Gastrointestinal Physiology Flashcards
Functions of GIT
digestion, absorption, excretion, host defense
Digestion
form absorbable molecules from food through GIT motility, pH changes, and biologic detergents and enzymes
Absorption
movement of digestive food from intestine into blood or lymphatic system
Excretion
non-absorbable components of food, bacteria, intestinal cells, and hydrophobic molecules, cholesterol, steroids are excreted out of body
Host defense
GIT forms a barrier with the outside environment and contains a highly developed immune system
Components of the GIT
mouth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine
3 accessory organs
pancreas, liver, gallbladder
Structure of the intestine
- lumen is the inside of the tube
- contains many folds to increase surface area
- circular fold where inner surface folds in on itself
- contains villi
- below the surface is called a crypt or invagination
Layers of GIT
mucosa, submucosa, muscularis externa, serosal layer
Layers of the mucosa
epithelium, lamina propria, muscularis mucosa
Epithelium layer
- apical surface - inside of the tube or lumen
- basolateral surface - closest to the blood surface, away from tube
- selective uptake of nutrients, electrolytes, water
- prevent passage of harmful substances
- surface area is amplified by villi and crypts
- stem cells within the crypts divide and produce daughter cells which differentiate into variety of cells
- replaced every 5 days
Paracellular pathway
- chemicals move between cells across the cell junctions
- limited by tight junctions
- water and small ions diffuse through tight junctions
Transcellular pathway
- cross the cell and require transport proteins
Lamina propria layer
- includes everything above the muscle layer
- connective tissue, blood vessels, nerve fibers, lymphatic vessels/lacteals, immune, inflammatory cells
Muscularis mucosa
- thin layer of smooth muscle
- not involved in GIT contraction and may function to move the villi
Submucosa
- contains blood vessels, lymphatic vessels, connective vessels and SUBMUCOSAL NERVE PLEXUS
- relays info to and away mucosa
Muscularis externa
- circular muscle - fibers are orientated in circular pattern and contract an relax to open and close tube
- myenteric nerve plexus - regulate the muscle function of the GIT
- longitudinal muscle - lengthens and shortens to control the tube length
Serosa
- connective tissue layer that incases the intestine and forms connections with intestine and abdominal wall
Blood is _____ oxygenated entering the GIT but ___ oxygen as it perfuses the intestine
highly; loses
Where is blood taken from the GIT before going back to heart
the liver via the portal vein
Portal circulation
- carries blood from intestinal tract to the liver
- blood is nutrient rich
- liver removes harmful substances
- processes nutrients
- hepatic artery and hepatic portal vein mix blood supply that is nutrient rich and poorly oxygenated
Hepatic artery
contains fully oxygenated blood that perfuses the liver
Hepatic portal vein
carries blood to the liver that has already perfused the stomach, pancreas, SI, LI
Liver is perfused in
series
- perfused by blood that has already perfused another organ
Reflexes regulating GI processes
- distension of the GIT wall
- osmolarity of contents
- pH of contents
- concentrations of specific digestion contents
Reflexes are propagated by these receptors
mechanoreceptors, osmoreceptors, chemoreceptors
Intrinsic regulation of GI processes
- to control motility and secretion
- contained wholly within organ
- occurs through nerve plexi located in GIT wall
- enteric nervous system - controls activity of secretomotor neurons, BRAIN OF THE GUT, independent of CNS, involuntary functions
- two nerve networks - myenteric plexus and submucosal plexus
Myenteric plexus
- found between two muscle layers (circular muscle and longitudinal muscle) of muscularis externa
- influencing and regulation smooth muscle
Submucosal plexus
- found in the submucosa
- influences secretion
Extrinsic regulation of GI processes
- outside of GIT wall
- ANS - sympathetic and parasympathetic
- smell of food signals through brain to GIT by ANS
- different emotional states influence appetite
Parasympathetic response
- stimulates large volume of watery saliva
- stimulates peristalsis
- stimulates secretion
- stimulates bile released from liver
Sympathetic response
- stimulates small volume of thick saliva
- inhibits peristalsis
- inhibits secretion
Long reflex
- extrinsic pathway
- smell of food/emotional state –> stimulates CNS –> efferent autonomic neurons fire and interact with same nerve plexus –> stimulates smooth muscle to contract or a gland to secrete –> causes response in GIT
Short reflex
- intrinsic pathway
- eating a meal activates receptors in GIT wall –> stimulus from receptors feeds into nerve plexus –> stimulates smooth muscle to contract or gland to secrete –> causes response in GIT
4 types of chemical messenger regulation
endocrine, neurocrine, paracrine, autocrine
Endocrine regulation
hormone secreting gland cell releases a hormone across its basolateral surface into the blood
hormone enters the blood and travels to target cell
Neurocrine regulation
nerve cell produces an electrical signal resulting in the release of a neurotransmitter which travels across a synapse and acts on a post-synaptic target cell
Paracrine regulation
local cell releases a paracrine substance which diffuses through the interstitial fluid to act on target cells in close proximity to the site of paracrine substance relase
apical surface of cell in lumen of gland
Autocrine regulation
local cell releases a substance which acts on the cell that released it
Endocrine cells
- product hormons
- found in the epithelium of the stomach and small intestine
- enteroendocrine cells release hormons which control GI functions
3 GI hormones
secretin, cholecystokinin (CCK), gastrin
- all peptide hormones
- participates in feedback control system
Intestinal motility
- stimulated by contraction and relaxation of the two muscle layers in the outer portion of the GIT, contents move along tract
- peristalsis - main driving force
- circular muscle contracts on the oral side of a bolus of food (longitudinal layer relaxes)
Segmentation
- mixing of food
- contraction and relaxation of intestinal segments with little net movement toward large intestine
- allows mixing of GIT contents with digestive enzymes
- slows the transit time to allow absorption of nutrients and water
Pacemaker cells
- cells in the GIT that are distributed smooth muscle cells
- constantly under spontaneous depolarization-repolarization cycles called slow waves under any circumstance
- slow waves give GIT basic electrical rhythm
- propagated through circular and longitudinal muscle layers through gap junctions
an increase of action potentials fired ____ the force of contraction
increases
3 phases of neural and hormonal GIT control
cephalic phase, gastric phase, intestinal phase
Cephalic phase
- initiated through stimulation of receptors in the head by sight, smell, taste, chewing of food and emotional state
- regulated by parasympathetic fibers that activate neurons in the GIT nerve plexus
Gastric phase
- receptors in the stomach are stimulated by distension, acidity, amino acids, peptides
- responses of stimuli are mediated by short (gastrin hormone) and long (acetylcholine neuron) neural reflexes
Intestinal phase
- receptors in the intestine are stimulated by distention, acidity, osmolarity, digestive products
- mediated by short and long neural reflexes, and hormones secretin, CCK, and GIP
Regulation of food intake
- hypothalamus
- contains a feeding center in the lateral region - increases hunger
- contains a satiety center in the ventromedial region - decreases hunger
Orexigenic factor
- increase intake
- neuropeptide Y –> neuropeptide in hypothalamus that stimulates hunger or appetite
- ghrelin –> synthesized and released from endocrine cells in stomach during fasting, released into blood and travels to hypothalamus stimulating release of neuropeptide Y
Anorexigenic factors
- decrease intake or cause loss of appetite
- leptin –> produced by adipose or fat tissue
- insulin –> produced by the pancreas
- peptide YY –> released from the intestine
- Melanocortin –> released from the hypothalamus
Lack of leptin results in
- no appetite regulation
- overeating
- obesity
Regulation of water intake
- hypothalamus
1. increased plasma osmolarity
2. decreased plasma volume
3. dry mouth or throat stimulated thirst
4. prevention of over-hydration
- increased plasma osmolarity
- osmoreceptors
- stimulated thirst and release of hormones called vasopressin or anti-diuretic hormone to conserve water in kidneys
- decreased plasma volume
- large blood loss or diarrhea and vomiting can cause dehydration
- arterial baroreceptors will alter sympathetic and parasympathetic to increase arterial pressure to kidneys
- activate renin-angiotensin system to produce angiotensin II which effects hypothalamus
- prevention of over-hydration
- person stops drinking well before water is absorbed by GIT
- mediated by stimulus from mouth, throat, GIT
3 pairs of salivary glands
- parotid
- submandibular
- sublingual
What is saliva made of?
hypotonic and slightly alkaline
- water
- electrolytes (K and bicarbonate)
- digestive enzymes (amylase and lipase)
- glycoproteins (mucin)
- antimicrobial factors (lysozyme and lactoferrin)
Functions of saliva
- moistens and lubricates food to make it easier to swallow
- initiates digestion with digestive enzymes
- dissolves small amount of food to allow it to diffuse to taste buds
- prevents microbial colonization
- aids in speec
- buffer to neutralize acid
3 cells that make up salivary gland
- acinar cells - secrete the initial saliva
- ductal cells - create the alkaline and hypotonic nature of saliva
- myoepithelial cells - mix of smooth muscle and epithelial cells
Path of saliva through the ducts
- saliva moves from the acinus to the striated duct
- myoepithelial cells contract to constrict the acinus end of the ducts
- move the components of saliva toward straited duct
- ductal cells modify the initial saliva to hypotonic, alkaline state
Tight junctions
- acinar cells are leaky and allow water and small ions through
- ductal cells do not allow the passage of water through
Acinar cells
- water
- proteins are released by exocytosis
- Cl, HCO3, K are secreted
- Na, H2O follow via leaky tight junctions
Ductal cells
- net loss of Na and Cl
- addition of K and HCO3
Regulation of saliva production
- no hormonal
- regulated by parasympathetic and sympathetic pathways (both stimulate salivary secretion)
- dominant pathway - parasympathetic
- increased blood blow to salivary glands results in increased secretion of saliva
Parasympathetic pathway for salivary gland function
- stimulated by smell, taste, pressure receptors in the mouth, and during nausea
- inhibited by tiredness, fatigue, sleep, fear, dehydration
- some drugs have dry mouth side effect
Sympathetic pathways for salivary gland function
- minor compared to parasympathetic
- increases saliva flow
- increases protein secretion from the acinar cells and stimulate myoepithelial cells to contract to increase flow
Amylase
- found in saliva
- enzyme that breaks down starches
- also called ptyalin
- inhibited by stomach acid
- 95% of carbs digested in small intestine by pancreatic amylase
- can only cleave alpha-1,4 linkages
Amylose
- straight chain of glucose with alpha-1,4 linkages
- breakdown leads to formation of maltose and matotriose
Amylopectin
- chain of glucose with alpha-1,4 linkages and alpha-1,6 linkages
- breakdown leads to formation of maltose, matotriose and alpha-limit dextrin
Lingual lipase
- found in saliva
- stable in acid, active in stomach
- breaks down lipids
Conditions where salivary secretion is impaired / dry mouth
- hereditary
- sjogrens syndrome - immune system destroys salivary glands
- side effects of drugs
- radiation treatment
Consequences of dry mouth - xerostomia
- dry mouth
- decreased oral pH
- difficulty in lubricating and swallowing food
Treatment for xerostomia / dry mouth
- frequent sips of water
- fluoride treatment to combat microbial populations
What initiates swallowing
- pressure receptors in wall of pharynx
- stimulated by food or liquid entering the pharynx and send signals to swallowing center in the brainstem
Steps of swallowing
- chew food
- tongue pushes it to the back of throat
- soft palette elevates to stop food from entering nose
- impulses from swallowing center inhibit respiration, raise larynx, and close the glottis
- epiglottis covers the trachea to prevent fluid or liquid from entering the trachea
- food descends into esophagus
Structure of the espohagus
- top 1/3 is skeletal muscle and bottom 2/3 is smooth muscle
- no absorption
- mucus is secreted to lubricate food
- stratified squamous epithelium (flat layers)
Structure of esophagus sphincters
- upper esophageal sphincter just below pharynx - skeletal muscle
- lower esophageal sphincter just below stomach - smooth muscle
- both are closed except when swallowing, vomiting, burping
Main force of swallowing phase down the esophagus
peristalsis
Heart burn
- lower esophageal prevents gastric contents from reaching esophagus (equal pressure of lower esophagus and stomach)
- acid gets into esophagus and peristalsis pushes acid back down, increased salivary secretion
- common when lower esophageal sphincter does not close properly, a big meal, pregnancy
Functions of the stomach
- storage of food
- mechanical breakdown of food
- chemical breakdown of food - pepsinogen, HCl
- reduces food to chyme
- partial sterilization of food
- controls rate food enters small intestine
- secretes intrinsic factor important for absorbing vit B12
- very little absorption - alcohol and little water
Stomach compartments
fondus and body
- upper part, thin smooth muscle
- mucus, pepsinogen, HCl are secreted
antrum
- lower part, thick smooth muscle, physical breakdown
- mucus, pepsinogen, gastrin are secreted
Pyloric sphincter
controls emptying of the stomach
Exocrine system
- chemical messenger secreted into ducts and then onto an epithelial surface without passing into the blood
Major exocrine secretions in stomach
- mucus - protects stomach epithelium from acid and digestive enzymes, helps to avoid self-digestion
- HCl - hydrolysis of proteins into amino acids, dissolving food, digesting macromolecules, sterilization
- pepsinogen - precursor to enzymes pepsin which is important for protein digestion
Mino secretions in stomach
- intrinsic factor for vit B12 absorption
- gastrin - endocrine - stimulates HCl
- histamine - paracrine - stimulates HCl
- somatostatin - paracrine - inhibits HCl