Gastrointestinal Physiology Flashcards
digestion
breakdown of food into simpler molecules for absorption and transportation in bloodstream
what are 4 basic processes of the digestive system?
- motility
- secretion
- digestion
- absorption
what defence mechanisms exist to repel foreign invaders?
mucus, enzymes, acid, lymphoid tissue (GALT - gut-associated lymphoid tissue)
function of mouth, pharynx, esophagus
mechanical breakdown of food
function of stomach
acidic compartments, main role is to break down complex protein molecules
- reservoir (storage)
- partial protein digestion
- disinfection
- formation of chyme = bolus + gastric juices
function of upper or small intestines
digestion and absorption
function of lower or large intestines (colon + rectum)
absorption of water to prevent dehydration, watery chyme is converted to semisolid feces, distention of rectal wall triggers defecation reflex
function of anus
release of indigestible material
major regions of the stomach
- fundus (predigestion storage)
- body (where most digestion takes place)
- antrum
- two sphincters
what are the 3 sections that comprise the small intestine?
- duodenum (where all enzymes are released from)
- jejunum
- ileum
what modifications exist to increase lumen surface area?
- rugae in the stomach
- plicae in the small intestine
- microvilli in small intestine
what are the 4 layers of the GI tract wall?
- mucosa (closest to the lumen/outside of the body)
- submucosa
- muscularis - smooth muscle
- serosa - connective tissue
mucosa
consists of a single layer of epithelial cells, lamina propria (capillaries + vasculature), muscularis mucosae (smooth muscle whose function is to open up epithelial cells to increase surface area)
what are 4 types of epithelial cells in the stomach?
- mucus cells
- parietal cells
- chief cells
- G cells
mucus cells
secrete mucus (these cells are replaced often)
parietal cells
secrete HCl
chief cells
secrete pepsinogen
G cells
secrete gastrin (a hormone which is released into the circulation/lamina propria layer)
what are the dominant epithelial cells in the small intestine?
- absorptive cells (brush-border membrane)
- endocrine cells
- goblet cells
- other secretory cells (e.g. mast cells which secrete histamine)
absorptive cells
move nutrients to the extracellular fluid
goblet cells
small intestine version of mucus-secreting cells
lamina propria
- subepithelial connective tissue
- contains nerve fibers
- contains blood and lymphatic vessels
- wandering immune cells
- macrophages and lymphocytes
- Peyer’s patches
Peyer’s patches
collection of lymphoid tissue in the mucosa layer of the stomach
muscularis mucosa
- separates mucosa from submucosa
- contraction of this smooth muscle alters effective surface area
submucosa
a thick connective tissue layer
- contains nerves, glands, blood vessels
- contains the submucosal plexus
enteric plexus is comprised of:
the submucosal plexus (submucosa layer) and myenteric plexus (muscularis layer)
muscularis
- two layers of smooth muscle
- inner layer is circulation (constriction/dilation)
- outer layer is longitudinal (shorten/length)
- stomach also has a third layer (oblique for sideways contraction/movement)
- contains the myenteric plexus (neuronal network that regulates muscles of muscularis)
serosa
- the visceral peritoneum, holds GI tract in place
- connective tissue and simple squamous epithelium
- continuous with the mesenteries in some areas
interocytes
single epithelial cells in small intestine that are important for nutrient absorption
smooth muscle
contracts spontaneously and regulated by enteric nervous system and hormones, dictated by the type of food you eat
single unit smooth muscle
responsible for pacemaker/spontaneous contractions, generates basal electrical rhythm
interstitial cells of Cajal (ICCs)
network of cells that produce the slow waves in circular muscle of musculars (spontaneous slow waves of graded depolarization)
basic electric rhythm (BER)
frequency of slow waves generated by ICCs, varies in different areas of the GI tract, propagation because of electrical coupling via gap junctions, affected by neural and hormonal input
tonic contractions
sustained, occur in smooth muscle sphincters and stomach, keep bolus from moving backwards
phasic contractions
last a few seconds, peristalsis moves bolus forward, segmentation mixes (mixes food in both directions)
peristaltic reflex
waves triggered by distention of wall
peristalsis
- progressive wave of contraction that promotes forward movement
- mediated through the enteric nervous system
- influenced by hormones, paracrines, autonomic nervous system
segmental contractions
- mixing contractions
- circular muscle contracts while the longitudinal muscle relaxes while the opposite pattern occurs in the receiving segment
- mixes food while keeping them in contact with absorptive epithelium
zymogens
inactive proenzymes: chymotrypsinogen - chymotrypsin procarboxypeptidase - carboxypeptidase procolipase - colipase prophospholipase - phospholipase trypsinogen - trypsin pepsinogen - pepsin
trypsinogen is converted to trypsin by:
enteropeptidase (which is stored in the brush border membrane of the small intestine)
what enzyme is necessary for cells that secrete acid/bicarbonate?
carbonic anhydrase
where does bicarbonate secretion occur?
pancreatic duct cell or duodenal cell
where does chloride secretion occur?
intestinal and colonic crypt cells
(chloride ions enter lumen through CFTR channel), also causes sodium secretion by paracellular pathway
amylase
found in the mouth, breaks down glucose polymers to disaccharides
disaccharidases
found in the intestinal epithelium (on the brush border membrane), breaks down disaccharides into monosaccharides
how does glucose enter the body?
- enters interocytes via SGLT (sodium-glucose transporter)
- enters bloodstream via GLUT (glucose transporters)
proteases
breaks down proteins into smaller peptides, usually secreted as inactive proenzymes, secreted by stomach, intestine, pancreas
endopeptidases
targets inside of protein for cleavage, produces smaller peptides
exopeptidases
cleaves from the ends of protein, produces single amino acids
-can be further subdivided into amino- and carboxy-peptidases
how do amino acids enter the body
- enter interocytes via sodium-coupled transporters on the apical membrane (symport)
- facilitated diffusion into ECF and circulation
transcytosis
formation of vesicles (endocytosis) that allows proteins to enter interocytes and then circulation via exocytosis (basis of food allergies, proteins aren’t supposed to enter circulation, otherwise it may activate the immune system)
fat digestion is aided by 2 types of secretions:
- bile
2. colipase
fat digestion
- mouth: lingual lipase breaks down triglycerides into monoglyceride and free fatty acids (released by serous glands in the tongue)
- stomach: lingual and gastric lipases
- small intestine/pancreas: bile salts break up large globules of lipids into tiny droplets called micelles, pancreatic lipase breaks down triglycerides into monoglycerides and free fatty acids stored in micelles
absorption via simple diffusion of monoglycerides and micelles
bile
- assist in fat breakdown for digestion and absorption
- contains salts, pigments, cholesterol
- stored in the gallblader
bile salts
bile acid combines with amino acids
this is what coats fat droplets
chylomicrons
formed in intestinal cells/interocytes by combination of absorbed fat with cholesterol and proteins, chylomicrons are then released into the lymphatic system via lacteals
nucleic acid
- not a significant part of most diets
- digested by pancreatic and intestinal enzymes
- breaks down into nucleotides, then bases + monosaccharides
- absorption of bases via active transport, monosaccharides enter via the same mechanisms as glucose uptake
absorption of vitamins A, D, E, K
(fat soluble), absorbed with fats
absorption of water soluble vitamins
mediated transport
absorption of vitamin B12
absorption depends on intrinsic factor secreted by stomach, if deficiency of B12 absorption = pernicious anemia
absorption of minerals
active transport (linked to their concentrations in the body because they can contribute to the electrical gradient)
CNS integrates:
long reflexes
ENS integrates
short reflexes (motility, secretion, growth)
cephalic phase of digestion
reflex outside of ENS, i.e. CNS-mediated reflexes
gastric pits
invaginations in the stomach that increase surface area
gastrin family of GI hormones
- gastrin and cholecystokinin (CKK)
- gastrin is synthesized by G cells in the stomach
- CCK is synthesized in intestinal mucosa
secretin family of GI hormones
- secretin
- vasoactive intestinal peptide (VIP)
- glucose-dependent insulinotropic peptide (GIP)
- glucagon-like peptide 1 (GLP-1)
secretin
- synthesized in intestinal mucosa
- released in response to acidic chyme in small intestine
- causes release of pancreatic buffers (bicarbonate ions) to neutralize small intestine environment
vasoactive intestinal peptide (VIP)
- synthesized in the ENS
- contracts/relaxes muscle
glucose-dependent insulinotropic peptide (GIP)
- released in resopnse to carbohydrates
- synthesized in intestinal mucosa (epithelium interocytes of duodenum)
- in the presence of glucose, it stimulates insulin release (a trophic hormone, acts on pancreas)
glucagon-like peptide-1 (GLP-1)
- functions similarly to GIP
- stimulates insulin-release by pancreas
motilin
regulates motility, peristalsis, segmentation contractions
cholecystokinin
- released in response to fatty food
- synthesized in intestinal mucosa
- stimulates gallbladder contractions for bile release
- delay of gastric emptying (helps fat digestion)
- enhance satiety
gastrin
- synthesized in the stomach
- released following parasympathetic activity after a meal or by stimulation from enteric nervous system
- stimulates acid secretion by direct action on parietal cells in stomach or indirectly through histamine
procolipase
split into colipase (signals to act on fat) and enterostatin (released into duodenum)
enterostatin
satiety signal for fat ingestion
histamines
secreted into ECF on the basolateral side, effects acid secretion
- secreted by enterochromaffin cells in gastric-mucosal lumen
serotonin
increased luminal pressure, stimulates reflex
what are the gastric events following a meal?
cephalic phase (brain involvement, long reflex), gastric phase (short reflex), intestinal phase
reflux esophagitis
heartburn
what is the negative feedback signal that modulates acid and pepsin release?
somatostatin release by D-cells (triggered by increased H+) - causes inhibition of cells that stimulate digestion
stomach ulcers
erosion of stomach lining by gastric juices
peritonitis
inflammation of the peritoneum caused by perforating ulcers (perforation of mucus layer), can be fatal
effect of aspirin and other non-steroidal anti-inflammatory drugs
inhibit mucus and bicarbonate secretion
helicobacter pylori
stomach bacteria that can precipitate an immune response that destroys the stomach lining making it more vulnerable
solutions for stomach ulcers
- buffer stomach acid (milk, antacids)
- proton pump inhibitors (omeprazole)
what comprises the intestinal phase?
reflexes that direct digestion action
- limit chyme entrance rate and motility (chyme in the small intestine inhibits gastric motility and secretion)
- neutralize HCl, add bile and enzymes
- most of digestion/absorption occurs in the small intestine
- large intestine concentrates waste for excretion
- mass movement triggers defection
- absorption of bile salts, water, and some nutrients
- microbe-mediated digestion of complex carbohydrates
