exit cells via exocytosis into interstitial fluid and enter the lymphatic system via central lacteals.
Describe protein absorption
Chemical digestion begins in the stomach with pepsin (10-20% of digestion occurs here)
Rest of digestion happens in the duodenum
A sodium potassium pump at the basolateral membrane of the epithelial cell establishes a concentration gradient which drives transport of sodium + amino acids into the cell
Small peptides enter the epithelial cell alongside hydrogen (which is being pumped out against sodium)
Amino acids move via passive diffusion into the interstitial fluid and into the blood stream
Describe iron absorption
We need 10mg of iron per day in diet (large proportion will not be absorbed)
Absorbable iron is either heme iron or ferrous iron (Fe2+)
Note: ferric iron (Fe3+) cannot be absorbed and must be broken down first
Heme iron enters the epithelial cell via heme protein carried 1
Ferrous iron enters the epithelial cell via divalent metal transporter 1
Iron then moves into the plasma bound to transferrin via the basolateral cell membrane through ferriportin carrier
Any iron not needed will be stored as ferritin
Describe what is happening in the pancreatic acinar cells
Sodium bicarbonate symporter on the basolateral membrane moves sodium & bicarbonate from plasma through the pancreatic duct cell into the pancreatic duct lumen
Sodium hydrogen exchanger on the basolateral membrane moves hydrogen out and sodium in to the pancreatic duct cell
Sodium potassium ATPase channel on the basolateral membrane moves sodium out and potassium into the pancreatic duct cell
Potassium is being pumped out of the pancreatic duct cell back into the plasma via potassium channels on the basolateral membrane to be recycled by the ATPase channel
Bicarbonate moves from the pancreatic duct cell into the lumen while chloride moves into the cell. The chloride is then manipulated by a cystic fibrosis transmembrane regulator which moves chloride back into the lumen. This brings sodium and water into the lumen as well.
Sodium also enters the pancreatic duct lumen via paracellular transport through leaky tight junctions between pancreatic cells
What are the segments of the large intestine called? What about the strip of muscle that runs the length of the large intestine?
Haustrums
teania coli
What is the function of the large intestine?
To reabsorb water and store fecal matter
Enteric nervous system controls what reflexes?
Gastrocolic reflex: reacts to food (stretch receptors) and increases motility in the GI tract
Othocolic reflex: standing upright first thing in the morning stimulate muscle contractions in the large intestine
List the 4 steps of defaecation
- Colonic distension (stretch receptors trigger sensory nerve fibres)
- Parasympathetic reflex (a spinal reflex which stimulates contraction of the rectum and relaxation of the internal anal sphincter)
- Reinforcement (voluntary motor neurones are inhibited)
- Sphincter relaxation (external sphincter relaxes allowing defaecation)
How much water is reabsorbed in the colon per day?
6L, most within the first half of the large intestine
How is water reabsorbed in the large intestine?
Sodium enters colonic cells (colonocytes) via multiple pathways
The sodium potassium ATPase pumps sodium into the interstitial fluid
Water follows sodium
What is the role of bacteria in the GIT?
To cause fermentation (hence if there are too many, too much gas will be produced)
Produce vitamin B & K
Recycle bile
Prevent growth of pathogenic bacteria
What are the two types of bacteria responsible for nutrient breakdown in the large intestine?
Bacteroides & escherichia
What is ulcerative colitis, which part of the GI tract does it affect and how does it present?
a chronic relapsing inflammatory bowel disease with symptoms of diarrhea and no known cure
affects the colon & rectum and can be either genetic or autoimmune in origin
seen in younger age groups (15-30 years)
presents with ulcerations in the mucosa layer of the large intestine
Describe the aetiology of ulcerative colitis/inflammatory bowel disease.
may be due to:
-a diet high in processed meats/food due to carcinogenic properties of these foods
-repeated infections in the GIT where pro-inflammatory mediators encourage an influx of macrophages and other immune cells which lead to inflammation
at a cellular level:
-unknown factor changes the viscosity of the mucous layer of the colonic lumen and increases the permeability to bacteria
- once the mucous layer is impaired, dendritic cells are exposed to the lumen and will be activated by antigens within the lumen
- activated dendritic cells generate cascades leading to the destruction of the mucosal cells and eventually the mucosal wall
- activation of dendritic cells, macrophages & T cells triggers an influx of pro-inflammatory markers
- inflammation increases the permeability of the epithelial barrier to bacteria, increasing this response more and more
What is Crohn’s disease, which part of the GI tract does is affect and how does it present?
an inflammatory bowel disease with an unknown cause (could possibly be genetic, from tobacco use or living in an urban residence)
affects multiple segments of the GIT spanning from the mouth to the anus
presents as:
fat wrapping over the taenia coli, muscle hypertrophy, fissures in the lining, and cobblestone appearance in the lumen
How is inflammatory bowel disease managed?
management of symptoms as there is no known cure
through medications for fever, muscle spasms, diarrhoea, constipation
immunosuppressants
nicotine (in a controlled environment)
Which cranial nerve supplies parasympathetic fibers to the digestive system?
vagus nerve
Which enzyme secreted by the salivary glands helps digest fats?
lipase
Salivary gland centres are located in which part of the brainstem?
medulla
Gastric acid secretion by a parietal cell requires efflux of … ions into the gastric lumen and influx of … ions into the cell.
efflux of hydrogen and influx of potassium
If the ileum was to be removed, which vitamin deficiency would result?
vitamin B12
