processes in GI tract Flashcards
functions of motility in GI tract
Propulsion - at a controlled rate
Mechanical digestion - increase SA
Mixing - food, secretions and enzymes
Exposure to absorptive surfaces -> intimate contact
Basic electrical rhythm (slow wave)
- These are spontaneous variations in membrane potential (pacemakers)
- If they surpass threshold, we get action potentials and therefore contraction of smooth muscle
regulation of motility
- BER determines the frequency of contractions
- Strength of contraction regulated by nervous and hormonal reflexes
- depolarise or hyperpolarise RMP to make it easier or harder to get to threshold
role of enteric nervous system
- determines type of motility that occurs: peristalsis or segmentation
- modified by stretch, nerves, hormones, loacl feedback loops
fasting motility pattern
- migrating motor complex
- coordinated movement from stomach to SI to LI for 4-5 hours after a meal
- repeated every 2 hours
- 3 period: inactivity, intermittent activity, intense activity
- function = housekeeping
feeding motility patterns in GI tract
- storage
- peristalsis
- segmentation
features of storage
Mainly stomach
Relaxation of smooth muscle (-> distension)
features of peristalsis
Esophagus, stomach, small & large intestine
Propulsive
Proximal squeeze, distal relax
features of segmentation
Small and large intestine
Mixing & exposure to absorptive surfaces - churning and squishing
Alternate contraction and relaxation of circular muscle
function of swallowing (deglutition)
rapid transfer of material from mouth to stomach via oesophagus
function of chewing (mastication)
Reduce size of food
Mix food with saliva
Taste
motility in esophagus
- Oesophageal peristalsis transfers food from the mouth (following mastication and deglutition) to the stomach
- May or may not be aided by gravity (i.e. still happens when we’re upside down)
- Oesophageal sphincters prevent backflow of food
control of chewing
- Under both voluntary and reflex control
- We have to voluntarily masticate (skeletal muscle)
- Reflexes control the strength, frequency, occlusion (how the teeth match up during biting), and side of chewing
control of swallowing
- Initiated voluntarily but proceeds via reflexes (involuntary)
- Involves the coordination of multiple muscles in order to pass the food to the stomach, whilst still protecting the airway
fasting stage of gastric motility
shrinks to small volume, MMC
fed stage of gastric motility
- storage in proximal stomach
- peristalsis
- controlled delivery of chyme to the duodenum
storage in stomach
- nervous reflex initiated by swallowing
- relaxation of proximal gastric smooth muscle leads to increase in volume - distension of rugae
peristalsis in stomach
- initiated on greater curvature that spread to antrum
- propulsion moves food down to antrum
- retropulsion moves food back from pyloric sphincter - helps mix food with secretions, and also mechanical digestion
controlled delivery of chyme to duodenum
- rate depends on composition of food (solids slower than liquids, fats move slowly)
- gastric inhibitory polypeptide (hormone) and enterogastric reflex (neural) are feedback from duodenum that influence gastric emptying
functions of small intestine motility
- Facilitate the chemical digestion of food
- Facilitate the absorption of nutrients, salts, and water
- Mix chyme with intestinal secretions
- Exposure chyme to absorptive surfaces
- Propel chyme along the GI tract
fed small intestine motility
Mainly segmentation to aid in contact digestion and mixing of chyme with digestive enzymes and other chemicals
Also peristalsis to propel chyme through the small intestine
functions of large intestine motility
Temporary storage of faeces
Regulation of the salt and water content of faeces
motility patterns in large intestine
Inactivity
Segmentation to aid in mixing and slow propulsion
Peristalsis to move masses of faeces into the rectum for defecation. Occurs 3-4 times per day
what is chemical digestion
chemical hydrolysis of food by digestive enzymes into molecules that can cross epithelial lining of GI tract
functions of secretions in GI tract
digest food
dilute food
maintain optimal pH
protect and lubricate GI tract
features of digestive enzymes
extracellular
organic catalysts
display specificity (i.e. they will only act on a specific substrate).
Function optimally at a specific pH (different for different enzymes)
two stages of chemical digestion
- luminal digestion - initial digestion of food in the lumen of the GI tract
- contact digestion - completion of chemical digestion by brush border enzymes in the small intestine
luminal enzymes secreted by salivary glands, chief cells of stomach and acinar cells of pancreas
salivary glands: amylase
chief cells of stomach: pepsinogen
acinar cells of pancreas: trypsinogen, chymotrypsinogen, lipase and amylse
composition of carbohydrates
composed of chains of monosaccharides
main polysaccharides in our diet are starch and glycogen - joined by alpha-1,4-glycosidic bonds
a lof of cellulose in in our diet which we cant digest because our digestive enzymes cannot cleave beta-1,4-glycosidic bonds
some disaccharides such as sucrose, lactose and maltose
luminal digestion of carbohydrates
Polysaccharides are converted to disaccharides by salivary and pancreatic amylase (in lumen)
contact digestion of disaccharides
Disaccharides are converted to monosaccharides by substrate-specific enzymes (on brushborder)
Sucrose is broken down by sucrase
Lactose is broken down by lactase
Maltose is broken down by maltase
composition of proteins
exogenous (ones we ingest) and endogenous (ones we produce) proteins that the intestine digests
long chains of amino acids joined by peptide bonds
approximately 10 essentials amino acids
luminal digestion of proteins
Proteins are converted to polypeptides by a number of luminal enzymes
Pepsin in the stomach
Trypsin and chymotrypsin in the small intestine (but secreted by the pancreas as inactive precursors - trypsinogen and chymotrypsinogen)
contact digestion of polypeptides
Converts polypeptides into individual amino acids by numerous peptidases attached to the brush-border of the small intestine
composition of lipids
composed mainly of triglycerides which are composed of a glycerol backbone with 3 fatty acids attached by ester bond
contain fat soluble vitamins
fatty acids have variable chain length
type of digestion of lipids and why is this the case
only digested by luminal digestion because of their insolubility in water so must be dissolved in luminal fluid
4 stages of lipid chemical digestion
- Emulsification - motility
- Stabilisation - bile
- Hydrolysis - lipase
- Formation of micelles
emulsification of lipids
must be broken down into small uniform particles
motility separates large fat droplets into smaller droplets to increase SA for further digestion
retropulsion in stomach and segmentation in SI
stabilisation of lipids
lecithin and bile salts secreted in bile stabilise the emulsion droplets because they are amphipathic (have hydrophobic and hydrophilic part)
increases SA even more as allows formation of even smaller droplets
occurs in SI
production and secretion of bile salts and lecithin
produced in the liver, stored and secreted by the gallbladder
hydrolysis of lipids
occurs in SI lumen at emulsion droplet surface
involves lipase and colipase (cofactor) which are both secreted by pancreas
colipase anchors to the surface of emulsion droplet
Lipase converts triglycerides into monoglyceride and two free fatty acids
micelle formation of lipids
Because products of fat digestion (fatty acids and monoglycerides) are insoluble in water, they are kept in solution by the formation of micelles (i.e. they don’t reform large insoluble fatty clumps)
micelle
small droplets (4-6nm) that are composed of amphipathic compounds (bile salts and lecithin), fatty acids, and monoglycerides
2 stage process of saliva production
- Primary secretion: isotonic, serous solution produced by acinar cells
- Hypotonic saliva: reabsorption of various molecules (including Na+ and Cl-) in the salivary ducts to produce the final hypotonic saliva
volume and rate of saliva secretion
volume = 1.5L/day
basal rate = 0.3 mL/min
stimulated rate = 1.5 mL/min
composition of saliva
mucus for lubrication
dilute solution of NaHCO3 and NaCl to aid in dilution and maintain optimum pH
enzymes including alpha-amylase and lipase
regulation of saliva secretion
autonomic nervous control in response to thought, smell and sight of food, as well as chewing
Parasympathetic NS: acetylcholine produces large amounts of saliva
Sympathetic NS: adrenaline produces small volumes of viscous saliva
functions of saliva
lubrication to aid in mastication and deglutition
hygiene - irrigates mucosa
digestion - dissolves food to allow taste
volume and rate of gastric secretions
volume = 2-3L/day
basal rate = slow - 15-30mL/hr
stimulated rate = 150mL/hour
composition of gastric secretions
basal: mainly mucous
stimulated: gastric acid (150mM HCl), mucous, pepsinogen, intrinsic factor
3 phases of gastric secretions
3 phases:
- cephalic phase - preparation
- gastric phase
- intestinal phase
cephalic phase secretions
- 30-40% of secretion associated with a meal
- Parietal cells stimulated to produce secretions
- Gastrin produced - upregulates parietal cell function
stimuli for cephalic phase
This is the preparation phase - controlled by higher centres (brain) in response to the thought, smell, and sight of food. Also stimulated by mastication and taste
Parasympathetic NS controls gastric secretions via the ENS
gastric phase secretions
60% of secretion associated with a meal
Ensures sufficient secretions are present to handle the ingested food
Parietal cells stimulated to produce secretions. Gastrin production is also stimulated, which upregulates parietal cell function
control of gastric phase
Acts in response to stretch/distension of the stomach and also the presence of products of digestion
Local reflex: controlled by the ENS
External reflex: controlled by the parasympathetic NS
intestinal phase secretions
5-10% of secretion associated with a meal
Controls the amount of acid delivered to the small intestine
gastric secretions inhibited by negative feedback processes
control of intestinal phase
Acts in response to distension of the duodenum and arrival of acid and products of digestion into the duodenum
Gastric secretions are inhibited by negative feedback processes caused by hormones gastric inhibitory polypeptide (GIP), cholecystokinin (CCK), secretin, and nervous enterogastric reflex (via vagus nerve)
function of intrinsic factor
facilitates absorption of vitamin B12 in the SI
function of pepsinogen
converted to pepsin by acidic environments and starts digestion of proteins
function of gastric acid
denatures protein, activated pepsinogen, optimum pH for pepsin, protection, dilute food
volume, rate and composition of pancreatic solutions
volume = 1.5L/day
basal rate = slow during fasting
stimulated = higher when stimulated by hormones
composition = enzymes produced by acinar cells and HCO3 produced by duct cells
cholecystokinin
Produced by duodenal ENDOcrine cells in response to digestive products (amino acids, fats, carbohydrates etc.) in the lumen
Stimulates enzyme secretion by pancreatic acinar cells
secretin
Produced by duodenal ENDOcrine cells in response to an increased [H+] in the duodenal lumen
Stimulates HCO3 secretion by pancreatic duct cells
enzymes in pancreatic secretions
lipolytic - lipase, phospholipase
amylytic - amylase
proteolytic - trypsin, chymotrypsin, carboxypeptidase
nucleolytic - ribonuclease, deoxyribonuclease
activation of proteolytic enzymes
Proteolytic enzymes are secreted as inactive precursors (e.g. trypsinogen and chymotrypsinogen
In the small intestine, trypsinogen is converted to trypsin by the enzyme enterokinase. Trypsin then activates other enzymes
function of pancreatic enzymes
multiple different types of enzymes that chemically digest all classes of food. Produced by acinar cells
functions of pancreatic bicarbonate
basic, so neutralises the acidic chyme from the stomach. This creates an optimum pH (basic) for pancreatic and intestinal enzymes.
Bicarbonate is produced by duct cells
volume and rate of biliary secretions
volume = 0.5L/day
rate = bile is secreted constantly and stored in gallbladder, which is delivered to SI when food is present
composition of biliary secretions
excretory products (bile pigments and cholesterol) and digestion-aiding products (bile salts and lecithin)
regulation of biliary secretions
delivery of bile from the gall bladder to the SI is controlled by cholecystokinin (CCK) in response to the products of digestion in the duodenum
CCK stimulates contraction of the gallbladder and relaxation of the sphincter of Oddi (opens up into the duodenum)
functions of bile salts and lecithin
help to emulsify fats so that can be absorbed and digested
small intestine secretion volume and composition/function
volume = 1.5L/day
composition/function = mucus for lubrication and NaHCO3 solution to neutralise chyme
what is absorption
net passage of substances from the lumen, across the lining of the GI tract, into the into interstitial fluid and then into the blood or lymph
absorption in mouth, oesophagus and stomach
minimal absorption - lipid soluble substances
absorption in small intestine
main site of absorption - absorbs 90% of water and Na, and all nutrients
absorption in large intestine
9% of water and Na absorption
how does motility affect absorption
Absorption requires the correct rate of propulsion for digestion and sufficient exposure to absorptive surfaces
(too fast = not enough time to absorb, too slow = too much absorption)
how does surface area affect absorption
The rate of absorption is proportional to the surface area available for absorption to occur
Anatomical adaptations maximise surface area (e.g. plicae circulares, villi, microvilli)
how does transport affect absorption
molecules need to travel across interstitial epithelium in order to be absorbed.
Two pathways for absorption are paracellular and transcellular
paracellular pathway of absorption
solutes travel through tight junctions between cells rather than crossing cell membrane
relatively non-selective - only selective based on size
passive so requires gradient
transcellular pathway of absorption
solutes travel through cells, crossing two membranes (apical and basal)
substances which are not lipid soluble require transport proteins
can be passive or active, and is more specific (can be through specific transport proteins)
how does removal of substances affect absorption
need to remove substances from interstitial space in order to maintain concentration gradients
requires large blood flow
lacteals in intestinal villi faciliate efficient removal of various substances
absorption of water
via osmosis - passive movement down conc gradient
osmotic gradients set up by absorption of salts
secrete around 7-8L/day so important that it is absorbed again
absorption of sodium
absorbed both passively (paracellular pathway) and actively (transcellular pathway)
3 mechanisms:
- Na+ transport alone
- glucose cotransport through SGLT-1
- amino acid cotransport
absorption of carbohydrates
absorbed both actively and passively
- passive absorption through paracellular pathway - diffusion of monosaccharides through tight junctions down conc grad
- active absorption via transcellular pathway - cotransport with Na+
absorption of proteins
both actively and passively
- passive absorption through paracellular pathway - diffusion of amino acids through tight junctions down conc grad
- active absorption via transcellular pathway - cotransport with Na+
absorption of lipids
Monoglycerides and fatty acids are delivered to the brush border via micelles and diffuse into the cell without need for transporter- whole micelle not absorbed
In the cell, monoglycerides and fatty acids are synthesised into triglycerides and packaged into chylomicrons. Chylomicrons exit the cell via exocytosis, and enter the lacteals
active absorption of glucose
Na+/K+ pump actively pumps Na+ out of epithelial cells into interstitial fluid so conc inside epithelial cell is low and Na+ diffuses in with glucose through SGLT-1
Then glucose can diffuse out through GLUT2 protein
absorption of bile salts
majority of secreted bile salts are reabsorbed in the distal portions of the small intestine (e.g. ileum) to promote fat absorption, as bile salts stabilise micelles
- Bile salts are reabsorbed via active transport through apical Na-dependent bile acid transporter in terminal ileum
- Passive absorption in the jejunum
absorption of vitamins
fat soluble vitamins (A, E, D and K) absorbed with fats
water soluble vitamins mostly have Na-dependent absorption
Vitamin B12 requires intrinsic factor in order to be reabsorbed
Receptors in the ileum bind the intrinsic factor/vitamin B12 complex - allowing vitamin B12 to be actively absorbed
components of ENS
submucosal plexus - on inside, regulate secretions
myenteric plexus - on outside, regulate motility
what initiates GI ENS reflexes
works independently of CNS and operate via a number of short, local GI reflexes:
distension
acidity of chyme
osmolarity of chyme
presence of products of digestion etc
how are GI reflexes mediated
GI reflexes are mediated by receptors in the GI mucosa that sense (afferent signals) distension of the GI tract (mechanoreceptors)
chemical composition of the lumen (chemoreceptors)
osmolarity of the lumen (osmoreceptors)
role of CNS in GI regulation
CNS modulates activity of CNS via long autonomic neural reflexes
Parasympathetic NS: in general stimulates motility and secretion
Sympathetic NS: in general inhibits motility and secretion
CNS regulation is bidirectional - has both afferent and efferent signals
e.g. vagovagal nerve
gastrointestinal peptide hormones
gastrin
gastric inhibitory peptide (GIP)
secretin
cholecystokinin
gastrin - produced and target
Produced by gastric G cells
Targets parietal cells and gastric muscle to stimulate gastric secretion and motility
gastric inhibitory peptide - produced and target
Produced by small intestinal K cells
Targets gastric G cells and gastric muscle to inhibit gastric secretion and motility
secretin - produced and target
Produced by duodenal S cells
Targets pancreatic duct cells to stimulate HCO3 secretion
Also targets gastric parietal cells to inhibit H+ secretion
cholecystokinin - produced and target
Produced by small intestinal I cells
Targets pancreatic acinar cells to stimulate enzyme secretion
