4. Digestion and Absorption Flashcards
What is a zymogen?
Describe fluid movement in digestion.
Why is so much fluid secreted during digestion (e.g. 2L into stomach, 1.5L into SI)? (3 reasons)
Inactivated form of enzyme.
- 2L intake/day and 200ml out in faeces. Reabsorb lots e.g. 8L from SI and 500ml from LI.
1) Lubricate digestive process
2) Dissolve nutrients and molecules into solution so can pass across borders
3) Enzymes largely catalyse hydrolysis = need H2O to break bonds
Describe the 3 main salivary glands?
Describe the structure of a typical salivary gland.
How much saliva is produced/day?
Parotid (serous saliva, watery, amylase-rich), Sublingual and submandibular glands (mix of serous and mucus secretions, more of antimicrobial role than amylase)
Compartmentalised structure. Serious acinus contain amylase in zymogen granules. Mucus acinus secrete mucus. Ionic composition altered at striated and excretory ducts by addition of HCO3 (amylase works best in alkaline pH).
1.5L
How is salivary secretion controlled?
What is amylase, and what does it do and produce?
Secretion in glandular cells contrlled via autonomic (parasympathetic) NS (ACh, M3). Secretions are modified via producion of NaHCO3 and fluid coming from blood flow (controlled by vasoactive intestinal peptide, VIP)
Enzyme in saliva (alpha-amylase in humans), catalyses hydrolysis of alpha 1,4 glycosidic bonds in starch (amylose, amylopectin, glycogen) (but not terminal 1,4 or branched 1,6 bonds). Produces di and trisaccharides (maltose, maltotriose) and branched oligosaccharides. Activity limited by low pH of stomach. Converts about 40% of starch.
Describe the different parts of the stomach.
How much gastric juice is produced/day and what does it contain?
Fundus: storage compartment. Cardia. Corpus: digestive unit. Antrum: motor part of stomach where mixing/grinding occurs. Pyloris: anatomical sphincter.
2L. Contains HCl, salts, pepsin, mucus, H2O, IF (B12 absorption!), bicarbonate (maintains pH7). K+ conc is higher than plasma (Prolonged vomiting can cause hyperkalaemia), H+ conc increases as secretion rate increases
Describe the 6 types of cell found in a typical gastric pit in e.g. the stomach.
1) Superficial epithelial cell
2) Mucous neck cell: injects lots of HCO3 in to produce buffer between stomach acid and itself
3) Stem cells: constantly replenish gut lining
4) Parietal cells: produce acid = optimal env for enzymes
5) Chief cells: produce pepsinogen
6) Endocrine cells: release peptide hormones, concentrated in corpus region
Describe the control of gastric acid secretion.
Driven by gastrin which is released from G cell (antral) in response to peptide -> stimulates CCKB receptors on parietal cells (produce acid = direct) and enterochromafin like cells (ECL, in corpus) (release histamine = indirect). Histamine stimulates acid secretion from parietal cells via H2 receptors.
D cells can produce somatostatin = inhibits G cells, ECL cells and parietal cells via SST2 receptors
Also descending excitatory control via vagus to G cells, ECL and parietal cells (M1 receptor ECL cell)
How is pepsin produced, and what does it do?
What is secretin?
Secreted as inactivated proenzyme pepsinogen from chief cells. Converted to pepsin by cleavage of acid liable linkages (lower pH = faster). Pepsin converts pepsinogen to pepsin (forward loop). Enzymatic activity highest at pH3 and below. Catalyses hydrolysis of peptide bonds.
Reaction is driven by acid presence, stretch receptors (ACh - parasympathetic), gastrin (from G cells in response to peptides)
Secretin released from duodenal S cells when acid leaves stomach to SI.
What is in a pancreatic secretion?
Up to 1.5L/day. Aqueous HCO3-rich component (promoted by secretin/VIP) and enzymatic component (promoted by CCK (released from duodenum I cells by fat and peptide) and ACh (neuronal, parasympathetic (gastro and duodenopancreatic ACh), AND local reflexes e.g. stretch)).
NB: lots of bicarb to convert acid -> alkali. Nutrients detected by sensory nerves triggers pancreatic release.
List some pancreatic enzymes secreted as zymogens.
What happens to them in the duodenum?
How is premature enzyme activation in the pancreatic ducts prevented?
Trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidase.
Trypsinogen specifically converted to trypsin by enteropeptidase (secreted by duodenal mucosa). Trypsin drives conversion of more trypsin from trypsinogen and converts chymotrypsin, chrboxypeptidase and elastase.
Pancreatic juice contains trypsin inhibitor
List some pancreatic enzymes secreted in active form.
How is fat digested?
Pancreatic amylase, pancreatic lipases (glycerol ester hydrolase, cholesterol ester hydrolase etc.) V. active = all proteins and sugars can be digested in duodenum by pancreatic enzymes in absence of gastric/salivary activity
Bile salts promote fat emulsification. Synthesised from cholesterol: cholic acid, chenodeoxycholic acid. Chemical structure (hydrophobilc and hydrophilic side) means can encapsulate lipid.
How is bile produced and stored?
What stimulates it’s secretion?
Composite of bile acids formed from cholesterol - conjugated/unconjugated with glycine/taurine. Secreted from hpatocytes into canaliculi and ducts, stored in gall bladder, stimulated by CCK from I cells IRT lipid presence. Isotonic then HCO3 secretion in ducts - alkaline.
Emptying of gall bladder (sphincter of Oddi relaxes) by ACh, gastrin and CCK stimulation
We only have a certain amount of bile; what happens to it?
Where are the following found, and what do they do:
a) Brunner cells
b) Peyer’s patch
How is absorption increased in the small intestine?
Bile salts are reabsorbed in ileum and recirculated to liver by hepatic portal vein - enterohepatic circulation of bile.
a) duodenum - secrete bicarbonate rich mucus
b) Ileum - aggregates lymphoid tissue (b/c start to get concentrations of pathogens)
Villi and microvilli. Folding of SA. Brush border of villi have digestive enzymes on enterocytes = final digestion
Describe fat breakdown.
Describe fat absorption.
Hydrolysis of emulsified fats -> micelles consisting of 2 monoclycerides on triglycerides or FA and free cholesterol or lysophospholipids
Micelle diffuses through layer of mucus/bicarb to brush border -> FA/chol/phospholipids absorbed (free diffusion) -> bile salts retained in lumen and reabsorbed in ileum. 50% cholesterol pumped back out into lumen by ABC multidrug resistance transporters.
Once in enterocyte, FA binding protein and sterol binding protein transport fat products to SER where they’re reconstituted to triglycerides/cholesterol ester/phospholipids -> packaged by lipoproteins into chylomicrons -> secreted to intracellular space and taken up by lacteals of lymph circulation -> rejoin venous circulation at subclavian vein
Describe carbohydrate digestion.
Describe carbohydrate absorption.
Brush border enzymes: lactase (lactose -> glucose + galactose), sucrase (sucrose -> glucose + fructose), alpha-dextrinase (alpha limit dextrins -> glucose), glucoamylase (maltooligosaccharides -> glucose)
Not fat soluble so need carrier systems/transport proteins. Na+/glucose cotransporter SGLT1 in apical membrane carries glucose and galactose. Facilitated fructose transporter GLUT5 in apical membrane. Facilitated sugar transporter GLUT2 in basolateral membrane transports all 3 sugars. SGLT1 is 20 active transport: energiesd by electrochemical Na+ gradient maintained by basolateral Na+/K+ pump which extrudes Na+.
Describe peptide digestion.
Describe peptide absorption.
Fat drains to lacteals -> lymph -> subclavian vein. What path do sugars and AAs take?
On brush border. Amino-oligopeptidase converts oligopeptides -> aa/di/tripeptides. Aminopeptidases convert di/tripeptides -> aas. Dipeptidylaminopeptidase covert oligiopeptides -> di/tripeptides.
Di/tripeptides are also taken up into enterocyte and broken down by cytoplasmic peptidases e.g. prolidase/di/tripeptidase
90% of aa that enterocytes acquire are transported across basolateral membrane (Diff types of aa transporters, some Na+ dependant). 10% serves intracellular protein synthesis.
Capillaries -> hepatic portal vein to liver which stores things/controls release.
