Digestive System Flashcards
Digestive system
process ingested food into a form that the body can use
Use food for:
- fuel
- raw materials
- essential nutrients (iron, etc)
basic processes
- motility
- secretion
- digestion
- absorption
motility
mixing/stirring and propulsion
secretion
exocrine (digestive juices) and endocrine (hormones)
digestion
mechanical (physical) and chemical (enzymes)
absorption
designed to maximize (not regulated)
carbohydrates
- monosaccharides
- –> glucose
- –> fructose
- –> galactose
proteins
amino acids
triglycerides
monoglyceride plus 2 free fatty acids
serosa
makes fluid to decrease friction
deepest muscle layer to superficial
- serosa
- muscularis externa
- submucosa
- mucosa
muscularis externa contains
- circular muscle
- myenteric plexus
longitudinal muscle
circular muscle
- narrowing motions
- mix contents
myenteric plexus
regulates motility
longitudinal muscle
shortening motions = peristalsis
submucosa contains
- major blood and lymphatic vessels
- submucosal plexus
submucosal plexus
regulates secretion
mucosa contains
- epithelium (superficial)
- lamina propria
- muscularis mucosa (deep)
mucosa
- increase surface area
- rapidly dividing (constantly replaced)
gastrointestinal histology
- high SA for absorption
- many blood vessels to transport absorbed nutrients
- highly innervated to regulate / coordinate = enteric nervous system
GI hormones
- gastrin
- CCK
- secretin
intrinsic
enteric nervous system (within gut)
extrinsic
autonomic nervous system (increase / decrease function within GI tract)
ex: histamine, somatostatin
PSNS activity
- increase activity
- increase motility
- increase secretions
- relax sphincters
SNS activity
- decrease activity
- decrease motility
- decrease secretion
- contract sphincters
phases of GI control
- cephalic
- gastric
- intestinal
cephalic (feed-forward)
- sight, smell, taste, chewing
- extrinsic nervous system
- increase PSNS (via Vagus n.)
- increase exocrine secretions in mouth and stomach
- increase gastric motility
gastric (food in the stomach)
- stretch, acidity, amino acids/peptides
- intrinsic and extrinsic nervous system
- gastrin and histamine
- increase PSNS + Enteric NS
- increase gastrin + histamine
- both = increased gastric secretions and increased motility
intestinal (food in small intestine)
- stretch, acidity, osmolarity, digestive products
- intrinsic and extrinsic nervous system
- CCK secretion
- “Enterogastric relfex”
- increased secretion + increased CCK
- –> decreased secretions
- –> decreased motility
- –> slows down stomach emptying to give duodenum time to work
- increased SNS + decreased PSNS
- –> increased secretions into the intestine
- –> increase intestinal motility
mouth/pharynx/esophagus secretions
- saliva (mouth)
2. mucus
salvia
- about 1.5 L/day
- activated during cephalic phase (both PSNS and SNS stimulate)
- SNS = mucus
- PSNS = watery
water secretion
moistens food, and dissolves molecules to facilitate chemoreception
mucus secretion
lubricates food and facilitates formation of a bolus for a swallowing
bicarbonate secretion
neutralizes acids in foods and bacterial metabolism
lysozyme secretion
kills bacteria to maintain heath of gums and teeth
amylase secretion
begins in the digestion of polysaccharides - salivary amylase
lipase secretion
begins the digestion of triglycerides - lingual lipase
- activated in the stomach
xerostomia
- no saliva
- difficulty chewing, speaking, swallowing, halitosis (bad breath), and tooth decay (but no trouble digesting)
mucus
- secreted through the tract (esophageal glands)
- forms slippery, protective surface, and buffers pH (bicarbonate)
digestion
minor digestion of carbohydrates (salivary amylase)
absorption
virtually no foods, some drugs like nitroglycerine
what is the main function of the mouth/pharynx/esophagus?
ingestion
chewing
- mastication
- break-up food, increase surface area, mix with saliva, chemoreception (taste receptors)
- voluntary (skeletal) but coordinated by pattern generator in brainstem
- doesn’t affect rate of digestion –> mainly for taste and prevent choking
swallowing
- deglutition
- 100x / hour
- 2400x / day
motility
- feeding
- chewing (mastication)
- swallowing (deglutition)
upper esophageal sphincter
anatomic sphincter
lower esophageal sphincter
true physiologic sphincter
swallowing process
- begins with voluntary closing of lips and elevation of lips and elevation of the tongue becomes an irreversible reflex when bolus hits back on the oral cavity
- usual blocks nasal passage
- epiglottis down & glottis up and closed
- peristalsis
gag reflex
initiated by object in throat that wasn’t put there by swallowing
peristaltic wave
- takes about 5-9 seconds
- local stretch activates secondary peristaltic wave
how does fluid to the stomach
fluid gets to the stomach without peristalsis in 1-2 seconds via gravity
how does solid food get to the stomach
peristaltic is needed for solid food
LES sphincter
relaxes so the bolus can enter the stomach
weak LES means
acids reflux –> heart burn
stomach layers of muscle (superficial to deep)
- longitudinal muscle
- circular muscle
- oblique muscle
oblique muscle
“churns”
gastric rugae
- increases surface area
- allows for distention
pyloric sphincter
controls movement of chyme into duodenum
where has the least amount of muscle in the stomach
the upper portion (cardiac region)
where has the most amount of muscle in the stomach
the lower portion (pyloric region)
- increase contraction strength
chyme
gastric juice + food
gastric juice
about 2 L / day
mucous cells
secrete mucus
parietal cell
secretes hydrochloric acid and intrinsic factor
chief cell
secretes pepsinogen
enteroendocrine cells
- secretes gastrin
- histamine
- somatostatin
HCl functions
- dissolves food / denatures proteins (mechanical digestion)
- kills bacteria
- activates pepsin + lipase (lingual + gastric)
frilosec
proton pump inhibitor used to treat acid reflux and gastric ulcer
stomach lumen
excessive vomiting leads to metabolic alkalosis and K+ depletion
gastrin
- gastric hormone
- increase gastric secretions + motility (gastric phase)
ACh
neurotransmitter (PSNS)
histamine and somatostatin
paracrine agents
histamine
most important because it potentiates response of gastrin and ACh
pepsin + zantac
H2R (histamine receptor) blocks to treat acid reflux / gastric ulcer
cephalic phase pathway
parasympathetic nerves to enteric nervous system
cephalic phase result
increase HCl secretion
gastric phase pathways
long and short neural reflex and direct stimulation of gastrin secretion
gastric phase result
increase HCl secretion
intestinal phase pathways
long and short neural reflexes; secretin, CCK, and other duodenal hormones
“enterogastric reflex”
intestinal phase result
decrease HCl secretion
pepsin
only works at low pH
intrinsic factor
binds to and facilitates absorption of vitamin B12 in the distal ileum
pernicious anemia
- no intrinsic factor
- vitamin B12 needed for red blood cell production
receptive relaxation
- prepares the stomach to receive food
- mediated by serotonin and nitric oxide and released from enteric neurons
- storing “food” is a key function; contractions are weak in the fundus body
peristaltic waves
- waves are smooth muscle contraction mix and propel food
- pyloric sphincter only allows small amount of chyme into duodenum after each wave
- –> opening is size of tip of little finger
where is the electrical rhythm (slow waves) produced?
longitudinal smooth muscle
—> frequency = 3/min
what does the neural/hormonal inputs modulate membrane potential
determine force
NOT frequency
gastric emptying stimulated by
- stomach distension
- gastrin
= gastric phase
gastric emptying inhibited by
“enterogastric reflex” = intestinal phase
hypertonicity
can be problem if no stomach = increased water = decreased blood volume = decreased blood pressure
types of digestion in stomach
- mechanical
2. chemical
mechanical digestion in stomach
- physical grindage
- oblique layer of muscle
- HCl
chemical digestion in stomach
- 15% protein (pepsin)
- 10-15% fat (gastric lipase) + lingual lipase
absorption in stomach
- not much:
- –> aspirin (and other weak acids)
small intestine contains
- duodenum
- jejunum
- ileum
duodenum
- first 10 inches
- receives secretions from liver and pancreas
small intestine
about 9-12 feet long
jejunum
- next 40%
- most Digestion and absorption occurs here
ileum
- last 50%
- less muscle and fewer vessels
- vitamin B12 absorption
intestinal secretions
- hormones: CCK, secretin
- mucus
- Na+, Cl-, bicarbonate –> water follows
- Pancreas:
digestive enzymes –> (lipases, amylase, nucleases, proteases)
Bicarbonate - Liver: bile
endocrine cells of pancreas
insulin and glucagon into blood
exocrine cells
(digestive enzymes secreted)
- pancreatic lipase
- pancreatic amylase
- proteases (ex: trypsin)
- -> secreted as zymogens
- CCK stimulates release
duct cells
(secrete bicarbonate)
- neutralizes HCl from stomach
- secretin stimulates release
- mechanism of bicarbonate production = opposite of HCl
hepatopancreatic sphincter
regulates passage of secretions into duodenum
accessory duct
always open
what does bile contain
- bile salts and lecithin
- cholesterol
- bilirubin
bile salts and lecithin
- emulsify fats
- reabsorbed in ileum
- 500-1000 mL/day
cholesterol
excreted
bilirubin
- from RBC breakdown
- excreted
- metabolites makes urine yellow and feces brown
bile
- constantly secreted by liver
- stored and concentrated in gallbladder
secretin and CCK
- decrease gastric motility + secretions
- enterogastric reflex
CCK
- increase bile flow
- increase pancreatic enzymes
secretin
- increase bicarbonate production / secretion
what is the small intestine made for
absorption
what increases the surface area within the small intestine
- circular folds
- villi
- microvilli (brush border)
capillary network
nutrients absorbed
lacteal
lymphatic capillary = fats + fat soluble vitamins are absorbed in the lymph
digestion and absorption of carbs
- some digestion in mouth by salivary amylase
- most by pancreatic amylase and brush boarder enzymes in small intestine
- all (fructose, glucose, galactose) converted to glucose in the liver
- absorption in the small intestine
digestion and absorption of proteins
- some digestion in stomach by pepsin
- most by pancreatic proteases and brush boarder enzymes in small intestine
- absorption in small intestine
micelles
keep most of the insoluble fat digestion products in small, soluble aggregates
digestion and absorption of fats
- bile salts emulsify fat
- pancreatic lipase digests fat
- micelles are in equilibrium
- fatty acids and monoglycerides diffuse accords the intestinal membrane
- resynthesizes into triglycerides and packaged into chylomicrons fro transporting the lymph/blood
liver
- ‘detoxifies’ the blood and plays a major role in nutrient metabolism
- NOT directly involved in digestion
- play important role in metabolism
carboyhdrate metabolism
regulates blood glucose (glycogenesis, glycogenolysis, gluconeogenesis)
lipid metabolism/transport
- regulates fat metabolism and transport of fat / cholesterol to / from tissues
- synthesis of lipoproteins (VLDL, LDL, HDL)
fluid into tract
- 8-9 L/day
- 7 L from secretions
fluid absorbed
- 99%
- 80% in small intestine via osmosis
how many GI cells per day?
+17 billion cells / day
–> entire epithelium replaced every 8 days
motility in small intestine
- segmentation
2. peristalsis
segmentation
- during meal (when food is present)
- basic electrical rhythm (slow waves) produced in circular smooth muscle
- frequency:
12/min –> duodenum
9/min –> ileum - neural/hormonal inputs modulate membrane potential: determines force
small intestine peristalsis
- migrating myoelectric complex (MMC)
- after a meal
- before more “food” reaches small intestine
- stops when food reaches stomach (receptive relaxation)
- removes remaining food and prevents bacterial growth
- “the intestinal housekeeper”
large intestine
- about 4 ft long
- less SA and no villi
large intestine function
- temporary storage and concentration of undigested material
- reclaim more water
large intestine secretions
- protective alkaline mucus
- bicarbonate
- K+ ions
large intestine digestion
bacterial digestion of complex polysaccharides (fiber)
large intestine absorption
- Na+ (water follows)
- product of bacterial metabolism
- -> vitamins (B5, B7, K), fatty acids, sugars
large intestine motility types
- segmentation (haustration)
- mass movements
- defecation
haustration
- segmentation
- about every 30 min
mass movements
- gastrin stimulates (gastrocolic reflex)
- long-range, rapid movements that occur during a new meal
- pack the rectum and stimulate the defection reflex
rectal stretch
(defecation)
- stimulates contraction of rectum
- relaxes the internal anal sphincter
- external anal sphincter (skeletal muscle)
- –> voluntary relaxation when ready
