Digestive system

Question 1

what is the structure of the GI tract

Answer 1

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

Question 2

how are digestive functions regulated?

Answer 2

autonomous smooth muscle activity

enteric nervous plexus

extrinsic nerves

gastrointestinal hormones

Question 3

what are functions of digestive system?

Answer 3

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

Question 4

what are the 2 parts of motility?

Answer 4

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

Question 5

what is autonomous smooth muscle activity?

Answer 5

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

Question 6

how are ICC signals transmitted to functional syncytium?

Answer 6

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

Question 7

what is the intrinsic nerve plexus?

Answer 7

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

Question 8

what are extrinsic nerves?

Answer 8

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

Question 9

what are the 4 gastrointestinal hormones?

Answer 9

gastrin of G cells from stomach, secretin, cholecystokinin, gastric inhibitory polypeptide (GIP)

Question 10

function of gastrin and secretin

Answer 10

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

Question 11

function of cholecystokinin and GIP

Answer 11

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

Question 12

what are the different gastric secretions?

Answer 12

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

Question 13

what are the type of gastric disorders?

Answer 13

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

Question 14

what are the 4 factors that regulate gastric mobility?

Answer 14

gastric factors, duodenal factors, neural factors and hormonal factors

Question 15

how do gastric and duodenal factors affect gastric mobility?

Answer 15

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

Question 16

how do neural and hormonal factors affect gastric mobility?

Answer 16

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

Question 17

what is gastroparesis

Answer 17

condition with delayed gastric emptying

can be caused by damage in vagus nerve

diabetic patients at increased risk

Question 18

how does vomiting occur

Answer 18

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

Question 19

what are the different pancreatic secretions?

Answer 19

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)

Question 20

what are the general functions of the liver?

Answer 20

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

Question 21

how does blood circulation occur with the liver?

Answer 21

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

Question 22

what is bile?

Answer 22

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

Question 23

how is bile secretion regulated?

Answer 23

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

Question 24

how is bile stored and released from the gallbladder?

Answer 24

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

Question 25

what are bile salts?

Answer 25

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

Question 26

what are gallstones caused by?

Answer 26

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

Question 27

what is the structure and function of the small intestine?

Answer 27

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

Question 28

what are the main intestinal enzymes?

Answer 28

enterokinase, aminopeptidase and dissacharides

Question 29

what is the function of the large intestine?

Answer 29

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