Gastrointestinal System Flashcards
what are the structures of the GI tract?
- oesophagus
- stomach
- small intestine
- large intestine
- anus
other accessory organs such as the gall bladder
what is the function of the GI tract?
- digestion and absorption of nutrients
how does the GI tract digest and absorb food?
1 motility to propel ingested food
- secretions from associated glands
- digestion/hydrolysis into absorbable molecules
- absorption into bloodstream of nutrients, electrolytes and water
what is contained in the structure of the GI wall?
- Lumen: where food is in the centre
- Mucosal layer
- submucosal layer
- muscularis externa: circular and longitudinal muscle layers
- serosa
- submucosal plexus and the myenteric plexus
what is the mucosal layer comprised of?
- epithelial layer for secretory/absorption function
- lamina propria made of connective tissue containing blood and lymph vessels
- muscularis mucosae: layer of smooth muscle cells which contract to change shape of epithelial layer
what is the submucosal layer made up of?
- collagen for structural integrity
- elastin for expansion and recoil of tract
- glands for secretions
- blood vessels for transport and removal of metabolites
what are the circular and longitudinal muscle layers?
muscularis externa:
- important for motility
- sandwiched between submucosa and mucosa
circular: thick and densely innervated with nerves
- contraction causes diameter of tube to decrease
longitudinal: thin with few nerve fibres
- contraction causes an increase in segment length
what is the serosa and what is it comprised of?
- outermost layer
- made of connective tissue
- can be surrounded by mesothelium to reduce friction during digestive movements
what are the 2 main plexuses of the GI tract?
- submucosal plexus:
- underneath mucosa
- between muscularis mucosae and circular muscle - myenteric plexus:
- between circular muscle and longitudinal muscle
what is the enteric nervous system (ENS)?
- branch of the ANS so is involuntary
- innervates the GI tract between the layers
- collection of nerve plexuses surrounding the GI tract, including the pancreas and biliary system (gall bladder and liver for bile)
- more than 100 million neurons (more than spinal cord)
- cells are connected to axons and dendrites
- secretes many chemicals
- modified by the brain and solely affects the GI tract
how is the GI regulated by the ANS?
- extrinsic: sympathetic and parasympathetic
- vagus nerve and pelvic nerve - intrinsic: ENS
- primary mechanism
- ganglia with submucosal and myenteric plexuses
ENS can direct all function of the GI tract, even in absence of extrinsic innervation
what is the parasympathetic innervation of the GI tract?
- vagus/pelvic nerve
- parasympathetic postganglionic neurons release ACh (cholinergic) or peptides (P or VIP)
- parasympathetic ganglia with plexuses coordinate info received from PNS and relay to smooth muscle, endocrine and secretory cells
what is the sympathetic innervation of the GI tract?
- sympathetic postganglionic fibres release NA (adrenergic)
- nerve fibres are mixed afferent and efferent
- sensory and motor info is relayed between GI tract and CNS, coordinated by plexuses
what is the role of GI regulatory substances/secretions?
- regulate contraction/relaxation of cells and sphincters
- secretion of fluids, enzymes, electrolytes
- stimulation of tissue growth
- can also regulate secretion of other GI peptides
what are the 3 classifications of GI peptides?
- Hormones
- paracrines
- neurocrines
what are the characteristics of hormones in the GI tract?
- secreted from GI endocrine cells e.g. GIP
- secreted into portal circulation, then liver, then systemic circulation to act on a target cell with the appropriate receptor
- endocrine cells exist as single cells or groups dispersed over the mucosal layer
what are the characteristics of paracrines in the GI tract?
- secreted from endocrine cells e.g. somatostatin and histamine
- act locally on the same tissues that secrete them by passing through interstitial fluid or travelling short distances through capillaries
what are the characteristics of neurocrines in the GI tract?
- released from neurons following an AP
- includes nitric oxide, ACh, VIP, NA and GRP
what enables the motility of the GI tract?
- most contractile tissue is the unitary smooth muscle (involuntary)
- cells are electrically coupled via gap junctions
- rapid spread of APs leading to coordinated contraction
what are the two kinds of contractions during GI motility?
- phasic: period of contraction and relaxation
- 3-12 per minute depending on the section of GI tract
- APs cause stronger contractions - tonic: constant level of contraction/tone
- subthreshold slow waves produce weak contraction
what are the slow waves of GI contraction?
- subthreshold membrane depolarisation via influx of Ca2+
- repolarisation via outflow of K+
- oscillating phases of membrane voltage in subthreshold range
- low frequency and several cycles per minute
what controls the contraction of the GI tract?
GI pacemaker interstitial cells of Cajal (ICC)
- extension to threshold generates AP
- contraction is preceded by electrical activity
what structures are involved in GI tract motility?
- low pressure organs are separated by sphincters
- sphincters are specialised circular muscles with positive resting pressure
- there are 6 sphincters and the sphincter of Oddi
- can contract and act as barriers to flow if there is a positive pressure at sphincter relative to adjacent organs
- regulate anterograde and retrograde movement
what are the structures of the mouth?
- teeth and tongue mechanically breakdown food
- saliva is mixed to produce a bolus food which is lubricated for swallowing
- saliva contains alpha-amylase to breakdown starch
- amylase is released from parotid, submandibular and sublingual glands
- mastication is innervated by mandibular nerve (V3) of CN V (trigeminal nerve)
- contains mechanoreceptors which relay sensory info to brain stem
what are the 3 phases of swallowing?
- oral: tongue forces bolus towards the pharynx
- area is highly populated with somatosensory receptors which signal to medulla which stimulates the pharyngeal - pharyngeal: when soft palette is pulled upwards and epiglottis covers the opening of larynx
- upper oesophageal sphincter relaxes
- allows food to pass into oesophagus - oesophageal
what is the structure of the oesophagus?
- lumen is lined with stratified squamous epithelia
- top layers are easily removed through abrasion by food to prevent damage to underlying tissue
- lower oesophageal sphincter opening is mediated by vagus nerve releasing VIP neurotransmitter
what is the swallowing reflex?
- closing of the oesophageal sphincter
- initiates primary peristaltic wave which causes coordinated contractions
- moves bolus down the oesophagus
what happens when some bolus gets stuck in the oesophagus?
- secondary peristaltic wave is triggered by ENS
- starts from point of distension (not from top of oesophagus)
what are the key areas of the stomach?
- fundus: at the top
- body
- antrum
- pylorus: where stomach leads into small intestine (pyloric sphincter)
- orad region: fundus and top of body
- caudad region: bottom of body and antrum
what are the 3 phases of motility in the stomach?
- receptive relaxation in thin-walled Orad region of stomach to receive food
- 3 muscular layers of caudad region contract to mix food with gastric juice to form chyme
- gastric emptying through pyloric sphincter into small intestine
what occurs during the receptive relaxation of the Orad region in stomach motility?
- fundus and Orad relax and expand in size to enable stomach to receive food due to reduced pressure and increased volume
- stomach can accept 1.5L of food
- as soon as bolus enters stomach, orad increases in size and lower oesophageal sphincter closes to prevent backflow
how do the layers of the caudad region form chyme?
- thick walls generate strong contractions to propel food to ileum and mix food
- neuronal input from ENS increases AP frequency so there is stronger contraction
- forms gastric juice
what is in gastric juice?
- H+ ions and HCl to eradicate pathogens in food
- pepsinogen is activated to cleave other digestive enzymes
- intrinsic factor enables absorption of B12
- mucus
what occurs during gastric emptying in the stomach?
- occurs via pyloric sphincter
- fat content and H+ slow rate of emptying
- rate is regulated to allow time for neutralisation of acid in duodenum and digestion/absorption of nutrients
- transit through sphincter cannot produce lots of pressure
- decreased pressure in stomach causes closing of pyloric sphincter
what is the structure of the small intestine?
- lined with epithelial cells
- 6.5cm long
- digestion and absorption of nutrients
- chyme is mixed with digestive enzymes and pancreatic secretions
what are the 3 sections of the small intestine?
- duodenum (20cm long)
- receives pancreatic enzymes and bile from gall bladder
- chyme enters here from the stomach - jejunum (2.5cm long)
- contains villi - ileum (3m long)
- absorbs any nutrients that haven’t been absorbed in the jejunum
- especially absorbs B12 and bile acids
how is the surface area of the small intestine increased?
- by plicae, villi and microvilli
what is the action of the small intestine?
- peristaltic contractions propel the chyme
- segmentation contractions split and expose the chyme to secretions via coordinated actions
- enterochromaffin cells release serotonin: peristaltic reflex
- material not absorbed passes through the ileocaecal sphincter into the caecum of the large intestine
what are the accessory organs of the GI tract?
- pancreas
- liver
- gall bladder
what is the role of the pancreas?
- 1L of exocrine pancreatic secretion into duodenum per day
- solution is rich in bicarbonate, secreted by centroacinar and ductal cells to neutralise HCl from stomach
- enzymes secreted by acinar cells breakdown macromolecules
- parasympathetic stimulation secretion, sympathetic inhibits secretion
what are the secretion phases of the pancreas?
- cephalic: initiated by smell, taste and conditioning
- gastric: initiated by distension of stomach
- intestinal: stimulates 80% of pancreatic secretions
- achieved by chyme stimulating the enteroendocrine cells in the small intestine which release hormones to stimulate the pancreas
what are the roles of the liver and gall bladder?
- liver hepatocytes produce bile and secrete it into gall bladder
- gall bladder stores, concentrates and ejects bile into duodenum
what are the roles of the liver and gall bladder?
- liver hepatocytes produce bile and secrete it into gall bladder
- gall bladder stores, concentrates and ejects bile into duodenum
- epithelial cells absorb ions and water into the bile
what is bile?
- mixture of bile salts, pigments, cholesterol and bilirubin
- amphipathic bile salts are used for emulsification and solubilisation of lipids into micelles
- aids digestion and absorption
how is bile secreted from the gall bladder into the duodenum?
- CCK is secreted by duodenum when chyme reaches the small intestine
- CCK causes ejection of bile from gall bladder within 30 mins of eating a meal
- occurs in response to presence of amino acids and fatty acids
- contracts the gall bladder and relaxes the sphincter of Oddi to allow ejection in a pulsatle manner
- 95% of bile acids are recirculated to liver via enterohepatic circulation
how do small intestine contents enter the large intestine?
- they enter the caecum and the proximal colon
- the ileocaecal sphincter contracts to prevent reflux back into the ileum
what are the 3 primary functions of the large intestine?
- absorption of water and electrolytes (Na+, Cl-, bicarbonate and K+)
- aldosterone increases Na+ absorption - makes and absorbs vitamin K and B due to bacteria
- forming and propelling faeces
- ferments indigestible food by bacteria to be excreted
By the time indigestible material has reached the colon, most nutrients and 90% of water has already been absorbed
what is the structure of the large intestine?
- surface columnar epithelial cells which are absorptive
- epithelia are interspersed with secretory glands e.g. secretes mucus to protect lining
- Taenia Coli: 3 bands of longitudinal muscle
- muscles are shorter than length of large intestine to cause ruching, forming pouches along the length called haustra
how is the large intestine motile?
- segmented contraction in caecum and proximal colon to mix contents in haustra
- mass movement: 1-3L/day over large distance
- called gastrocolic refex
what are the secretions of the mouth?
- alpha-amylase – secreted from 3 parted salivary gland and hydrolyses amylose and amylopectin to maltose
- lingual lipase – breaks down triglycerides to diglycerides and fatty acids, used at low pH so continues into the stomach
- kallikrein
- mucus – lubricates food in mouth to move along tract
what are the secretions of the oesophagus?
- mucus: from submucosal glands
2. bicarbonate: protects against reflux of gastric acid
what are the secretions of the stomach?
- HCl: initiates protein digestion
- pepsinogen: initiates protein digestion
- gastric intrinsic factor (GIF): used for B12 absorption
- mucus: protects stomach lining from corrosive action of HCl and lubricates contents
what are the secretions of the small intestine?
- mucus
2. bicarbonate: to neutralise HCl
what are the secretions of the large intestine?
mucus for protection of luminal lining and transit of stools
what are the secretory cells in the body of the stomach?
Oxyntic glands:
- epithelial cells: secrete bicarbonate
- mucous neck cells: secrete mucus
- parietal/oxyntic cells: secrete HCl and GIF
- chief/peptic cells: secrete pepsinogen
- enterochromaffin cells: secrete histamine and regulate action of parietal cells
what are the secretory cells in the antrum of the stomach?
pyloric glands (deeper pits than oxyntic):
- G cells: secrete gastrin which stimulates secretion of gastric acid by parietal cells
- D cells: secrete somatostatin to suppress release of hormones from GI tract
contains no parietal cells
what is the function of parietal cells in the stomach?
- secretes HCl to acidify gastric contents to pH 1-2
- allows conversion of pepsinogen to pepsin for protein digestion
what occurs on the apical membrane of parietal cells in the stomach?
- hydrogen-potassium ATPase
- chloride channel
- in the intracellular fluid, CO2 combines with water to form carbonic acid, catalysed by carbonic anhydrase
- carbonic acid dissociates into H+ and bicarbonate
- H+ and Cl- are secreted across apical membrane out of the cell into the lumen
- HCl is secreted into the lumen
what occurs on the basolateral membrane of parietal cells in the stomach?
- sodium-potassium ATPase: maintains conc gradient
- bicarbonate-chloride exchanger
- bicarbonate is absorbed into blood by bicarbonate-chloride exchanger
- bicarbonate causes alkaline tide due to high pH in venous blood
secretion of HCl, absorption of bicarnobate
how does stimulation of H+ secretion occur in HCl regulation?
- histamine: paracrine hormone from enterochromaffin cells diffuses to parietal cells
- binds to H2 metabotropic receptors
- causes adenylyl cyclase to upregulate cAMP, activating PKA
- this leads to H+ secretion into the lumen to combine with Cl- and form HCl - ACh: released from Vagus nerve
- innervates M3 metabotropic receptors
- activate IP3 and Ca2+ which stimulate H+ secretion into lumen
- also activates enterochromaffin cells to secrete histamine - gastrin: secreted by G cells and binds to CCKB receptor (acts in same way as ACh)
how is HCl secretion inhibited?
- low pH: inhibits secretion of H+ if there is no need for digestion as chyme no longer buffers the stomach
- any H+ ions secreted would have excessive lowering of pH - somatostatin: secreted by D cells and inhibits H+ secretion by binding to receptors on parietal cells
- coupled to adenylyl cyclase which binds to inhibitory G protein
- inhibits release of histamine and downregulates H+ secretion - prostaglandins: same as somatostatin
how do the small intestine and large intestine have similar functions?
- both absorb fluid and electrolytes via villi/surface epithelia respectively
- crypt cells secrete fluid and electrolytes as protection from bacteria/toxins
- polar cells have apical and basolateral membranes separated by tight junctions
- transport can be transcellular or paracellular
how do the SI and LI differ?
- SI also absorbs hydrolysed food after digestion via luminal and brush-border enzymes
- SI have larger surface area due to containing villi
- LI doesn’t contain any villi
how does secretion occur across the apical and basolateral membranes of the intestines?
Apical:
1. chloride channels: Cl- diffuses into lumen from cell via the cotransporter on basolateral side bringing Cl- into the cell
basolateral:
- sodium-potassium-chloride transporter brings Na+, K+ and Cl- into cell from blood
- sodium-potassium pump maintains conc gradient
- hormones/neurotransmitters bind to basolateral side and activate cAMP which opens chloride channel on apical side
water and Na+ diffuse from blood to lumen paracellularly
how does absorption across the jejunum occur?
Apical:
- sodium contransport symporter: Na+ enters via cotransport with glucose/amino acids
- sodium-hydrogen exchanger: driven by CO2 and water combining to form carbonic acid, which dissociates to H+ and bicarbonate
- H+ is secreted into lumen
basolateral:
1. sodium-potassium ATPase: maintains Na+ electrochemical gradient to aid Na+ cotransporter on apical side
how does absorption occur across the ileum?
Apical:
- chloride-bicarbonate exchanger: when H+ and bicarbonate is generated intracellularly, H+ is secreted into lumen by sodium-hydrogen exchanger
- bicarbonate is secreted into lumen
- chloride enters cell
basolateral:
1. chloride transporter: absorption of chloride into blood
net absorption of NaCl
what is secreted during pancreatic secretion?
- secretion of bicarbonate into small intestine to neutralise HCl
- enzymes are secreted to hydrolyse macromolecules
- acinar cells secrete enzymes and have receptors for CCK
- centroacinar cells secrete isotonic aqueous component
how does secretion occur across the apical and basolateral membranes of the pancreatic cells?
apical:
- chloride-bicarbonate exchanger
- bicarbonate is secreted into pancreatic juice into the lumen
- Cl- is moved into the cell
basolateral:
- sodium-hydrogen exchanger
- allows H+ to be absorbed into the blood to acidify it
carbonic acid is split in the ICF to bicarbonate and H+ by carbonic anhydrase
how does absorption occur in the large intestine?
Apical:
- Sodium channel: Na+ moves into cell, induced by aldosterone
- potassium channel: K+ secretion into lumen
- aldosterone increases Na+ absorption and K+ secretion as it forms more channels
Basolateral:
- Sodium-potassium ATPase:
- increase of K+ inside the cell which is then secreted across apical membrane
- absorption of Na+ into blood
how are digestive products absorbed:
occurs across the brush-border of the jejunum and ileum:
- Carbohydrates: alpha-amylase, alpha-dextrinase, maltase, sucrase, trehalase, lactase
- products absorbed into villus blood - proteases: pepsin, trypsin, chymotrypsin elastase, carboxypeptidases
- products absorbed into villus blood - lipases and bile salts
- products into lacteals within villus
brush-border is highly folded so has large SA
what is the process of carbohydrate absorption?
glucose, galactose and fructose are end-products
- glucose absorbed by sodium-dependent cotransport or facilitated diffusion
- Na+ and galactose moves across apical via SGLT1 against conc gradient due to low intracellular Na+ by Na-K ATPase on basolateral side
- fructose enters cell via faciliated diffusion on apical membrane via GLUT5 and through basolateral membrane by GLUT2
- GLUT2 allows diffusion of glucose and galactose from high to low conc
what is the process of protein absorption?
- digested as amino acids, dipeptides and tripeptides
- amino acids absorbed by sodium-amino transporter driven by Na+ electrochemical gradient formed by Na-K ATPase
- amino acids diffuse into blood via facilitated diffusion
what is the process of lipid absorption?
- pancreatic lipase, colipase, milk lipase, aided by bile salts, completes lipid hydrolysis in duodenum and jejunum
- produces cholesterol, lysophospholipids and monoglycerides
- free fatty acids are solubilised in micelles
- micelle exterior is lined with amphipathic bile salts and are absorbed across luminal surface of the enterocyte
- products are re-esterfied with free fatty acids in SER and packaged into chylomicrons
- they cross basolateral membrane and absorbed into lacteals
