Digestive System (3) Flashcards
Where does large intestine extend from?
- from ileocecal valve to the anus
Major digestive function of large intestine
- “absorb” most remaining water from indigestible food residues
- “store” the residues temporarily
- “eliminate” them from body as feces
Subdivisions of the large intestine
CACRA
- cecum
- appendix
- colon
- rectum
- anal canal
What is ileocecal valve?
- sphincter muscle located at the junction of ileum and colon
What is cecum?
- saclike first subdivision of large intestine
- lies below ileocecal valve
Function of the appendix
- contains masses of lymphoid tissue
- plays important role in immunity (part of MALT: mucosa associated lymphoid tissue)
- bacteria storehouse
Shortcoming of appendix
- its twisted structure provides ideal condition for enteric bacteria to accumulate and multiply
Distinct regions of the colon
- ascending colon
- transverse colon
- descending colon
- sigmoid colon
The rectum
- after sigmoid colon
- runs posteroinferiorly just in front of sacrum
What is the sacrum?
- shield shaped bony structure at the base of lumbar vertebrae
- connected to the pelvis
Anal canal
- last segment of large intestine
- lies in perineum
- opens to exterior at the anus
2 sphincters of the anal canal
- involuntary internal anal sphincter (composed of smooth muscles)
- voluntary external anus sphincter (skeletal muscle)
Bacterial Flora
- How is bacteria beneficial for us?
In the large intestine
- colonize the colon
- synthesize vitamin Bs and some vitamin Ks liver needs to produce several clotting proteins
- metabolize some host derived molecules (mucin, heparin, hyaluronic acid)
- ferment some indigestible carbohydrates (cellulose, xylan), releasing irritating acids and a mixture of gases
Mechanism of digestion
- breakdown food to monomers smol enough to be absorbed by GI tract linings
- catabolic
- done by enzymes secreted into lumen of the GI tract by intrinsic and accessory glands
- enzymatic breakdown is hydrolysis
3 common monosaccharides in our diet
- glucose, fructose, galactose
Where does digestion of starch (carbohydrate) begin?
- in the mouth
- “salivary amylase” splits starch into oligosaccharides
What are oligosaccharides?
- smaller fragment of 2 to 8 linked glucose molecules
- salivary amylase breakdown starch into that in the mouth
When does salivary amylase work best?
- slightly acidic to neutral environment (pH 6.75-7)
Until when does starch digestion continue?
- until amylase is inactivated by stomach acid and broken apart by stomach’s protein-digesting enzymes
How are carbohydrates that escape salivary amylase broken down?
- by pancreatic amylase in the small intestine
- starch converted to oligosaccharides
Which enzymes digest oligosacchiarides into monosaccharides?
- intestinal brush border enzymes
most important brush border enzymes that act on oligosaccharides
- dextrinase and glucoamylase (act on oligosaccharides with more than 3 sugars)
- maltase, sucrase, lactase (hydrolyse maltose sucrose, lactose)
Where does protein digestion begin?
- in the stomach
- pepsinogen is activated by HCl into pepsin
Where does pepsin function optimally?
- in the acidic pH range found in the stomach 1.5-2.5
- cleaves bonds involving amino acids tyrosine and phenylalanine
Protein fragments in small intestine
- trypsin and chemotrypsin secreted by pancreas cleave proteins into smaller peptides -> become grist for other enzymes
- pancreatic and brush border enzyme “carboxypeptidase” splits of one by one amino acid from carboxyl end
- “aminopeptidase” and “dipeptidase” liberate final amino acid products one by one from amine end
Primary site of lipid digestion
- small intestine because pancreas is the major source of lipases (fat-digestion enzymes)
Stable fat emulsion
- lipid digestion in small intestine
- fatty droplets are pulled off the large fat globules (because of bile too) forming stable emulsion
- aqueous suspension of fatty droplets
pancreatic lipases catalysis of fats
- cleaves off two of fatty acid chains, yielding free fatty acids and monoglycerides (glycerol with one fatty acid chain attached)
site of digestion of nucleic acids
- small intestine
enzymes that digest nucleic acids
- pancreatic nucleases in pancreatic juice => nucleic acids to nucleotide monomers
- intestinal brush border enzymes nucleosidases and phosphatases => break nucleotides to release free bases, pentose sugars and phosphate ions
Which vitamins does small intestine absorb?
- dietary vitamins
Which vitamins does large intestine absorb?
- K and B vitamins made by enteric bacterial guests
How are most water-soluble vitamins absorbed?
- by diffusion or via specific active/passive transporters
Absorption of vitamin B12
- intrinsic factor produced by the stomach binds to B12
- complex binds to specific mucosal receptor sites in terminal ileum
- triggers uptake by endocytosis
Why is the small intestine primary site of lipid digestion?
- pancreas is the major source of lipases (fat-digesting enzymes)
Which structure absorbs vitamins?
- small intestine, large intestine
- K and B vitamins made by enteric bacterial guests - large intestine
- dietary vitamins - small intestine
What is intrinsic factor?
- produced by stomach and binds to vitamin B12
- intrinsic-B12 complex binds to mucosal receptor sites in ileum
- active uptake of B12 by endocytosis
