Exam 3: Digestive System Flashcards
major processes of the digestive system
digestion
absorption
secretion
mobility
primary functions of the digestive system
digestion, absorption, elimination
digestion
process of breaking down food into smaller chemicals used as nutrients by the body
mechanical digestion
physical breakdown of food into smaller pieces
chemical digestion
enzymes break down food into small molecules that the body can use
absorption
absorption of digested food (nutrients) from the tract to the blood stream in the body (from GI lumen to ECF)
elimination
excretion of waste from the body
mouth
movement: chewing
digestive juice: saliva
food particles broken down: starches
esophagus
movement: swallowing
digestive juice: none
food particles broken down: none
stomach
movement: muscle mixes food with digestive juices
digestive juice: stomach acid
food particles broken down: protein
small intestine
movement: peristalsis
digestive juice: small intestine digestive juice
food particles broken down: starches, protein, and carbohydrates
pancreas
movement: none
digestive juice: pancreatic juice
food particles broken down: starches, fats, and protein
liver
movement: none
digestive juice: bile acids
food particles broken down: fats
motility
movement of material through the GI tract as a result of muscle contraction
secretion
movement of material from cells into lumen or ECF
which GI processes are the primary regulated functions? why?
motility and secretion
motility: food cannot move too rapidly or else not everything will be digested and absorbed
secretion: allows digestive enzymes to break down food into an absorbable form when the food is at the correct location
what controls motility, secretion, and growth of the digestive tract?
enteric nervous system
this takes place INDEPENDENTLY of control from the central nervous system
enteric nervous system
200-600 million nerve cells
spans from esophagus to the anal sphincter
branches extend to the liver, gall bladder, biliary tract, and pancreas
aka “second brain”
connections exist between it and the CNS and are known as the Gut-Brain Axis
part of the autonomic nervous system (controls bodily functions that are not consciously directed); consists of the parasympathetic, sympathetic, and enteric nervous systems
parasympathetic nervous system
rest and digest: controls the body’s ability to relax
constricts pupils
stimulates flow of saliva
constricts bronchi
slows heartbeat
stimulates peristalsis and secretion
stimulates bile release
contracts bladder
sympathetic nervous system
fight of flight: controls the body’s response to an attack
dilates pupils
inhibits salivation
relaxes bronchi
accelerates heartbeat
inhibits peristalsis and secretion
stimulates glucose production and release
secretion of adrenaline and noradrenaline
inhibits bladder contraction
stimulates orgasm
chewing
masticating
physical breakdown by teeth and tongue
hydrolysis: enzymatic breakdown via amylase
makes the bolus
salivary secretion
humans have over 200 different proteins and peptides in the saliva
water and mucus soften and lubricate food for easy swallowing/dissolves food for taste
what glands secrete saliva
sublingual gland: under the tongue
submandibular gland: under the jawbone
parotid gland: near the hinge of the jaw
salivary amylase
breaks down amylose (starch) into maltose (sugar made of two glucose molecules bound together)
inactivated in the stomach by gastric acid
salivary lingual lipase
functional only at very low pH
degrades fats but only with stomach acid
protective effect of saliva
damages bacterial cell wall to disable bacteria and viruses
lysozyme
antibacterial salivary enzyme that damages bacterial cell wall
salivary immunoglobulins
disable bacteria and viruses
bolus
crushed foods and salivary juices including enzymes
esophagus
fibromuscular tube to propel bolus from mouth to stomach via peristalsis
esophageal sphincter
circular localization of ring-shaped muscle
uses close-open motions by the skeletal muscle (volunatary control)
importance of motility
- moves food from mouth to anus
- mechanically mixes food to break it into uniformly small particles –> maximizes exposure of the particles to digestive enzymes via increased surface area
how is GI motility determined/modified?
determined: properties of the GI smooth muscle
modified: chemical input from nerves, hormones, and paracrine signals
slow wave potentials
cycles of smooth muscle contraction and relaxation that are associated with cycles of depolarization and repolarization; spontaneous depolarizations in GI smooth muscle
where do slow wave potentials originate from?
network of cells called the interstitial cells of Caja aka ICCs
ICCs are pacemakers for slow wave activity in the GI tract
the longer the duration of the slow wave…
the more action potentials fire and the greater the contraction force in the muscle
the likelihood of a slow wave firing an action potential depends primarily on input from the…
enteric nervous system
action potentials fire when…
slow wave potentials exceed threshold
the force and duration of muscle contraction are directly related to…
the amplitude and frequency of action potentials
peristalsis
progressive waves of contraction that move from one section of the GI tract to the next
circular muscles just behind the bolus contract to push the bolus forward into a receiving segment (where the circular muscles are relaxed), then this segment contracts and so on
segmental contractions
mainly occur in the large and small intestines
short (1-5 cm) segments of intestine alternately contract and relax (circular muscles contact, longitudinal muscles relax)
this movement churns the intestinal contents to mix them and maintain its contact with the absorptive epithelium
gastroesophageal reflux disease
GERD
occurs when stomach acid frequently flows back into the tube connecting your mouth and stomach
stomach
breaks down bolus by chemical (gastric juice), biological (by pepsin hydrolysis), and mechanical (churning) functions
produces chyme (a mixture of degradation products of bolus including enzymes and gastric juices)
long reflexes
originate outside the digestive system and includes feedforward reflexes called cephalic reflexes (originate in the brain)
begin with stimuli from sight, smell, sound, or thought of food –> prepare the digestive system for food that the brain is anticipating
short reflexes
originates in the enteric nervous system within the wall of gut
activation of g-cells and gastrin release is triggered by…
neural reflexes
regulation/secretion in the stomach
activation of g-cells and gastrin release is triggered by neural reflexes
long reflexes: parasympathetic neurons from the vagus nerve stimulate g-cells to release gastrin into the blood
short reflexes: mediated by an ENS neurotransmitter called gastrin-releasing peptide (GRP); stimulated by the presence of amino acids and peptides in the stomach
example of regulation/secretion in the stomach
- activation of g-cells through long and short reflexes
- g-cells release gastrin
- gastrin activates ECL cells
- ECL cells release histamine and increases mobility
- histamine activates parietal cells
- parietal cells release HCl
- HCl activates chief cells
- chief cells release pepsinogen
- pepsinogen is activated to pepsin by HCl
pepsinogen –> pepsin (via HCl)
acid secretion in the stomach
parietal cells deep in the gastric glands secrete HCl into the stomach lumen
H+ from water inside the parietal cell is pumped into the stomach lumen by an H+-K+-ATPase (K+ enters the cell and Cl- follows H+ through open chloride channels)
cytoplasmic pH of the parietal cells
7.2
normal salivary flow rate
0.5 mL/min
maximal secretion of saliva after rinsing with 0.5 M citric acid
7.4 mL/min
bicarbonate effect in the stomach
bicarbonate is absorbed into the blood
buffering action makes blood leaving the stomach less acidic –> alkaline tide that can be measured as a meal is being digested
alkaline tide that occurs after a meal creates a temporary increase in pH
bicarbonate secretion
carbonic anhydrase (CA) converts OH- and CO2 to bicarbonate ions
HCO3- ions are pumped out to ECF from cytoplasm of parietal cells by the basolateral Cl–HCO3 exchanger
Cl- are absorbed by Cl–HCO3- exchanger
proteases for protein digestion
proteases are secreted as inactive proenzymes (zymogens) from epithelial cells in the stomach, intestine, and pancreas and are activated in the GI tract lumen
protease types
endopeptidase + exopeptidase
