Describe the basic process of the digestive system:
Motility: mixing, propulsion
Secretions: exocrine (digestive juices) and endocrine(hormones)
Digestion: mechanical and chemical
Absorption:designed to maximize (not regulated)
Distinguish between mechanical and chemical digestion:
Mechanical: the physical breakdown of food into smaller particles, exposes more food surface to the action of digestive enzymes
Chemical: a series of hydrolysis rxns that break down dietary macromolecules into their monomers,; carried out by digestive enzymes produced by the salivary glands, stomach, pancreas, and small intestine
Put the following tissues in order from the lumen of the GI tract outward.
fxns: holds abdomial viserca in their proper relationship to each other, provide passageway for blood vessels and nerves, contain lymph nodes for immunity, isolates infection
lesser omentum: extends from liver to the superior margin of the stomach
greater omentum: lager, fatty "apron" from the inferior margin of the stomach, loosely covering the small intestines
describe the structureal and fxn features of the layers of the GI tract:
mucosa
inner lining of digestive tract
epithelium
lamina propria - loose connective tissue
musclaris mucosae - thin layer, smooth muscle
increase surface area for absorption
submucosa
thick layer of loose conncetive tissue containing blood vessels and lymphatics
transport nutrients
submucosal nerve plexus - regulates secretions
muscularis externa
circular smooth muscle - narrowing motions
longitudinal smooth muscle - shortening motions
myenteric nerve plexus - regulates motility
serosa
visceral peritoneum (reduces friction)
*plexuses - regulate, coordinate enteric nervous system (ENS), a subdivison of the ANS
describe the regulation of the digestive tract:
short (myenteric) reflexes: stretching or chemcial stimulation, myenteric nerve plexus stimulate contactions, such as peristalic contractions of swallowing, completely in the GI tract, separate of CNS
long (vagovagal) reflexes: autnonmic nerve fibers, sensory signals to brain, and motor commands back to GI tract, PSNS increased activity, and SNS decreased activity
mumps is a parotid inflammation and swelling caused by a virus
describe the pharynx and esophagus as accessory organs of the GI tract:
upper esophageal sphincter: closes + allows food bolus to pass down, keeps air out, pharyngeal constirctors muscles - force food downward when swallowing
lower esophageal sphincter: relaxes to admit food + protects the esophageal mucosa from teh erosive effects of stomach acid ("heartburn")
describe the process of swallowing:
1. tongue compresses food against palate to form a bolus
2. bolus passes into pharynx. misdirection of bolus is prevented by tongue blocking oral cavity, soft palate blocking nasal cavity, and epiglottis blocking larynx
3. upper esophageal sphincter constricts and blous passes downward
4. peristalisis drives bolus down esophagus. esophagus constricts above bolus and dilates and shortens belown it
5. lower esophageal sphincter relaxes to admit bolus to stomach
2400x/day
gravity: how most goes to stomach, 1-2 seconds
peristalisis: how large food goes to stomach, 4-8 seconds
*acid reflux: weak LES, aka "heartburn"
define peristalsis:
a wave of muscular contraction that pushes the bolus ahead of it, triggered by stretch receptors of the short reflex
describe the fxns of the stomach:
food storage organ
digestion- mechanically breaks up food, liquifies it, begins chemical diegestion
*smooth muscles: lontiguidonal, circular, oblique
pyloric sphincter: "gate watcher"; regulates the passage of chyme into the duodenum
describe the histology of the stomach walls, including the fxn of the cells:
rugae: mucosa and submucosa are flat, smooth when stomach is full, but as it empites, these layers form longitudinal folds called rugae, allows for digestion and absorption
mucous cells: secrete mucus
parietal cells: secrete hydrochloric acid and intrinsic factor
chief cells: secrete the enzymes gastic lipase and pepsinogen
G cells: hormones - gastrin + paracrine messangers (histamine)
secretions: 2-3L/day
food + gastric secretions = chyme
Know the fxn of HCl as a gastric secretion:
HCl has a pH ~ 1
stimulated by: PSNS (ACh), histamine, gastrin
fnxs: dissolves food/denatrues proteins, kills bacteria - non specific resistanc, activates pepsin and gastric lipase
ACh (secreted by PSNS), histamine, and gastrin stimulate (increase) gastric secertions and increase motility
food stretches the stomach and activates myentertic nerve plexus and vagovagal reflex
describe the intestinal phase of gastric regulation:
doudenum responds to arriving chyme, moderates gastric activity
stimulators: stretch, amino acids, fats, acid
enterogastric reflex: duodenem sends inhibitory signals to teh stomach by way of enteric nervous system
intestinal gastrin stimulate the stomach - secretin, cck, enterogastric relex inhibit gastric secretions and motility, while duodenum process chyme. sympathetic nerve fibers suppress gastric activity while vagus (PSNS) stimulation of stomach is inhibited
decreased PSNS, increased SNS
increase secretin and increase CCK
decreace gastric secretion, decrease motiltiy
slow stomach emptying of chyme, gives duodenum time to work
describe the liver as a accessory organ to the small intestine:
largest gland: 3 lbs
fxns: secretes bile, detoxifies blood before leaving GI tract
*blood flows to liver via hepatic portal vein
describe the fxn of the gallbladder:
stores/concentrates bile
describe the secretions of the liver:
bile: contains minerals, cholesterol, neutral fats, phospholipids, bile pigments, and bile acids
bilirubin: decomposition of hemoglobin, color stool brown
bile acids (bile salts): steroids synthesized from cholesterol
lecithin: a phospholipid that aid in fat digestion and absorption
* bile flow: recycled several times, only way to eliminate cholesterol
about 5% of bile salts are lost in feces
describe the pancreas as an accessory organ to the small intestine:
pancreatic duct: runs lengthwise, joins bile duct at hapatopancreatic sphincter
hepatopancreatic sphincter: controls the release of both bile and pancreatic juice into the duodenum (sphincter of Oddi)
describe the activation of pancreatic enzymes:
pancreatic zymogens (enzyme precursor) trypsinogen, converted to trypsin by enterokinase (secreted by mucosa of small intestine)
CCK (cholecystokinin): stimulatory effect on the gallbladder, increases bile flow, stimulation of pancreatic enzyme secretions, positive feedback mechanism on pacrease, gallbladder, negative feedback mechanism on stomach
secretin: stimulates an abundate bicarbonate solution
HCO3-: flueshes enzymes from pancreas
describe the gross anatomy of the small intestine:
9-12 ft. long, almost all digestion occurs here
duodenum: first 10 inches, receives stomach contents, pancreatic juice, and bile, and enzymes take over chemcial digestion
jejunum: next 40% of small intestine, thick, muscular, most digestion and absorption occurs here
ileum: last 60%, thin, less muscular, less vascular, B12/bile salt absorption
ileocecal sphincter: regulates passage of food residue into the large intestine; prevents feces from backing up
describe intestinal secretions:
in addition to CCK + secretin, secrete intestinal juice
intestinal juice: water, mucus, NaCl, HCO3-
1-2 L secreted/day
goblet cells: secrete mucus
intesintal cypts (pores): epithelial cells are sloughed off and digested
describe the panacreatic secretions
both endocrine and exocrine secretions
endocrine: insulin and glucagon secreted into the blood
exocrine: carry thur a duct - digestive enzymes into duodenum (lipase, amylase), zymogens (becoming proteases), duct cells secrete sodium bicarbonate, the bicarbonate buffers HCl arriving from the stomach
describe the microscopic anatomy of the small intestine, in particular the strcutures that support absorption:
circular folds: promotes more thorough mixing and nutrient absorption
villi: finger-like projections, increase surface area 600x(200-300m2), increase absorption efficency
microvilli: brush border, increase absorptive surface area, carry out final stages of chemcial digestion, contact digestion - chyme must contact brush border, enzymes are not secreted into lumen
lymphatic vessels: absorbe fats/soluble vitamins, absorbed into lymph, "milky" consistancy
describe the digestion and absorption of carbohydrates:
most diegstiable carbohydrate is starch
first digested into oligosaccharides ( 8 glcuose residues long), then 2nd into disaccharide maltose, finally into glucose, which is absorbed by small intsetine
process begins in the mouth: salivary amylase hydrolizes starch inot oligosaccharides
pancreatic amylase: chyme mixes with pancreatic amylase, converted to oligosaccharides an maltose
describe the digestion and absorption of proeints:
Mouth: no digestion
stomach: pepsein hydrolyzes certain peptide bonds, breaking them into shorter polypeptides and small amount of free amino acids, about 10-15% of dietary protein digestion
pancrease: trypsin and chymotrypsin take over protein digestion by hydrolyzing polypeptied chains into short oligopeptides
brush border enzymes: dipeptidase, aminopeptidase take apart one amino acid at a time
absorption: action by the brush border enzymes, contact digestion, absorbed as amino acids
*absorptive cells of infacts can take up intact proteins, and release them into the blood, this allows IgA from breastmilk to pass into infants bloodstream and confer passive immunity from mother to infant
describe the digestion of fats:
lingual lipase: digests small amount of fat in the mouth
gastric lipase: 10-15% of dietary fat digested
pancreatic lipase: 85-90% fat digestion, emulsification droplets - contain lecithin and bile acids - attracted to the surface of fat globule, agitation by intestinal segmentation breaks up fat into droplets and coating of lecithin and bile acids keeps it broken up, exposing more surface area for enzymatic action
products of lipase action: two free fatty acids, and one monoglyceride
describe the absorption of fats:
micells: hydrophobic side faces inward, bile phospholipids and cholesterol diffuse into the enter of the micelle, micelle passes down bile duct into duodenum, they absorb fat soluble vitamins, more choesterol, and the FFA and monoglycerides of fat digestion, then absorbed in small intestin
within brush border, FFA and monoglycerides are resynthesized into triglycerides, golgi complex forms chylomicrons, they are taken into the lymph, thur lymphatics to blood
describe the absorbtion of vitamins, minerals, and water:
fat soluble vitamins: A, D, E - absorbed with other lipids as described in lipid absorption
water soluble vitamins: C, B - absorbed by simple diffusion
vitamin B12: absorbed only if bound to intrinsic factor
minerals: electrolytes, absorbed by simple diffusion, and active transport
water: absorbed by osmosis, follwing the absorption of salts and nutrients that create an osmotic gradient
describe in general the seretions and absorptions of the GI tract:
fluids: 8 - 9L/day - 85%
secretions 7L/day
food and drink: 99% absorbed, 1200 mL water, 800g solids per day ingested
salivary secretions: 1500 mL
gastric secretions: 200 mL
bile: 500 mL
pancreatic secretion: 6700 mL
excretion of cells: 17 billion/day, whole epithelium replaced in 5 days (susceptical to cancer drugs)
describe the motility of the small intestine:
segmentation: a movement in which stationary ringlike constricitons appear at several places along the intestine and then relax as new constrictions form elsewhere
used to churn chyme and bring it into contact with the mucosa for contact digestion and nutrient absorption
12x/min in the duodenum, 8-9x/min in the ileum
less frequent distally, slow progression of chyme toward colon, intensity is modified by nervous and hormonal influences
migrating motor complex: peristalic wave, become successive and overlapping, milk chyme toward colon over 2 hr period, removes remaining food, limits bacterial infection
enhanced segmentation in the ileum relaxs the sphincter, cecum fills with residue, sphincter shuts prevents reflux of cecal contents
describe the gross anatomy of the large intestine:
about 4 ft. long, less surface area, no villi, trillions of bacteria (800 species)
cecum: ileocecal sphincter
appendix: populated with lymphocytes, source of immune cells
ascending, transverse, decending colon
sigmoid colon: s-shaped
rectum: straight
internal anal sphincter: smooth muscle
external anal sphinctre: skeletal muscle
describe the roles of the large intestine:
temporary storage
removes water
concentrates undigested material (500ml - 150 ml)
secretions: mucus (goblet cells)
digestion: bacteria digestes cellulocse, absorbable sugars > absorption > sugars from bacterial metabolism > nacl, water vitamins K and B
describe the motility of the large intestine:
haustral contractions: occur every 30 mins, churns, mixes the residude promotes water and salt absorption, passes residue distally to another haustral
mass movement: occur 1-3 times/day, triggered by the filling of the stomach and duodenum, occur in the transverse and sigmoid colon
describe the defecation reflex including neural control:
stimulated by the stretching of the rectum
intreinsic defecation: myentereic nerve plexus, stretch singals of plexus in descending and sigmoid colon and rectum, activates peristalic wave that drives feces downward, relaxes internal anal sphincter
parasympathetic defecation reflex: stretch singal to spinal cord, motor signal returns by way of pelvic nerve to intensify perilstalsis in decending and sigmoid and rectum, and to relax internal anal sphincter
**these are involuntary control, infants and spinal cord injuries
voluntary control
external anal sphincter: must be voulunarily relaxed, prolonged distortion > reversed peristalis > contents moved backwards until next mass movement
monosaccharides are a product of ____ digestion.
B. carbohydrate
which of the following is NOT consicered an accessory organ/structure of the digestive system:
A. spleen
which type of muscle in the GI tract is invlolved in shortening motions, like those that occure during perstalsis?
D. longtiudinal
which of the following is NOT a fxn of saliva?
D. digest protein
in the stomach, ________ binds to vitamin B12 and ____ digests proteins.
a. pepsin, Hcl
b. gastric lipase, intrinsic factor
c. gastrin, trypsin
d. intrinsic factor; pepsin
intrinsic factor; pepsin
heartburn can result from a weak:
A. lower esophageal sphincter
the sight, smell, and thought of food increases as the secretions and motility of the stomach. This is called the ____ phase of gastric regulation:
A. cephalic
which of the following is FALSE?
A. CCK and secretin are hormones secreted by the pancrease
CCK (cholecystokinin) does all of the following EXCEPT:
B. stimulates the pancrease to release HCO3-
bile is made by the _____, stored in the _____, and aids in the digestion of ______.
