Gastrointestinal System Flashcards
What are the layers of the GIT? (4)
- Mucosa
- Submucosa
- Muscularis externa
- Serosal layer
Subsections of the mucosa (3)
- Epithelium: thin layer of polarized cells that line all body cavities and surfaces. Functions for the selective uptake of nutrients and water, also keeps things out.
- Lamina propria: Everything above muscle layer. Has connective tissues and all vessels and nerve fibers.
- Muscularis mucosa: thin layer of smooth muscle. Functions in moving the villi, but is NOT involved with GIT contraction
The 2 pathways that molecules can use for epithelial level movement
- Paracellular pathway: movement between cells across cell junctions. Limited by tight junctions, therefore only water and small ions can diffuse
- Transcellular pathway: cross the cell and require transport proteins
Submucosa
- layer beneath the mucosa layer.
- contains connective tissue, blood vessels, lymph vessels, and submucosal nerve plexus
Muscularis externa
Contains:
- circular muscle: contract and relax to open the tube
- myenteric nerve plexus: regulate muscle function of GIT
- longitudinal muscle: lengthen and shorten to control length of the tube (does not change diameter)
Serosa
Connective tissue layer that encases the intestine and forms connections with the intestine and abdominal wall
Portal circulation
- circulation that carries the blood from the intestinal tract to the liver
- blood here is low in oxygen, but nutrient rich
What ae the reflexes regulating GI processes initiated by? (4)
- Distension of the GIT wall by the volume of luminal contents
- Osmolarity of the contents
- pH of the contents
- The concentrations of the specific digestion contents
Enteric nervous system
- intrinsic nerve regulation
- controls the activity of the secretomotor neurons which play a role in secretion and motility
- can function independently of the CNS
- myenteric plexus is responsible for regulating smooth muscle, submucosal plexus is responsible for secretions
Categories of chemical messenger regulation (4)
- endocrine regulation: hormone secretion across a gland’s basolateral surface.
- Neurocrine regulation: neurons and electrical signals.
- Paracrine regulation: local cell releases paracrine substance that diffuses through interstitial fluid and acts on another cell in close proximity. Happens across the apical surface and into lumen of the gland.
- Autocrine: local cell releases a substance that acts on the cell that released it.
Endocrine cells
- produce hormones that control GI functions through blood vessels in the lamina propria
- found in epithelium of the stomach and small intestine
Peristalsis
- main driving force for food moving through intestinal tract
- circular muscle contracts on the oral side of a bolus of food (Longitudinal layer relaxes)
- circular muscle contracted moves toward the anus, propelling the contents of the lumen in that direction
- as the ring moves, the circular muscle on the other side relaxes, facilitating passage of the bolus
Segmentation
- important for the mixing of food
- involves contraction and relaxation of intestinal segments with very little net movement of the food towards the large intestine
- occurs mostly in small intestine
- allows the mixing of contents with digestive enzymes
- slows transmit time to allow for the absorption of water and nutrients
Pacemaker cells
- cells distributed throughout the smooth muscle cells that constantly undergo spontaneous depolarization-repolarization cycles called slow waves
Slow waves
- give the GIT the basic electrical rhythm
- propagated through the circular and longitudinal muscle layers through gap junctions
- they do not result in any contraction when there is no neural or hormonal input. When there is a stimulus, further depolarization occurs enough to reach the threshold and muscle can contract
- the number of action potentials fired is proportional to the force of conraction
Phases of gastrointestinal neural and hormonal control (3)
- Cephalic (head): initiated by sight, smell, taste, chewing, emotional state stimuli.
- Gastric (stomach): initiated by distension/stretching stimulation, acidity, amino acids, and peptides
- Intestinal: initiated by distension, acidity, osmolarity, and digestive products stimulation
Feeding centre
- found in hypothalamus
found in the lateral region - activation increases hunger
Satiety cener
- found in hypothalamus
- found in ventromedial region
- activation makes you feel full
Orexigenic factors
- factors that increase food intake
- neuropeptide Y in the hypothalamus
- Ghrelin from endocrine cells in stomach (travels to hypothalamus to stimulate release of neuropeptide Y)
Anorexigenic factors
- factors that decrease food intake
- Leptin: produced by adipose/fat tissue
- Insulin: produced by pancreas
- peptide YY: released from intestine
- Melanocortin: released from hypothalamus
Thirst center and what it is stimulated by (4)
- increased plasma osmolarity: stimulates release of vasopressin or anti-diuretic hormone to conserve water in kidneys
- Decreased plasma volume: from pathophysiological conditions (vomiting, large blood loss, diarrhea), will activate renin-angiotensin system
- Dry mouth and throat
- Prevention of over-hydration: mediated by stimulus from mouth, throat, and GIT
Renin-angiotensin system
- activated in the kidneys when Juxtaglomerular cells (act as pressure receptors) sense low blood pressure
- will produce angiotensin II, which has a direct effect on the hypothalamus and increases thirst
Main pairs of salivary glands (3)
- Parotid: watery/serous secretion
- Submandibular: serous/mucous secretion
- Sublingual: Mucous secreation
Salivary gland composition (3)
- acinar cells: secrete initial saliva
- Ductal cells: create the alkaline and hypotonic nature of saliva
- Myoepithelial cells: have characteristics of both smooth muscle (can contract) and epithelial cells (it contracts to move components of saliva towards striated ducts from acinus duct)
Sjogrens syndrome
autoimmune disease that destroys the salivary glands
Xerostomia
- Dry mouth syndrome due to impairment of salivary secretion
- causes decrease in oral pH which causes tooth decay and esophageal erosions
- treated via water and fluoride treatment to combat the microbial poplations
Swallowing
- series of relfexes inistiated by pressure receptors in the pharynx
- receptors are stimulated by food or liquid entering phaynx and sends signal to swallowing centre in brainstem to signal muscles
esophagus structure
- skeletal muscle in top 1/3
- smooth muscle in lower 2/3
stratified squamous epithelium (flattened cells to protect underlying regions of esophagus from food) - upper and lower sphincters which are closed except for when swallowing, eating, vomiting, burping
- no absorption
- mucus is secreted for lubrication
Heart burn
Occurs due to lower esophageal sphincter does not closer properly, after a big meal, or during pregnancy
Components of the stomach (3)
- Fundus and body (upper part of stomach, have thin layer of smooth muscle, secret mucus, pepsinogen, and HCL)
- Antrum (lower region, thicker smooth muscle, physical breakdown and grinding of food, secrets mucus, pepsinogen and gastrin)
- Pyloric sphincter (controls emptying of stomach)
Exocrine vs endocrine
- exocrine: chemical messenger secreted into ducts and onto epithelial surface without passing into blood
- endocrine: requires passage through blood
Major exocrine secretions of stomach
mucus, HCl, pepsinogen
Minor secretions of the stomach (4)
- intrinsic factor for Vitamin B12 absorption
- Gastrin (endocrine): hormone for stimulating HCl production and increasing stomach motility
- Histamine (paracrine): stimulates HCl production
- Somatostatin (paracrine): inhibits HCl production
Generalized gastric gland components (6)
- mucous cell: protect stomach lining
- parietal cell (oxyntic cell): has canaliculi to increase surface area, secretes intrinsic factor and HCl in body and fundus of stomach
- chief cell: secretes pepsinogen
- ECL cell: secretes histamine in all regions
- D cell: found in all regions, secretes somatostatin
- enteroendocrine cell: found in antrum, secretes gastrin
carbonic anhydrase
- mechanism for ensuring parietal cells in stomach don’t become basic when they make stomach lumen acidic
- H2CO3 is formed from H2O and CO2, then H2CO3 is dissociated into H+ and HCO3- and is secreted out of cell in a Cl-/HCO3- exchanger pump (secondary active transport)
Chemical messengers that regulate the insertion of H+/K+ ATPase into plasma membrane of parietal cell (makes stomach more acidic) (4)
- Gastrin: hormone released by G cells, stimulates HCL release
- Acetylcholine: neurotransmitter via increased parasympathetic activity, increases acid production
- Histamine: paracrine via ECL cell, stimulates acid production
- Somatostatin: paracrine from D cells, inhibits HCl, gastrin, and histamine
Messengers that have an indirect effect on acid secretion in parietal cells (2)
- Ach: stimulates ECL cells to release histamine, inhibits somatostatin production from D cells, stimulates G cells to produce gastrin
- Gastrin: stimulates ECL cells to release histamine
Location of the vomiting center?
Medulla oblongata
Metabolic alkalosis
A condition in which the pH of a tissue is elevated beyond the normal range, due to the loss of acid from the stomach after vomiting has occurred
Peptic ulcers
- damage to or erosion of the GIT mucosa
- occurs in regions which are acidic, like esophagus, stomach, or duodenum
- most common cause is infection from H pylori bacteria which results in inflammation and irritation
Endocrine and Exocrine parts of pancreas
- endocrine: important for some body hormones like insulin (not involved in digestion)
- exocrine: produces secretions into the GIT like enzymes for digestion of carbs, proteins, fats, nucleic acids. Also secretes bicarbonate into duodenum for neutralization of stomach acid (pancreatic enzymes are not functional/inactive under acidic pH)
When are problems with digestion and absorption noticed in pancreas?
When function falls below 10%
Sphincter of Oddi (hepatopancreatic)
- Sphincter common to the bile duct and to the main pancreatic duct
- regulates release of liver and pancreatic content into the SIP
Pancreatic islets (Islets of Langerhans)
- endocrine pancreas
- produce the hormone insulin and secretes it into blood
Pancreatic duct cells (2)
- Acinar cells: end portion of the duct, secretes digestive enzymes via exocytosis
- Ductal cells: secretes bicarbonate for neutralization, water is also secreted
CFTR (custic fibrosis transmembrane conductane regulator)
- Cl- channel in pancreas ductal epithelium cells
- Allow Cl- out of the duct cell in to the lumen, which is exchanged for HCO3-
Alkaline tide
Whatever is moving in the blood, so in the case of the stomach, it is the large amount of bicarbonate that is pumped across the basolateral surface into the blood
Acid tide
The large amounts of acid that are being pumped across the basolateral surface into the blood stream from the pancreas
Zymogens
- proenzymes or inactive precursor enzymes
- packaged by pancreas acinar cells to be stored at the apical pole until stimuli causes exocytosis
Enterokinase
- enzyme that is attached to the apical or luminal surface of epithelial cells in duodenum
- cleaves a pro-protease called trypsinogen into the protease trypsin to activate other enzymes
Endopeptidases (3)
- hydrolyze interior peptide
bonds of proteins and polypeptides
1. Trypsinogen: activated by enterokinase
2. Chymotrypsinogen: activated by trypsin
3. Pro-elastase: activated by trypsin
Exopeptidase
- hydrolyze bonds at C-terminal end
- Pro-carboxypeptidase A and B: activated by trypsin
S cells
- found in the epithelial of the duodenum
- stimulated by acid from the stomach and produces secretin
Secretin
hormone released into the blood by the duodenum S cells and find their way to the pancreatic duct cells to stimulate release of HCO3-
I cells
- found in the epithelium of the duodenum
- digest fats and proteins entering the upper SI from the stomach
- release cholecystokinin (CCK)
CCK
- released into the blood by the duodenum I cells
- acts on the acinar cells in the pancreatic duct to stimulate the zymogen granules to release their digestive enzymes
Cystic fibrosis
- a defective chloride channel in the pancreas
- pancreatic insufficiency
- HCO3- and water secretion becomes minimal
- autodigestion of the pancreas
- patients need supplements of digestive enzymes and antacids
Hepatic lobule
- functional unit of the liver
- hexagonal structure with a central vein running through the centre and a portal tried in each corned
Portal triad
-in the liver, consists of a hepatic artery, hepatic portal vein, and bile duct
Hepatocytes
- epithelial cells of the liver
- form tube like structures called canalicular networks that conduct bile produced by the hepatocytes
- they join together to form bile ducts
components of bile (6)
- bile acids (made within hepatocytes from cholesterol)
- cholesterol
- salts and water
- phospholipids
- bile pigments (bilirubin from heme)
- trace metals
Emulsification
- bile making lipid droplets smaller in order for lipase from the pancreas to be able to digest the lipids
- needs amphipathic bile acids and phospholipids which are found in bile
gallbladder importance
storing and concentrating bile between meals and releasing it when chyme enters the duodenum
Enterohepatic circulation
Recycles bile acids
Where are bile acids reabsorbed for recycling?
In the ileum of the small intestine
Gallstones
- caused by excess cholesterol
- cholesterol precipitating out of bile acids when it becomes too much
pigment stones
- when there is excess red blood cell breakdown (hemolysis), the amount of bile pigments are increased
- bile pigments precipitates with calcium
Cholecystectomy
- removal of the gallbladder to treat gallstones
- consequence is less ability to release the bile components in time of need
folds of kerckring
circular folds covered in villus in the small intestine to increase surface area and absorption
Crypt regions
Projections of folds in the opposite direction
potential cells derived from stem cells in the crypts (4)
- Absorptive cells (enterocytes): has a microvili brush border membrane
- Goblet cells: secretion of mucus for lubrication and protection from acid
- Enteroendocrine cells: hormone producing I and S cells
- Paneth cells: secrete antibacterial peptides to protect from bacteria
Chylomicrons
Extracellular fat droplets that are absorbed by the lymphatic system lacteals
migrating myoelectric complex (MMC)
- The peristaltic activity that moves food from the stomach to the SI, then dies out, then picks up again from the SI to the anus
- prevent bacteria from remaining in the SI
Motilin
- intestinal hormone
- initiates the MMC
- when you eat, motilin is inhibited to allow for segmentation to occur
Cholera
- occurs after eating food or drinking water contaminated with bacteria Vibrio cholera
- causes vomiting and excessive diarrhea
- causes an increase in cAMP production, which stimulates Cl- channels in the SI to open, allowing water to be lost as diarrhea
- treatment is to consume clean water with salts and glucose to replace fluids
Ileocecal valve
sphincter between the ileum and the LI
Large intestine functional domains
- appendix/cecum: no apparent function in humans
- ascending/transverse/descending/sigmoid colon: re-absorption of water, reservoir for waste storage and undigested materials prior to elimination, absorb products of bacterial metabolism
- rectum: reservoir for feces
- anus: internal anal sphincter (smooth muscle, involuntary), and external anal sphincter (skeletal muscle/voluntary control)
Epithelial stem cells of the LI (3)
- Absorptive (enterocytes): same as SI, but do not contain brush border enzymes
- Goblet cells (abundant)
- Endocrine calls and Paneth cells (very few)
