Chapter 20 - Gastrointestinal System Flashcards
describe the GI tract in order
mouth - pharynx - esophagus - stomach - small intestine - colon - rectum - anus
what do accessory glands do
there are several glands outside the GI tract that secrete fluids and enzymes into the lumen of the GI tract
describe the four layers of the GI tract wall
mucosa (lining of lumen), submucosa (connective tissue), muscularis externa (smooth muscle), and serosa (connective tissue)
describe the mucosa
a barrier that separates GI lumen from body’s internal environment. it consists of 3 layers: epithelial lining (entrocytes), lamina propria (connective tissue), and muscularis mucosae (thin smooth muscle). the features vary in different parts of the GI tract
what cells make up epithelial lining of mucosa
absorptive cells, exocrine cells, and endocrine cells
what cells make up lamina propria of mucosa
connective tissue, small blood vessels, lymphatic vessels, nerves and lymphoid tissue that defend against bacteria
what cells make up muscularis mucosae
thin layer of smooth muscle (local movement of mucosa itself?)
describe the submucosa
a thick layer of dense connective tissue deep to the mucosa. contains larger blood vessels and lymphatic vessels and nerves. contains the submucosal plexus (meissner’s plexus) which is a network of nerves of the enteric nervous system
describe the muscularis externa
two layers of smooth muscle: inner circular layer that changes diameter of GI tract and outer longitudinal layer that changes length. also contains the myenteric plexus (auerbach’s plexus) which is also a network of nerves and another part of enteric nervous system
describe the serosa
it is epithelium (and underlying connective tissue) that covers surface of GI tract. in the abdomen serosa is visceral peritoneum, which is continuous with parietal peritoneum and mesenteries.
name the structures that make the GI tract
mouth, pharynx, and esophagus. stomach, small intestine, colon, rectum and anus
describe mouth in digestion
digestion begins with mastication. mouth secretes saliva that is a lubricant and contains salivary amylase which digests starch and glycogen
describe pharynx in digestion
common passageway for air and food. food leaves the pharynx and enters the esophagus (air leaves pharynx and enters larynx but this is not a part of digestion)
describe the material that makes up esophagus
the upper 1/3 is skeletal muscles while the lower 2/3 is smooth muscle
describe upper esophageal sphincter
skeletal muscle it is between pharynx and esophagus. relaxes to allow food to pass into esophagus
describe lower esophageal sphincter
smooth muscle. between esophagus and stomach
describe the stomach in digestion
stores food after it’s swallowed. its empty volume is 50 mL but full volume is 1000 mL. it secretes gastric juice and releases food into intestine in controlled manner in small portions
name the 4 parts of the stomach
fundus, body, antrum, and pylorus with pyloric sphincter
describe processes of the stomach
mixes chyme, which is then passed through pylorus into duodenum (gastric emptying). stomach expands to acommodate food
what are rugae
folds in mucosa that flatten on expansion
what are gastric pits
they’re located in the stomach lining and lead to gastric glands
describe the different cells of the gastric pits
neck cells secrete mucus, chief cells secrete pepsinogen, parietal cells secrete acid
what does pepsinogen do
precursor for pepsin, and enzyme that digests proteins
what do hydrogen ions do
maintain acidic environment in stomach
what is intrinsic factor and what secrets it
it is necessary for absorption of b12 and parietal cells secrete it
what is gastrin and what secretes it
secreted from G cells (hormone)
describe the acidic environment of the stomach
ph = 2. this is necessary for activating pepsinogen. denatures protein and kills bacteria. there is a gastric mucosal barrier which is a protective layer of mucus and bicarbonate secreted from neck cells and goblet cells
what does pyrolic sphincter do
regulates passage of chyme between stomach and small intestine
describe the small intestine
coiled hollow tube 8-10 ft long between stomach and large intestine. primary site of digestion and absorption. it consists of three divisions: duodenum, jejunum, and ileum
describe secretions into dueodenum
pancreatic juice enters duodenum this consists of digestive enzymes from pancreas and bicarbonate which neutralizes acidic chyme. bile enters the duodenum as well which is secreted from liver, can be stored in gallbladder and contains bile salts which aid in fat digestion
describe absorption in small intestine
absorption completed within first 20% of intestine length. it is anatomically arranged for large surface area for absorption
describe the small intestine wall
villi increase surface area of epithelium (contain blood capillaries and lacteals for absorption of nutrients). microvilli increase surface area of epithelial cells (form brush border)
describe crypts of lieberkuhn
in the mucosa of small intestines epithelial cells in crypts secrete bicarbonate-rich fluid (secreted in proximal small intestine and absorbed in distal small intestine
describe the role of the liver in absorption
absorbed nutrients travel in blood to liver before entering general circulation. liver functions to detoxify substances and processes certain nutritents
describe the hepatic portal system
vasculature that delivers absorbed nutrients to liver before entering general circulation. nutrients are absorbed from small intestine into mesenteric veins, then nutrients travel from mesenteric veins to liver via hepatic portal vein
where do nutrients go after liver
nutrients travel from liver to heart (general circulation) via hepatic vein. hepatic artery provides liver with fresh blood to supply oxygen
describe the large intestine
consists of cecum, colon, and rectum. the ileocecal sphincter is between ileum and colon. the teniae coli are bands of longitudinal smooth muscle
describe the colon
a hollow tube from small intestine to rectum. functions in concentration of wastes into feces, absorption of most water, and storage of feces until defecation
name the 4 section of the colon
ascending colon, transverse colon, descending colon, and sigmoid colon
describe rectum and anus
colon empties into rectum, then feces are excreted through anus. the internal anal sphincter is made of smooth muscle, the external anal sphincter is skeletal muscle. relaxation of both sphincters is necessary to excrete feces
name 3 accessory glands and their function
salivary glands which secrete saliva, pancreas, which secretes pancreatic juice, and liver which secretes bile
name the 3 salivary glands and locations
paratoid gland near cheek, submandibular below the mandibal and sublingual gland under tongue
describe the characteristics of saliva
rich in bicarbonate ions, contains mucus, and has both salivary amylase and lysozyme enzymes
describe the pancreas
has exocrine and endocrine portions. exocrine portion produces the pancreatic juice which is rich in bicarbonate and the following enzymes: pancreatic amylase and lipases, proteases, and nucleases
what is main function of liver in digestion
secretes bile
describe metabolic processing of nutrients in liver
glucose –> glycogen, amino acids –> fatty acids, triglycerides and cholesterol synthesis, and lipoprotein synthesis
name 4 extra functions of liver
removal of old red blood cells (catabolize hemoglobin and generate bilirubin), elimination of wastes and toxins (bilirubin eliminated as bile pigments), synthesis of plasma proteins, and secretion of modification hormones
describe the biliary system
consists of liver, gallbladder, and associated ducts. liver synthesis bile, gallbladder stores bile, and common bile ducts transport bile from liver and/or gallbladder to duodenum. they join with pancreatic duct before entering duodenum. the sphincter of oddi regulates flow from pancreas and gallbladder to duodenum
describe liver anatomy
materials to make bile taken from blood in sinusoids into hepatocytes. the hepatocytes secrete bile into bile caniculi on side opposite of sinusoids. caniculi drain into bile ducts and bile ducts drain into common hepatic duct.
how many grams/day of carbohydrates are typical
500 grams/day
how are most carbs consumed
as disaccharides or polysaccharides
what carbs can be absorbed
only monosaccharides. disaccharides and polysaccharides must be digested into monosaccharides
describe digestion of starch and glycogen
enzymes of digestion + salivary amylase and pancreatic amylase. the end product is disaccharides (maltose) and limit dextrins. amylases can only hydrolyze internal linkages between monomers. additional enzymes are needed to complete digestion of disaccharides and dextrins
name 4 enzymes of digestion
dextrinase (dextrins into glucose), sucrase, lactase, and maltase
where are digestion enzymes located
in brush border of small intestine. brush border enzymes are integral membrane proteins in the apical membrane, with their active sites facing lumen
describe absorption of monosaccharides
absorption is transport of nutrients from lumen to blood. glucose and galactose are absorbed in two step process: secondary active transport across apical membrane and facilitated diffusion across basolateral membrane. fructose is absorbed by facilitated diffusion in both steps
what ion allows glucose from maltose across apical membrane
sodium
how much protein/ day in typical diet
125 grams/day which is more than the necessary 50 grams/day
describe digestion and absorption of proteins
secreted into lumen of intestinal tract and sloughed off with cells lining intestinal tract
name 3 protein digestion products
amino acids, dipeptides, and tripeptides
name 2 types of proteases
endopeptidases (split polypeptides at interior peptide bonds product = small peptide fragments), and exopeptidases (cleave amino acids from one end of polypeptide product = amino acids)
describe zymogens
aka proenzymes. inactive storage form of proteases, stored in zymogen granules, secreted by exocytosis, and activated by proteolysis
describe protein digestion in stomach
protein digestion begins in stomach with the enzyme pepsin which produces peptides. the inactive (secreted form) of pepsin is pepsinogen, it is activated by acid.
describe activation of pepsin in stomach
chief cells secrete pepsinogen. parietal cells secrete Hcl which activates pepsinogen. active pepsinogen cleaves other pepsinogen to pepsin which is the active form
name the enzymes for protein digestion in small intestine
pancreatic proteases are: trypsin, chymotrypsin, and carboxypeptidase. brush border proteases are aminopeptidase and enterokinase
describe activates of proteases in small intestine
enterokinase activates trypsin which then activates both chymotrypsin and carboxypeptidase
describe absorption of amino acids
cross the apical membrane by sodium-linked secondary active transport or facilitated diffusion. cross the basolateral membrane by facilitated diffusion
describe absorption of dipeptides and tripeptides
cross apical membrane by active transport, and are broken down inside cells to amino acids. amino acids cross the basolateral membrane by facilitated diffusion
how many grams/day of lipids is usual
140 grams/day and 90% of that is triglycerides
describe the special problem facing lipids in digestion
they’re not water soluble, don’t mix with stomach or intestinal contents, and form fat droplets
describe lipid digestion
the enzymes of digestion are lipases. these are secreted from pancreas. lipases can only act on molecules at the surface of a fat droplet. bile salts increase the combined surface area of droplets by breaking each large droplets into several small ones. this is called emulsification
describe bile salts
synthesized in liver from cholesterol. secreted in bile to dueodenum. they’re amphipathic molecules that emulsify fat
discribe how pancreatic lipases digest lipids
a triglyceride is turned into a monoglycride and 3 fatty acids. some fatty acids and monoglycerides are absorbed while others form micelles. equilibrium between micelles and the fatty acids and monoglycerides is formed by the action of lipases
describe the enterohaptic circulation
the recycling of bile salts by the liver. gallbladder and liver share common bile duct into duodenum. bile salts travel from here to ileum where some are absorbed into capillaries and returned to the liver via the hepatic portal vein
how are free form fatty acids transported across epithelium
via simple diffusion
what happens inside epithelial cell to lipids
they enter the smooth ER and re-form triglycerides and other lipids. lipids enter golgi apparatus to be packaged into chylomicrons (aka lipoproteins)
what happens to chylomicrons
they’re secreted by exocytosis into interstitial fluid. chylomicrons enter the lymphatic system via lacteals
describe absorption of fat-soluble vitamins and name them
vitamins A, D, E and K are fat soluble and are absorbed with lipids. they dissolve in lipid droplets, micelles, and chylomicrons.
describe absorption of water-soluble vitamins
they require special transport proteins
describe absorption of vitamin B12
absorbed only if bound to intrinsic factor
describe absorption of sodium
solvent drag with water reabsorption, actively absorbed in jejunum, ileum, and colon
describe chloride absorption
passively follows sodium absorption
describe potassium absorption
passively absorbed, secreted when luminal concentrations are very low
describe absorption of calcium
actively absorbed in duodenum and jejunum. binds to brush border protein (calcium-binding protein) and transported into epithelial cells. transported out of cells across basolateral membrane via Ca pump. 1,25-(OH)2D3 increases calcium absorption by increasing concentration of calcium-binding protein
describe absorption of iron
transferrin is secreted by enterocytes into the lumen of the small intestine. transferrin binds iron. the transferrin-iron complex binds to receptor, and is taken into cells by receptor-mediated endocytosis. some iron is stored in enterocytes as ferritin. some iron is transported into blood: bound to transferrin
describe absorption and secretion of bicarbonate in both jejunum and the ileum and colon
in jejunum bicarbonate ions are passively absorbed. in the ileum and colon bicarbonate ions are secreted in exchange for chloride ions
describe absorption of water
7 liters of water/day is secreted in GI tract and 2 liters/day is taken in. water absorption is passive and follows absorption of solutes by osmosis
describe GI regulation
it’s not based on concept of homeostasis. GI function is regulated to maximize absorption, regardless of whether nutrients are needed. conditions in the lumen of GI tract are regulated to maximize absorption
describe neural control of GI
the enteric nervous system (ENS) consists of submucosal plexus, myenteric plexus, and reflexes mediated through ENS control many GI functions. the CNS contributes generally through communication of ANS with enteric nervous system
describe endocrine control of GI
GI hormones are secreted from endocrine cells in stomach and small intestine. sensory receptors in GI tract detect the environment in the lumen to initiate relflexes
describe gastrin site, stimuli, and action
secreted in stomach. activated by proteins and protein digestion products in stomach, distension of somach; parasympathetic input to stomach. its action stimulates gastric secretion and motility; stimulates ileal motility and relaxes ileocecal sphincter which stimulates mass movement of colon
describe cholecystokinin (CCK) site, stimuli, and action
secreted in duodenum and jejunum, activated by fat or protein digestion products in duodenum. it inhibits gastric secretion and motility by potentiating actions of secretin on pancreatic bicarbonate secretion, stimulates pancreatic enzyme secretion, stimulates bile secretion by liver, and stimulates gallbladder contraction and relaxation of sphincter of oddi
describe secretin site, stimuli, and action
secreted by duodenum and jejunum, activated by acid in duodenum, it inhibits gastric secretion and motility, stimulates pancreatic bicarbonate secretion, potentiates actions of CCK on pancreatic enzyme secretion, and stimulates bile secretion by liver
describe glucose-dependent insulinotropic peptide (GIP) secretion site, stimuli, and action
secreted by duodenum and jujenum, activated by glucose, fats, or acid in duodenum or distension of duodenum. it inhibits gastric secretion and motility, stimulates insulin secretion by pancreas
describe the steps of a short reflex pathway
stimulus in lumen - GI receptors - intrinsic nerve plexus (ENS) - effector organ in GI tract
describe the steps of a long reflex pathway
stimulus in lumen - GI receptors - central nervous system - intrinsic nerve plexus (ENS) - effector organ in GI tract
describe phases of GI control
cephalic phase are stimuli that originate in head: thoughts, taste, and smell. requires input from CNS. gastric phase stimuli originate in stomach: long and short reflexes and GI hormones. intestinal phase stimuli originate in small intestine: long and short reflexes and GI hormones
how does hypothalamus regulate feeding behavior
it is satiety center: the hunger (or feeding) center
describe satiety signal to hypothalamus
reduce sensation of hunger, reduce food intake, increase energy expenditure (stimulation of sympathetic activity), increase cell metabolism
describe orexigenic signals to hypothalamus
stimulate hunger, induce feeding behavior, increase parasympathetic activity, decrease metabolism
describe leptin
a hormone synthesized by adipose cells and released in proportion to amount of adipose tissue in body. it is released when calories intake exceeds body demands. leptin supresses hunger, decreases food intake, and increases metabolism it is the satiety signal.
how does leptin act on hypothalamus
leptin acting on hypothalamus initiates multiple pathways: stimulates aMSH and CARt, these both increase sympathetic activity and stimulate release of TSH and ACTH, both TSH and ACTH increase metabolic rate and reduce fat storage
what does a congenital deficiency in leptin lead to
hyperphagia and severe obesity
name 3 short term satiety factors
insulin, CCK, and neural input from mechano and chemoreceptors
name 1 short term orexigenic factor
ghrelin is a hormone released by the empty stomach
describe the pathway of saliva secretion
taste, smell and texture of food - mechanoreceptors and taste receptors in mouth as well as olfactory receptors - salivary center of medulla - autonomic nervous system - stimulate salivation
describe autonomic input to salivary glands
parasympathetic leads to large amount of watery saliva. sympathetic input leads to more mucus, thick viscous saliva, and only a small amount
describe gastric acid secretion
parietal cells secrete HCl, carbonic anhydrase catalyzes production of bicarbonate and H+, H+ are actively secreted into lumen of stomach, bicarbonate is transported into interstitial fluid for Cl-, Cl- diffuses into lumen of stomach through channels
what is the net result of acid production
H+ and Cl- are secreted into lumen of stomach, and bicarbonate enters the interstitial fluid and then blood
how is acid secretion stimulated
parasympathetic activity, gastrin, and histamine (paracrine)
how is pepsinogen secretion regulated in sync with acid secretion
parasympathetic nervous system, gastrin, and histamine
describe cephalic-phase regulation of secretion
stimuli that increase acid and pepsinogen secretion: sight of food, taste, smell, chewing, and swallowing. all of these activate parasympathetic nervous system and this stimulates gastrin secretion
describe gastric-phase regulation of secretion
proteins, peptides, and amino acids as well as distension of stomach stimulate gastric secretion. short and long reflex pathways trigger gastrin, acid, and pepsinogen release
describe inhibition of secretion in gastric phase
exit of food removes stimuli for secretion. increased acidity inhibits gastrin release
describe inhibition of secretion in intestinal phase
this is feedback regulation. the effects of food entry into duodenum leads to increased: osmolarity, fat and acid, and increased distension. long and short reflex pathways inhibit acid and pepsinogen secretion in the stomach
describe secretion of pancreatic juice
acinar cells makes a small volume of primary secretion and contains water, electrolytes, and digestive enzymes. duct cells are large volume bicarbonate-rich secretion
how are pancreatic enzyme and bicarbonate secretion regulated
independently. composition of pancreatic juice varies based on lumenal contents
describe CCK effect on regulation
stimulates acinar cell to secrete enzymes (small stimulant for bicarbonate release, CCK potentiates secretin effects on bicarbonate release)
describe secretin effect on regulation
secretin stimulates duct cells to secrete bicarbonate (small stimulate for enzymes release, secretin potentiates CCK effects on enzymes release)
describe daily fluid flow in GI system
9000 mL entering lumen daily and 8900 mL leaving lumen daily. so 100mL of fluid is excreted in feces
describe GI motility
this is the movement of the wall of the GI tract. due primarily to contractions of the muscularis externa (outer muscle layers) the function is to mix and propel
describe electrical activity in GI smooth muscle
spontaneous slow waves of depolarization are called slow waves. regular, constant frequency of waves = basic electric rhythm. BER varies in areas of GI. amplitude of BER affected by neural and hormonal input
describe parasympathetic and sympathetic effects on BER
parasympathetic excites and leads to increased contractile force while sympathetic inhibits which leads to decreased contractile force
describe relationship between electrical and mechanical activity in stomach smooth muscle
action potentials are not required for contractions. larger depolarizations of slow waves leads to stronger contractions. action potentials lead to even stronger contractions
describe relationship between electrical and mechanical activity in intestinal smooth muscle
actions potentials are required for contractions. strength of contraction varies with frequency of action potentials
what are peristalsis and segmentation
peristalsis propels contents forward while segmentation mixes the contents
describe peristalsis
requires circular and longitudinal muscle contractions. in proximal segment: circular muscle contracts; longitudinal relaxes and diameter increases. in distal segment: circular relaxes; longitudinal contracts diameter increases. contents are propelled forward from small diameter to large
describe segmentation
a type of motility of small intestine that requires circular muscle layer. alternation contractions between intestinal segments mixes chime with digestive juices and exposes it to mucosal lining
describe chewing and swallowing
control is both voluntary and involuntary. chewed food + saliva = bolus. tongue moves bolus to pharynx, this initiates swallowing reflex. integration center = swallowing center of medulla oblongata
describe swallowing reflex
bolus is pushed by tongue into pharynx, pressing on epiglottis. epiglottis cover glottis so food doesn’t enter trachea. larynx is pulled superiorly and anteriorly. glottis closes, breathing inhibited temporarily. upper esophageal sphincter relaxes, bolus enters esophagus, sphincter closes behind bolus. bolus stretches esophagus which triggers peristalsis which propels bolus to stomach. travel time is around 9 seconds
describe bolus after arriving at stomach
bolus arrives at stomach triggering relaxation of lower esophageal sphincter. bolus enters stomach. if necessary, additional peristalsis waves comes to move bolus into stomach
describe receptive relaxation of stomach
swallowing center also initiates relaxation of stomach smooth muscle as it prepares for receipt of bolus
how is gastric motility coordinated
by the enteric nervous system
describe gastric motility patterns
waves of peristalsis: upper body –> pylorus. they strengthen as they approach the pylorus. functions in mixing of chyme: pyloric sphincter closed. functions in gastric emptying: stronger contractions open pyloric sphincter volume of chyme in stomach and strength of gastric peristalsis govern the emptying rate
describe migrating motility complex
wave of intense contractions. travel short distances. occur between meals to clear the stomach
describe regulation of gastric motility (both increase force and decrease force)
gastrin increasing contraction force while CCK, secretin, and GIP decrease force of contraction
describe excitation and inhibition of cephalic phase
anger and aggression excites while pain, fear, and depression inhibits
where is vomiting mediated
through vomiting center in medulla
describe the vomiting reflex
abs contract and puts pressure on stomach contents. reverse peristalsis in stomach and small intestine. lower esophageal relaxes. epiglottis closes and contents come up and out
describe intestino-intestinal reflex
injury or severe stress inhibits intestinal contractions
describe ileogastric reflex
destinsion of ileum inhibits gastric motility
describe gastroileal reflex
presence of chyme in stomach increases motility in ileum
describe haustrations
takes place in proximal colon. it is like segmentation but slower
describe mass movement
propels bolus toward rectum. similar to peristalsis and contraction lasts longer than relaxation
describe colonocolinic reflex
distension of colon in one area causes relaxation of other areas.
describe gastrocolic reflex
food in stomach increases colonic motility
describe process of defecation
voluntary and involuntary control of colon distension - stretch receptors - rectum smooth muscle contracts - increase pressure in rectum - internal anal sphincter relaxes while external contracts - both sphincters relax - defecation
