Gastrointestinal Flashcards
What are the major functions of the gastrointestinal system? (4)
- digestion
- absorption
- excretion
- host defense
Components of the GIT (6) and accessory components (3)
mouth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine
accessory components: pancreas, liver, gallbladder
explain the structure of the lumen
- the inside of the GIT tube.
- many folds and processes to increase the surface area - circular folds are where the entire inner surface folds in on itself.
- folds contains villi that project into the lumen, and crypts that project below the surface
explain the structure and function of the layers of the GIT (4)
slide 14-17
1.Mucosa:
- epithelium: single layer of cells
- lamina propria
- muscularis mucosa: thin layer of smooth muscle
2.submucosa: beneath the mucosa layer, contains blood vessels, lymphatic vessels, submucosal nerve plexus, connective tissue
3.muscularis externa:
- circular muscle: fibers in a circular pattern, contract and relax to open and close tube
- myenteric nerve plexus: network of nerve cells that regulate muscle function
- longitudinal muscle: lengthens and shortens to control the length of the tube
4.serosa: connective tissue layer that encases intestine and forms connections with intestine and abdominal wall
functions and structure of the epithelial layer of the mucosa
slide 10
functions:
- selective uptake of nutrients, electrolytes and water
- prevent passage of harmful substances
- stem cells within crypts divide and produce daughter cells which differentiate into a variety of cells
structure:
- polarized cells; contains apical surface (lumen facing), and basolateral surface (blood facing). Transport proteins exist at each surface due to tight junctions.
- surface area amplified by villi (single layer of epithelial cells containing microvilli) and crypts.
2 pathways that chemicals or molecules can use to get across an epithelial layer
- paracellular pathway - chemicals move through cell junctions, tight junctions allow only water and small ions diffuse
- transcellular pathway - two step process; requires a transport protein on the apical and basolateral surface of the cell
How is blood supplied to the GI system?
- blood supply carries away water-soluble nutrients and other molecules for usage at other sites in the body
- blood entering the GIT is highly oxygenated, loses oxygen as it perfuses intestine
- liver: unusual organ, receives blood from both venous and arterial circulation. it is perfused mostly by blood that has already perfused another organ.
Portal circulation - process, functions, components
process: the portal circulation carries nutrient-rich blood from the GIT to the liver via the portal vein before returning to the heart
functions: removing harmful substances (liver acts as filter with many enzymes), and processing of nutrients
components: hepatic artery - contains fully oxygenated blood that perfuses the liver. hepatic portal vein -perfuses liver with blood that has already perfused the organsof the GIT
What is unusual or different about the portal blood circulatory pathway?
- liver is different from other organs in the body as it isn’t perfused by arterial (fully oxygenated) blood. (30% arterial blood when not eating, 10% when eating)
- hepatic artery contains fully oxygenated blood that perfuses liver and hepatic portal vein carries blood to the liver that has already perfused stomach, pancreas, SI and LI. These two blood supplies mix, so that the liver is perfused with nutrient-rich, poorly oxygenated blood.
reflexes regulating GI processes are initiated (4) and propagated (3) by:
initiated by:
1. distention of wall by volume of luminal contents
2. osmolarity of contents
3. pH of contents
4. concentrations of specific digestion contents (monosaccarides, fatty acids, peptides and aa’s)
propagated by:
1. mechanoreceptores (pressure, stretch)
2. osmoreceptors (change in osmolarity)
3. chemoreceptors (specific chemicals)
Intrinsic Neural Regulation of GI processes
occurs through enteric nervous system
- Myenteric plexus: found between circular and longitudinal muscle of muscularis externa, responsible for influencing and regulating smooth muscle
- submucosal plexus: found in submuscosa, predominantly influences secretion
constant communication between these two layers
Role of the ANS in the regulation of GI activity
extrinsic pathways - long GI reflex
- nerve fibers from PS and S pathways enter the GIT and synapse w/ neurons in both plexuses
influences motility and secretion of the GIT by:
1. hunger - 2. sight/smell of food - 3. emotional state
parasympathetic: stimulates flow of saliva, stimulates peristalsis and secretion, stimulates release of bile
sympathetic: stimulates flow of saliva (small amounts, thick), inhibits peristalsis and secretion
- be able to explain how the short and long reflexes act together to activate the GIT (refer to diagram on slide 29) *
What is meant by enteric nervous system
refers to intrinsic nerve regulation - short GI reflex
- controls the activity of the secretomotor neurons which play a role in secretion and motility
- contained completely in walls of GIT
- brain of the gut - large # of neurons
- can function independantly of CNS
- critical for involuntary functions; allows digestion without thought
Role of the GI hormones in regulation of GI activity? (4)
each participates in a feedback control system that regulates an aspect of the GI lumen and has an effect on more than one cell
Secretin:
- released by S cells in the SI
- release stimulated by acid in the SI
- decreases acid production and stomach motility, increases HCO3-/H2O from pancreas and in bile
Cholecystokinin (CCK):
- triggered by fatty and amino acids in the SI, secreted from I cells in SI into blood
- stimulates pancreas to increase digestive enzyme secretion, increases bile acids for fat breakdown, decreases HCl
- CCK release is stopped due to removal and uptake of fats and aa’s
Gastrin:
- released by by G cells in the stomach
- release stimulated by peptides/aa’s in stomach and parasympathetic nerves
- increases HCl production, and GIT motility
GIP:
- released by K cells in SI
- release stimulated by glucose or fat in SI
- increases release of insulin
What is meant by neurocrine, endocrine, paracrine and autocrine?
the 4 categories of chemical messenger regulation:
Neurocrine - a nerve cell sends an electrical signal that then releases neurotransmitters that act on another neuron or effector cell
Endocrine - a hormone-secreting cell releases hormones across the basolateral surface into the blood, which then travel through blood vessels to target cells in one or more distant places in the body
Paracrine - local cells release paracrine substances that travel through IF and target cells in close proximity; occurs across the apical surface of the cell into the lumen of the gland
Autocrine - local cells release autocrine substances that act on the same cell that released it
What type of muscle activity occurs in the GI tract? What are properties of smooth muscle?
Peristalsis (Propulsion)
- circular muscle contracts above a bolus (food) - longitudinal layer relaxed
- contracted CM moves toward the anus, contents propel in that direction; CM on the other side of the distended area relaxes - longitudinal muscle contracts = smooth passage of bolus
Properties of smooth muscle:
- ability to contract/relax causes contents to move along tract smoothly
Segmentation (mixing):
- contraction and relaxation of SI that allows mixing of contentx w digestive enzymes and absorption of nutrients + water. Little net movement toward LI occurs
basic electrical rhythm
- set by slow waves - pacemaker cells throughout GIT smooth muscle cells; constant spontaneous depolar./repolar.
- slow waves propagated through through gap junctions in muscle layers
- excitatory hormones/neurotransmitters further depolarize the membrane to threshold = muscle contraction
frequency of contractions = basic electric rhythm
force of contraction = neural and hormonal input
How is ingestion of food regulated in the digestive tract
- feeding centre: lateral region of hypothalamus ~ activation increases hunger. satiety centre: ventromedial region ~ activation results in fullness
orexigenic factors - increases intake
- neuropeptide Y (neurotransmitter)
- ghrelin: from endocrine cells in stomach during fasting + stimulates NPY release in hypothalamus
anorexigenic factors - decreases intake
- leptin: from fat tissue
- insulin: from pancreas
- peptide YY: intestines
- melanocortin: hypothalamus
How is water intake regulated? (4 factors)
- increased plasma osmolarity - eating salt/exersice (+ thirst)
- osmoreceptors&thirst centre stimulated
- vasopressin (antidiuretic) released; conserves water @ kidney - decreased plasma volume (+ thirst)
- stimulates baroreceptors in cardio. system to increase angiotensin II - dry mouth and throat (+ thirst)
- prevention of over hydration (- thrist)
- person stops drinking well before water is absorbed by GIT
Phases of Gastrointestinal Control (3)
- Cephalic (Head) - receptors in the brain stim (sight, smell, taste/chewing of food, emotional state) PS fibers activate neurons in GI nerve plexuses
- Gastric (Stomach) - receptors stim. by distension, acidity, aa’s and peptides. Short and long neural reflexes mediate the response (ie. gastrin and acetylcholine)
- Intestinal - receptors stim. by distention, acidity, osmolarity and digestive products. mediated by short and long neural reflexes and by hormones secretin, CCK and GIP.
aa’s - amino acids
Major salivary glands (3) and how is their structure related to function?
- Parotid (water/serous secretion)
- Submandibular (serous/mucous secretion)
- Sublingual (mucous)
- one of each gland on either side of the face
structure:
- glands are made up of many microspopic ducts that branch out from grossly visible ducts
- smooth muscle and epithelial cells push saliva from acinus into duct
functions:
- important for protein, electrolyte and water secretion
- important for creating alkaline and hypotonic secretion
What are major components of saliva and why are they important (5)
- Water (97-99.5%)
- hypotonic, slightly alkaline (contains bicarbonate) - Electrolytes
- rich in K+ and HCO3-
- poor in Na+ and Cl- - Digestive enzymes
- amylase (digests starch), lipase (digests fat) - Glycoproteins
- mucin [mucin + water = mucus] - Other components
- anti-microbial factors (lysozyme, lactoferrin)
Adult produces ~ 1500mL of saliva/day
How is saliva produced? Explain cells involved + their transport systems
- Acinar Cells ~ initially secrete an isotonic saliva (due to leaky cells)
- proteins released by exocytosis
- Cl-, HCO3- and K+ are released by AS
- Na+ and H2O follow paracellularly via leaky tight junctions - Myoepithelial Cells ~ contract & expel formed saliva from acinus into duct
- Ductal Cells ~ modify the initial saliva to a hypotonic, alkaline state
- loss of Na + & Cl- (AT/reabsorption)
- addition of K+ and HCO3- (AS)
- cells tightly joined, impermeable to water
AT - active transport
AS - active secretion
How is salivation regulated?
0.5mL/min, can increase by 10-fold following stimulation
- PS and S both stimulate salivary secretion; PS predominant pathway
- PS stim. increases blood flow to glands → increases secretion & release of proteins from acinar cells, and stim. myoepithelial cells for contraction → increases flow
PS pathway stimulated by:
1. smell & taste
2. pressure receptors in mouth
3. nausea
Inhibited by: fatigue, sleep, fear, dehydration, some drugs
- S effects: same as PS but with modest increase in saliva flow
What is the relationship between salivation and blood flow through the salivary glands? Are the salivary glands essential for life?
increased blood flow → increased salivation
salivary glands are not essential for life however conditions can arise from not being able to produce saliva (xerostomia) such as:
- dry mouth
- decreased oral pH leading to tooth decay or esophageal erosions
- difficulty lubricating and swallowing food
- poor nutrition (not related to poor digestion)
What roles/functions does saliva have in the digestion of food? Are they major or minor pathways for digestion?
-
starch digestion: enzyme amylase (ptyalin) ~ inhibited at acidic pH in the stomach
amylase cleaves internal alpha-1,4 linkages of glucose polymers amylose and amylopectin - lingual lipase: breaks down fat, acid stable & active in stomach
overall amylase and lingual lipase are minor pathways for digestion
