Lecture 20 Flashcards
Length of adult GI tract
9m or 30ft mouth to anus
- shorter in vivo due to tonic contractions
Digestion
Caused by enzymes released by the systems exocrine glands
Amylase
Polysaccharide digestion
Lipase
TAG digestion
Proteases
Protein digestion
Method of enzyme released into lumen
From exocrine cells via exocytosis into a duct that connects to the GI tract
Peristalsis (peristaltic waves)
Wavelike muscular movements in one direction
Stomach function
Storage, dissolution and partial digestion of food
- regulates the rate at which food moves into the small intestine
3 sections of small intestine
1) Jejunum
2) Duodenum
3) Ileum
Is the digestive system responsible for the homeostatic control of nutrients?
No, that is controlled by hormones from the endocrine system and kidneys
What makes up feces?
Bacteria and ingested material not digested or absorbed
Empty vs full stomach sizes
Empty: 50 mL, diameter the size of the small intestine
Full: 1.5 L
Receptive relaxation
Relaxation of smooth muscle in stomach wall via parasympathetic innervation
- vagal input from stomach
- efferent input from swallowing center
- mediated by NO and serotonin
Fundus
Uppermost part of the stomach
Antrum
Lower portion of stomach
- thicker layer of smooth muscle
- responsible for mixing and grinding stomach contents
Pyloric sphincter
Junction between antrum and small intestine
- ring of contractile smooth muscle
Intrinsic factor function
Binds and allows absorption of vitamin B12
What do Parietal cells secrete
Acid and Intrinsic Factor
Canaliculi
Maximize SA and thus secretion of parietal cells into lumen
G cells
Secrete Gastrin
- located in antrum
ECL cells
Enterochromaffin-like
- secrete Histamine (paracrine)
- in tubular glands
D cells
Secrete somatostatin
- in tubular glands
Stomach peristaltic wave
Begins in body as ripple
- larger wave as it approaches Antrum
- mixes luminal contents
- pyloric sphincter muscles close as wave arrives
Retropulsion
Food bouncing backwards from pyloric sphincter
What produces peristaltic waves
Pacemaker cells in longitudinal smooth muscle layer
Peristaltic wave rate
~3 per min
Basis electrical rhythm of the stomach
Spontaneous depolarization-repolarizatio cycles (slow waves) of smooth muscle cells
- through gap junctions
- also affect circular layer above it
Are the slow waves enough to bring smooth muscle to threshold
No, needs excitatory NTs or hormones
What determines the force of contraction
Neural and hormonal input
- determined by number of APs fired with each slow wave
Effect of increased Gastrin on antral smooth muscle contraction
Increases force of astral smooth muscle contractions
Effect of stomach distension
Increase force of antral contractions
- via long and short reflexes triggered via mechanoreceptors in stomach wall
4 things that trigger enterogastric reflex
1) Duodenum distension
2) Presence of fat
3) Low pH (high acidity)
4) hypertonic solutions in duodenum
Enterogastric reflex
Inhibition of gastric emptying by multiple factors
- prevents overfilling of duodenum
Most potent inhibitor of gastric emptying
Fat
Effect of fat rich meal on drug absorption
Slows it down, due to delay of drug entering small intestine through pyloric sphincter
Effect of hypertonic solution on gastric emptying
Prevents duodenum from becoming too hypertonic
- slows rate of entry of chyme
- decreases delivery rate of large molecules to be broken down
Effect of ANS on gastric motility
- increase in para increases gastric motility
- increase in sympa decreases gastric motility
Segmentation
Produces a continuous division and subdivision of intestinal contents
MMC
Migrating myoelectrical complex
- ceasing of segmenting contractions
- occurs after absorption of meal
- migrates about 2 ft each time down intestine
Function of MMC
1) Moves undigested material into large intestine
2) Prevents harmful accumulation of bacteria
Mass movement
3-4 times a day
- wave of intense contraction
- transverse section of large intestine towards rectum
Effect of para and sympa on large intestine
- Para increases segmental contractions
- sympa decreases colonic contractions
Internal anal sphincter
Smooth, involuntary
External anal sphincter
Skeletal, voluntary
Initiation of Defecation reflex
Sudden dissension of rectum walls from mass movement
Urge to defecate
Occurs due to mechanoreceptors activating with rectum distension
Mechanism of defecation reflex
1) contraction of rectum, relaxation of internal anal sphincter
2) contraction of external, increased colon motility
3) high pressure triggers reflex relaxation of external
Valsa maneuver
Closure of glottis, contraction of abdominal/thoracic muscles, transmits pressure to large intestine and rectum
