WEEEK 4- Physiology of the Lower Gastrointestinal Tract small intestine Flashcards
where is the pancreas and what kind of functions does it have?
-just behind stomach
-Exocrine and endocrine tissue
-endocrine islet of langerhans that secrete insulin and glucagon
Exocrine function
– Secretes pancreatic juice containing:
• Pancreatic enzymes actively secreted by acinar cells that
form the acini
• Aqueous alkaline solution actively secreted by duct cells that
line pancreatic ducts – rich in NaHCO3 for neutralising HCl
how are hormones able to control pancretic secretion?
- acid in duodenal stimulates secretin rlease in the duodenal mucosa
- secretic travels in the blood to pancretic duct cell that secretes bicarbonate and travels back in the duct and neutralises the acid in the duodenal and switches secretin off
- fat and protein in the duodenum allows secretion of CCK in the mucosa, it travels in the blood to acinar cells to increase scertion of pancretic digestve enzymes in to the duodemum travel in the ducts to duodeum and small intetsine and breakdown products
what kind of enzymes are produced by the pancreas that break down proteins?
-Trypsinogen - converted to active form trypsin by enterokinase
• Chymotrypsinogen – converted to active form chymotrysin • Procarboxypeptidase – converted to active form carboxypeptidase
-they all affect different types of bonds
what are the 3 main enzymes produced by the pancreas?
Proteolytic enzymes
Pancreatic amylase
Pancreatic lipase
how does amylase works?
– Converts polysaccharides into the disaccharide maltose
how does lipase work?
– Only enzyme secreted throughout entire digestive system that
can digest fat
what does the liver secrete?
bile salts
what is the livers blood supply like?
Hepatic artery -Oxygen and metabolites hepatic vein -away Hepatic portal vein -absorbed nutrients from digestive tract
what is bile? secreted, stored, contents
-made in heptaocytes of the liver, stored and concentrated in gallbladder
-actively secreted by liver and actively diverted to gallbladder between meals
-Aqueous alkaline fluid containing:
• Bile salts
• Cholesterol
• Lecithin
• Bilirubin
– After meal, bile enters duodenum
what is bile salts?
derived from cholestrol -Convert large fat globules into a liquid emulsion After participation in fat digestion and absorption, most are reabsorbed into the blood
what happens in the small intestine?
most digestion and absorition
what are the 3 segments of the small intestine?
– Duodenum
– Jejunum
– Ileum
how has the small intetstine adapted for absoprtion and digestion?
- large folds called plica have lots of villi on which increases SA
- Villi have epitheial cells which have microvilli which also increases SA
what is the mesentery?
a membrane that keeps the large and small intestine apart
what is the primary method of motility in the small intestine?
- where the circular muscle contracts and relaxes over and over
- causes mixing of enzymes and propels chyme through intestine lumen
what initiates segmentation?
by pacemaker cells in small intestine which
produce basic electrical rhythm (BER)
what does the migrating motility complex do?
Sweeps intestines clean between meals
what kind of secretion occurs in the small intestine?
– Juice secreted by small intestine does not contain any digestive enzymes – Synthesized enzymes act within brush-border membrane of epithelial cells • Enterokinase • Disaccharidases • Aminopeptidases
how does digestion occur in small intetstine?
– Pancreatic enzymes continue carbohydrate
and protein digestion
– Brush-border enzymes complete digestion of
carbohydrates and protein
– Fat is digested entirely within small intestine
lumen by pancreatic lipase
how often is the lining replaced?
every 3 days
how does carbohydrates get digested?
- carbohydrates is broken down to glucose galactose fructose
- they then need to be absorbed by the epithelial cells in brush boarder enzymes
- glucose and galactose enter the SGLT symporter into the epithelial cells
- fructose enters through passive facilitated diffusion via GLUT-5
- CAN BE TRANSPORTED through epithelial cells and exported through GLUT 2
- then enter blood and into liver
how does fat absorption occur?
- fat is broken down by bile salts and and lipase to monoglyerides and fatty acids
- travels through lipid membrane of the cell where they are made into triglycerides
- which then gets incorporated into chylomicrons which through exocytosis are taking up by the centeral lacteals and anre taken back into lymph into liver
how does protein get absorbed?
protein is broken down by pepsin and the pancreatic enzymes and the peptidase in the brush border down to amino acids and small peprides
-the acids are taken in again through symporteres and the small peptides they are exported through facilitated diffusuon into the blood to the liver
what does the large intestine consist of?
– Colon
– Cecum
– Appendix
– Rectum
what is the role of the large intestine and what happens there?
• Primarily a drying and storage organ
• Chyme from small intestine consists of
indigestible food residues, unabsorbed biliary
components, and remaining fluid
what happens in the colon?
– Extracts more water and salt from contents
– Feces – what remains to be eliminated
what is the main type of motility in the Large intetsine?
– Pouches or sacs in structure
– Actively change location as result of contraction of circular smooth muscle layer
• Haustral contractions
– Main motility
– Initiated by autonomous rhythmicity of colonic
smooth muscle cells
what is mass movement that occurs in the LI?
– Massive contractions
– Moves colonic contents into distal part of large intestine OCCURS 3 TIMES A DAY
what is the gastrocolic reflux?
– Mediated from stomach to colon by gastrin and by
autonomic nerves
– Most evident after first meal of the day
– Often followed by urge to defecate
what is the defecation reflux and when does it occur?
– Initiated when stretch receptors in rectal wall are
stimulated by distension
– Causes internal anal sphincter to relax and rectum and
sigmoid colon to contract more vigorously
– If external anal sphincter (skeletal muscle under voluntary
control) is also relaxed, defecation occurs
what is secreted in the LI?
alkaline mucus helps lubricate the feaces and protection of constipation
does digestion occur in the IL?
– no digestion as no digestive enzymes
– colonic microflora (>500 species) digest cellulose to short chain fatty acids
does absportion occur in the IL?
– salt and water
– Vitamin K synthesised by bacteria
WHAT IS EXCRETED OUT THE IL?
– approx 2/3 water
– undigested cellulose, bilirubin, bacteria and salt
why is bacteria from stomach important?
- fat storafe
- metabolism of drugs
- breaks down food compounds
- protection of epitheial injury
how does gastrin work as gastrointestinal hormones?
– Stimulated following presence of protein in stomach
– Inhibited by accumulation of acid in stomach
– Functions
• Increases secretion of HCl and pepsinogen
• Enhances motility - gastric motility, stimulates ileal
motility, relaxes ileocecal sphincter, induces mass
movements in colon
• Helps maintain digestive tract lining
how does secritin work as gastrointestinal hormones?
– Stimulated following presence of acid in duodenum
– Functions
• Neutralising acid by
– inhibiting gastric emptying
– inhibiting gastric secretion
– Stimulates NaHCO3 secretion in the pancreas
– Stimulating NaCO3-rich bile from the liver
• Along with CCK, is trophic to exocrine pancreas
how does secritin work as gastrointestinal hormones?
- Inhibits gastric motility and secretion
- Stimulates secretion of pancreatic enzymes
- Causes contraction of gallbladder to release bile
- Along with secretin, is trophic to exocrine pancreas
- Important regulator of food intake
