Digestive system Flashcards
what is the structure of the GI tract
mucosa:
- mucous membrane: simple columnar epithelium
- lamina propria: thin layer of connective tissues
- muscularis mucosa: thin layer of smooth muscles
submucosa: loose connective tissue with blood vessels, lymphatic vessels and nerves
muscularis:
- inner circular layer of smooth muscle
- outer longitudinal layer of smooth muscle
serosa: connective tissue covered with simple squamous epithelium
how are digestive functions regulated?
autonomous smooth muscle activity
enteric nervous plexus
extrinsic nerves
gastrointestinal hormones
what are functions of digestive system?
motility: movement of GI tract through muscle contraction that mix and move food through GI tract
secretion: includes both exocrine and endocrine secretions
digestion: biochemical breakdown of food stuff into small absorbable units
absorption: passage of small absorbable units into small intestine > into blood
what are the 2 parts of motility?
propulsive movement (peristalsis): push contents forward through digestive tract
mixing movement (segmentation): first mixing food with digestive juices > promote digestion and facilitate absorption by exposing all parts of intestinal contents to absorbing surfaces of tract
what is autonomous smooth muscle activity?
smooth muscle of digestive tract that undergoes spontaneous, rhythmic cycles of depolarisation and repolarisation
interstitial cells of cajal located throughout layers of muscularis external > generate slow rhythmic depolarisation > propagate via gap junctions to adjacent smooth muscle cells
depolarisation reaches threshold > burst of action potential which spreads to smooth muscle cells of functional syncytium via gap junctions > muscle contractions
autonomous activity of ICC influenced by mechanisms of nervous and hormonal factors
how are ICC signals transmitted to functional syncytium?
functions electrically and mechanically as a unit
action potential develops in ICC cells > spread quickly to other cells in a syncytium
functional syncytium found in digestive, reproductive and urinary tracts etc
what is the intrinsic nerve plexus?
also known as enteric nervous system containing 100 million neurons
contains synapse with autonomic nerves and sensory neurons, interneurons and motor neurons
consists of submucosal and myenteric plexuses:
- submucosal: sense the environment within the lumen and regulate gastrointestinal blood flow and gland secretion
- myenteric: controls mobility via contractions of muscles
what are extrinsic nerves?
synapse with intrinsic nerve plexus > act directly on smooth muscles and glands
parasympathetic activation stimulates gastrointestinal secretion and mobility:
- vagus nerves innverates tract from oesophagus to to transverse colon
- pelvic nerve from colon to anus
sympathetic stimulation inhibits/slows down digestive tract contraction and secretion and causes vasoconstriction
what are the 4 gastrointestinal hormones?
gastrin of G cells from stomach, secretin, cholecystokinin, gastric inhibitory polypeptide (GIP)
function of gastrin and secretin
gastrin:
- stimulate secretion by parietal and chief cells
- enhance gastric emptying and relaxed pyloric sphincter
secretin:
- released in response to acid in duodenum
- stimulates aq NaHCO3 secretion by pancreas and liver
- inhibit gastric emptying and secretion
function of cholecystokinin and GIP
cholecystokinin:
- most potent stimulus is fat in duodenum
- stimulates pancreatic secretion of digestive enzymes
- promotes contraction of gall bladder and relaxation of sphincter of Oddi
- inhibit gastric emptying and secretion
GIP:
- stimulates insulin secretion
- inhibit gastric emptying and secretion
what are the different gastric secretions?
parietal cells secrete HCl and intrinsic factor (glycoprotein):
- HCl activates pepsinogen into pepsin and denatures ingested proteins and kill bacteria
- intrinsic factor important for vitamin B12 activation in terminal ileum for RBC production
enterchromaffin- like cells secrete histamine
chief cells secrete pepsinogen
mucus cells secrete alkaline mucus to protect gastric mucosa from injury by acid and digestion by pepsin
G cells secrete gastrin
what are the type of gastric disorders?
reflux esophagitis: stomach contents reflux into oesophagus or beyond
gastric and duodenal ulcers: open sores that develop on lining of stomach
aspirin induced ulcers
gastronoma: neuroendocrine tumours characterised by secretion of gastrin with resultant excessive gastric acid production > severe peptic ulcer disease
what are the 4 factors that regulate gastric mobility?
gastric factors, duodenal factors, neural factors and hormonal factors
how do gastric and duodenal factors affect gastric mobility?
gastric: increase amount of chyme and fluidity stimulate gastric emptying
duodenal: delay gastric emptying
- fat is most potent in inhibiting gastric emptying
-acids, hypertonicity and distention inhibit gastric emptying
how do neural and hormonal factors affect gastric mobility?
neural:
- intrinsic nerve plexuses promotes mobility
- vagus nerve stimulates mobility and sympathetic activity inhibits it
- emotions like sadness and fear decreases mobility, anger increases it
hormonal:
- gastrin stimulates gastric mobility
- secretin and cholecystokinin and GIP from duodenum inhibit it
what is gastroparesis
condition with delayed gastric emptying
can be caused by damage in vagus nerve
diabetic patients at increased risk
how does vomiting occur
due to activation of vomiting centre in medulla
action involves contraction of diaphragm and abdominal muscles, and relaxation of oesophagus
and gastroesophageal sphincters
may cause dehydration and metabolic alkalosis
what are the different pancreatic secretions?
NaHCO3- neutralises acidic chymes from stomach
proteolytic enzymes:
- trypsinogen activated by enterokinase to trypsin
- chymotrypsinogen and procarboxypeptides activated by trypsin to chymotrypsin and carboxypeptidase respectively
pancreatic amylase digest starch to maltose
pancreatic lipase:
- major enzyme for lipid digestion
- hydrolyses triglyceride to monoglyceride and fatty acids
- pancreatic insufficiency can cause steatorrhoea (excessive amounts of fat in poop)
what are the general functions of the liver?
metabolic processing of nutrients
degradation and detoxification of hormones, body wastes, pharmacological agents etc
synthesis of all plasma proteins and clotting factors except immunoglobins
storage of glycogen, fats, iron, copper and vitamins
activation of vitamin D and TH
removal of worn out RBC and bacteria
digestion - bile
how does blood circulation occur with the liver?
hepatic artery from aorta delivers liver nutrients and O2
hepatic portal vein to liver: venous blood from digestive tract delivers absorbed substances
blood from liver returns to heart by hepatic vein
what is bile?
produced by hepatocytes
contains bile salts, cholesterol, lecithin, bilirubin, NaHCO3- etc
bile is collected by system of bile ducts to common bile duct which opens to the duodenum
how is bile secretion regulated?
chemical: bile salts are the most potent stimuli
hormonal: secretion stimulates secretion of aq NaHCO3 by biliary bile ducts
neural: vagus nerve stimulation plays minor role
how is bile stored and released from the gallbladder?
common bile duct to duodenum guarded by sphincter of oddi
sphincter of oddi closes between meals > bile diverted to gallbladder for storage
CCK stimulates contractions of gallbladder and relaxation of sphincter of oddi > delivery of bile into gut
what are bile salts?
synthesised in liver from cholesterol, conjugated with glycine
hepatic synthesis of bile salts accounts for majority of cholesterol breakdown in body
bile salts are amphipathic
what are gallstones caused by?
cholesterol stones:
- hyper secretion of cholesterol
- hypo secretion of bile salts
- impaired gallbladder function with incomplete emptying or stasis
pigment stones:
- composed of large quantities of bile pigments, along with less amounts of cholesterol and calcium salts
what is the structure and function of the small intestine?
includes duodenum, jejunum and ileum
place where most digestion and absorption takes place
inner surface contains many fold to increase surface of digestion:
- villi are projected from the folds to increase SA x10 times
- microvilli arise from surface of epithelial cells of villi to increase SA by another x20 times
what are the main intestinal enzymes?
enterokinase, aminopeptidase and dissacharides
what is the function of the large intestine?
absorption of water and electrolytes
microbial fermentation: digestion of cellulose that are not digested in small intestine
synthesis of vitamin K and biotin by bacteria
formation and storage of faces
