Week 8 Integrating Human Physiology Flashcards
Secretion during the pre-ingestive cephalic phase
Initiated when receptors in the head (cephalic) are stimulated by thought, sight, small and taste of food.
Before we even eat anything!
Involves the parasympathetic nervous system (Vagus) short: only lasts a few minutes
What is stimulated during this cephalic phase ?
Same responses as ACTUAL eating, but normally weaker)
Saliva production (24h a day, but 2/3 in early cephalic phase)
Gastric acid (1/3 of max capacity) (protein digestion)
Gallbladder contraction (start to release bile to absorb fat)
Pancreatic exocrine (enzymes) and endocrine function (hormones, e.g. insulin spike)
Why is it important ?
Feed forward mechanism:
Prepares the body for influx of food
Secretion stimulated by food in the stomach
Parietal cells:
• Secretion of HCl (breaks down food) • pH 1-2 (very acidic, corrosive!)
Chief cells:
• Secretion of pepsinogen (protein digestion) • Secretion of lipase (lipid digestion)
Mucous cells:
• Production of the mucus-bicarbonate layer (protect us from low gastric acid)
The bicarbonate-mucus layer
Continuously mucus production (goes down the gut) • Bicarbonatecatcheshydrogen,releaseofgasandwater
“Bicarbonate buffer system”
Critical for protection of the
mucosa (mucus barrier)
& makes it watery (good for digestion)
Secretion in the small intestine
Presence of food stimulates secretion
HCO3- secretions neutralise acidic chyme (stomach contents) (Bicarbonate can come from pancreas and duodenal glands)
Enzymes work in alkali range not low pH
Secretion of digestive enzymes and bile
Pancreatic exocrine secretion
Exocrine cells (Acini) connected to duct
Produce pancreatic juice, watery solution of digestive enzymes and HCO3- (optimizes pH for enzymes)
Enzymes:
Proteases secreted in inactive form to prevent auto-digestion Amylase (starch) / lipases (triacylglycerol) secreted in active form
Bile from the gall bladder – reservoir for bile
yellow/green bitter/alkaline solution
Contains: bile salts, bile pigments (bilirubin/biliverdin), cholesterol, neutral fats, phospholipids and electrolytes
Function of bile is emulsification of dietary fat
2 process to regulate/release bile secretion:
• Cephalic phase: Paras. NS (vagus)
stimulates the bladder to constrict of
smooth muscle & relaxation of sphincter
• Presence of nutrients triggers hormones (CCK) via blood to constrict bladder
Digestion & Absorption
Water absorption
Water crosses lipid-rich membranes by diffusion through aquaporins
lipid bilayer
Water absorption
Water crosses lipid-rich membranes by diffusion through aquaporins
lipid bilayer
Carbohydrate Digestion
Break down starch to glucose
glucose digestion begins in mouth with secretion of
salivary α-amylase (1,4 bond) (only short, then swallowed)
Enzymatic activity inhibited in stomach due to low pH
Most of digestion happens due
to pancreatic amylase,but doesn’t liberate glucose!!! only short-chain glucans
Absorption of glucose from duodenum stimulates pancreatic exocrine secretion (α-amylase) via release of hormones (CCK, secretin) “nutrient stimulated process”
Carbohydrate digestion & absorption at the brush border
Enzymes liberate monosaccharides from disaccharides in the diet and glucan from starch digestion
Only glucose, galactose and fructose can be absorbed in humans
Monosaccharides are released in really close proximity to their transporters (efficient absorption)
They don’t have to travel far, no malabsorption
Protein Digestion
e.g. piece of meat:
• Mechanical disruption by chewing and gastric
motility (increase surface and break down structure)
Stomach parietal cells
• Protein denaturation by HCl (break bonds and folding) leaving large polypeptides
Stomach chief cells
Pepsinogen pH < 5 Pepsin (inactive) (active)
Mix of small peptides and amino acids sensed in duodenum CCK-induced release of pancreatic peptidases into duodenum; endo & exo peptidases
Pancreatic Peptidases act in the Duodenum
We don’t wanna digest ourself!
Trypsinogen (inactive) > Enterokinase > Trypsin (active)
V
Proenzyme (inactive) > Enzyme (active)
Endopeptidases
(break middle of chain)
trypsinogen chymotrypsinogen proelastase
Exopeptidases
(breaking terminal ends)
procarboxypeptidase A
procarboxypeptidase B
70 % oligopeptides: 30% free amino acids
Protein Absorption: 3 Transport Mechanisms
Na+ dependent amino acid transport (4 types)
• H+ co-transport with small peptides e.g. di-tri peptides (Pept1)
• Larger peptide fragments moved by transcytosis (minor)
• Intracellular/Cytoplamic peptidases, most peptides leave cell as free amino acid
We can absorb free Amino acids and small peptide chains!
Fat digestion - Initiation
Whole process is heavy on enzymes…….
In the mouth: saliva containing lingual lipase (taste of fat (evolution?), not important nutritionally in humans)
Few digestion products stimulate the stomach to release enzymes and CCK from sI (feed forward mechanism)
In the stomach:
release of gastric lipase & motility-> Emulsification surface area increase
Fatty acids in duodenum-> Cholecystokinin (hormone)
(from action of gastric lipase)
/\
Bile release
(gallbladder contraction, relaxion of sphincter of odi)
3 Pancreatic enzymes
Triacylglycerol hydrolase Cholesterol ester hydrolase Phospholipase A2
Bile is important for Emulsification
Doesn’t contain enzymes, but important for digestion
• Bile salts crucial in increasing (and maintaining) the
surface area for lipase action
• Prevent droplets to merge into big one again
Hydrophobic tail and hydrophobic head, repel each other
Micelles
