Digestive System
Digestive Processes
- motility
- secretion
- digestion
- absorption
Motility
- contractions mix and move contents forward throuugh DT
2 types:
1) propulsive movements
- push contents forward through DT
2) mixing movements
- mix food wit digestive juices promos digestion of food
-facilitates absorption by exposing all parts of intestinal contents
Secretion
- consists of water, electrolytes and spef organic constituents
- released into DT lumen upon appeoperiate neural or hormonal stim
- reabsorbed back into blood
Digestion
- biochemical breakdown of complex food into small absorbable units
- by enzynmatic hydrolysis
Absorption
- small units result from digestion
- water, vits and electrolytes
- transfered from DT lumen into blood or lympth
Digestive Tract Structures
- mouth, throat, esphoagusm, stomach, small and large intestines and anus
Accessory Digestive Organs
- salivary glands
- exocrine pancreas
- biliary system
Digestive Tract Wall
- length from esophagus to anus
4 tissue layers:
1) muscosa
2)submucosa
3)musclaris externa
4) serosa
Regulation of Motility and Secretion
1. autonomous smooth muscle
2. instrinsic nerve plexuses
3. extrinsin nerves
4. gastrointentianal horms
Autonomous Smooth Muscle Function
- self-induced electrical activity
- slow wave potentials
- muscle like but noncontractile interstitial ells of Cajal
(Pacemaker cells)
Intrinsic Nerve Plexuses
- submucosal and myenteric plexus lie within DT wall and run entire length of DT
Extrinsic Nerves
- nerves from both branches of ANS
- influence dT motolity and secretion
- modfies activity of enteric nervous systems
- alters hormone secretion levels
- SNS inihibts digestion
- PNS stims digestion
Receptor Activation
- chemoreceptors
- mechanoreceptors
- osmoreceptors
Teeth
- chewing (mastication)
- 1st step of digestion
Functions:
1.
- grind and break food into smaller pieces to make swallowing easier
- increase food surface area on which salivary enzymes can act
2.
- mix food with saliva
3.
- stimulate taste buds
Saliva
- prod largely by 3 major pairs of salivary glands with 99.5% water and 0.5% electrolytes and protein (amylase, mucus, lysozyme)
Functions:
1.
salivary amylase begins digestion of carbs
2.
facilitates swalliwing by moistening food
3.
mucus lubricates
4.
antibacterial action
5.
solvent for molecules stiming taste buds
6.
aids speech by facilitating movements of lips and tongue
7.
helps keep mouth and teeth clean
8.
rich in bicarbonate buffers
Salivary Reflexes
- salivary secretion is con't and can increase reflexively
- both reflexes activate salivary centre of the brain stem, activating ANS to promo saliva prod
Simple Reflex
- occurs when food is present in the mouth
Conditioned Reflex
- occurs upon seeing and smelling food
All or None Reflex
- started voluntarily but cannot be stopped once begun
2 stages:
1) oropharyngeal stage
- lasts about 1s
2) Esophageal stage
- moves bolus from mouth through pharynx into esophagus
Perstaltic Waves
Primary Peristaltic Wave
- ring-like contractions of the circular smooth muscle
- peristaltic waves push food through esophagus toward stomach
Secondary Peristaltic Wave
- does not involve swallowing centre
- caused by distention of esophagus
Sphincters of Esophagus
1) Pharyngoesophageal Sphincter
- upper esophagus
- keeps entrance closed to prevent large vols of air from entering esophagus and stomach during breathing
2) Gastroesophageal Sphincter
- prevents refulx of gastric contents
- relaxes as peristaltic wave sweeps down esophagus so bolus can enter stomach
- not closed, acid reflux occurs
Esophageal Secretion
- secretions are entirely protective
- reduces the likelihood of damage by sharp edges
- secreted along the length of the DT
Stomach
- pyloric sphincter serves as a barrier between stomach and small intestines
Divided into 3 sections:
1. fundus
2. body
3. antrum
Functions:
1. stores ingested food until it can be emptied into small intestine
2. secretes hydrochloric acid and enzymes that begin protein digestion
3. mixing movements convert pulverized food into chyme
Food Storage and Digestion
Gastric Filling
- involves receptive relaxation
- enhances stomachs ability to accommodate the extra vol of food with little rise in stomach
- tiggered by act of eating
- mediated by cagus nerve
Gastric Storage
- takes place in body of stomach
Gastric Mixing
- takes place in antrum of stomach
Gastric Emptying
- largely controlled by factors in duedenum
Gastric Juice
2 Areas of Gastric Scretions:
1) oxyntic mucosa
- lines body and fundas
2) pyloric gland area (PGA)
- lines antrum
3 Gastric Exocrine Secretory Cells
Mucous Cells
- line gastric pits and entrance of galnds
- secretes tin, watery mucus that acts as lube, prevents mech injury and protects against self-digestion
Cheif Cells
- secrete enzyme precursor, pepsinogen
- converted into pepsin breaking down amino acid links (pepsinogen to pepsin)
Parietal Cells or Oxyntic Cells
- secretes HCL and intrinsic factor
- denatures protein
- aids in breakdown of fibers and connective fibers
- kills microorganisms in food
Control of Gastric Secretion
Cephalic phase
- increase secretion of HCL and pepsinogen in response to stim acting in the brain before food reaches stomach
Gastric Phase
- begins when food reaches stomahc
- presence of protein increases gastric secretions
Intestinal phase
- inihibitory Phase
- helps shut off flow of gastric juices as chyme
- begins to empty into small intestine
Pancreas Functions
Endocrine:
- islets of langerhans
- secrete insulin and glucagon
Exocrine:
- secretes pancreatic juice with 3 pancreatic enzymes
- lipase, amylase, protease
- secreted by acinar cells forming the acini
- aqueous alkaline solution secreted by duct cells lining the pacreatitc duct
Pancreatic Enzymes
Amylase: converts polysaccharides into disaccharide maltose
Lipase: secreted throuhgout entire digestive system to digest fat
Protease: breaks peptide bonds to convert proteins into AA
Liver Functions
1. metabolic processing of carbs,fats and proteins
2. syn plasma proteins
3. stores glycogen,fats,iron,copper and vits
4. activates vit D
5. Detoxifies wastes, horms,drugs and other foreign
6. removing bacteria and work out RBC
7. excreting cholesterol and bilirubin
Bile Release by Liver
- actively secreted by liver and diveted to gallbladder between meals
- stored and concentrated in gallbladder
- consists of bile salts, cholesterol, lecithin and bilirubin
- after meals, bile enter duodenum
Functions:
- converts large fat globules into liquid emulsion
- after fat digestion and absorption, most is reabsorbed into the blood
Micelle
- consists of hydrophilic shell and phydrophobic core
- outer shell is water soluble so products of fat digestion are carried through to aborptive surface of small intestine by dissolving in the core
Bile Secretion and Storage
Secretion increased by:
1. chemical mechs (bile salts)
2. horm mechs (secretin)
3. neural mechs (vagus nerve
Bile storage
- secreted bile stored in gallbladder between meals
- salt actively transported out with water following osmotically
- primary site for precipitation of bile constituents into gallstones
- presense of fat trigger the release of cholocytokinin that stims release of bile
Small Intestine
3 segments:
1. duodenum
2. jejunum
3. ileum
motility:
1. segmentation
2. migrating motility complex
Segmentation
- primary method of motility in SI
- has ringlike contractions along length of SI
- mixes chyme throughout SI lumen
Segmentation initiation and Control
- intitiated by pacemaker cells in SI proding basic electrical rhythms
- circular smooth muscle response influenced by distension of intestine, gastrin and extrinsic nerve activity
Segmentaiton Functions
1. mixing chyme w/ digestive juices secrete into SI lumen
2. exposing all chyme to absorptive surfaces of SI mucosa
3. slowly moving chyme thru digestive tract