Digestion and absorption Flashcards
gastrointestinal system
- One long tube
- Ingestion
- Secretion
- Motility
- Digestion
- Absorption
- Defecation
ingestion
taking food in
secretion
water, acids, buffers, enzymes
motility
contraction and relaxation of smooth muscle
digestion
mechanical and chemical
absorption
95% via small intestine
defecation
indigestible substances, cells, digested materials not absorbed
oral cavity
Mouth and pharynx - back of mouth
first step in digestion process
mastication (chewing)
what does mastication do?
o Grind up food for swallowing
o Mix food with saliva
o Stimulate taste buds
when is swallowing (deglutition) initiated?
when food bolus forced by tongue to rear of mouth to pharynx - stops food going into lungs
voluntary and involuntary - how hard and what side of mouth
saliva
- Serous (watery) solution containing electrolytes, mucous, proteins (immunoglobulin A, lysozyme) and enzymes (amylase, lipase)
- Produced by 3 pairs of branched glands that provide 95% of total salivary volume (1l/day)
glands that produce saliva
o Parotid (cheek, 60%) o Submandibular (under chin) o Sublingual (under tongue)
salivary gland secretion
- Acinar (mucous) cells produce primary secretion that is isotonic with plasma
- Duct cells actively reabsorb Na+ (and Cl-) ions, some secretion of K+ and HCO3- ions (protection from acid) - make cells more watery
- Impermeable to water, producing hypotonic saliva for lubricant)
stomach
• Stores food/delivers to gut
o Ingestion mins, digestion hours
• Chyme (3 waves/min)
• Secretes 2-3 l/day of gastric juice from mucosa
regulation of gastric secretion
- Cephalic phase
- Gastric phase
- Intestinal phase
cephalic phase
- (30% of response to meal) - start secretion
o Prior to arrival of food in stomach, vision, smell, chewing, taste stimulate gastric secretion (inhibited by fear or depression)
o Gastrin also released (hormone stimulates gastric secretion)
o Before anything goes to stomach - as chewing and when thinking and smell food
gastric phase
- (60% of response to meal) - ramp up secretion
o Stretch and products of protein digestion stimulate gastric secretions
o Gastrin also released by stretch - stimulates secretion of more gastric fluid
o All stimulate motility, larger meals increase rate of emptying
intestinal phase
- slow down secretion
o Chyme entering duodenum decrease gastric secretion and motility
o Removal of peptide fragments (no longer stimulate gastrin release)
o Removal of food leads to decrease in pH (pH < 2 inhibits gastrin release)
o Distention of duodenum leads to release of hormones (inc cholecystokinin (cck)) which inhibits gastrin releas
small intestine
- Absorption of nutrients
- Passes food cyme down system
- Mucosal lining and muscle layer round outside
- Mucosal layer has more folds
- Circular and longitudinal smooth muscle lining - propel food down intestine
- Vascularised villi
why does mucosal layer of SI have more folds?
increase SA
finger like villi
rich blood supply
lymphatic supply
epithelial cells
microvilli
increases SA
vascularised villi
blood supply
capillaries
arterial supply to intestine and venous supply to collect nutrients across mucosal layer into supply delivered to liver
epithelial lining
microvilli
on lumen of gut
nutrients pass
into venous circulation
small intestine motility
• Peristaltic, longitudinal type contractions main propulsive force over short distances (10-15cm)
o Increases when chyme enters - 12x min - upper part - 2/3x min in distal part
o Wave of contractions
o Happens when under tone - increases when chyme enters SI
- Segmentation is alternate contraction and relaxation of whole segments intestine and serve to mix chyme with pancreatic secretions and bile and increase exposure to mucosal surface - adapted for absorption
o Circular smooth muscle
control of small intestine motility
- GI tract has similar basic structure of outer longitudinal smooth muscle, inner circular smooth muscle and lumen lined with mucosa
- Muscle primed by pacemaker cells (interstitial cells of Cajal) which initiate a basal electrical rhythm - smooth muscle under tone and contracting - wave down gastral intestine system
- Intrinsic enteric NS
pancreas
- endocrine and exocrine organ
• Releases hormones
• Acinar cells secrete enzyme rich Cl- solution
• Duct cells secrete aqueous HCO3- solution (1.5 l/day) - take sodium and water with it
• CCK (released in response to fat and protein in duodenum) stimulates enzyme secretion
• Secretin (released in response to acid in duodenum) stimulates HCO3- production
o Protecting duodenal mucosa
• Duct cells secrete bicarbonate - neutralises acid - imp for enzyme function - digest food
what is HCO3- necessary for?
o Digestive enzyme activity
o Micelle formation
o Protecting duodenal mucosa
large intestine
- Storage of faeces - waste
- Chyme takes 12h to reach terminal ileum
- Ileal-caecal sphincter acts to prevent retrograde movement of chyme and bacteria from colon
- Peristaltic and segmental movement mostly slow and non-propulsive serving to improve salt and water absorption
- Mass movements occur infreq (2-3x day) and drive colonic contents towards rectum (high pressure moving 30cm/more)
function of LI
reabsorption of water and storage faeces, some bacterial met of carb to produce short chain FAs
overview
make sure to look through notes
carbohydrate digestion
• Mono- and disaccharides
• Ingested as complex carbs
o Starch (plants) - sweeter when broken into saccharides
o Glycogen (meat)
• Digestion begins in mouth (salivary a-amylase), but ceases in acidic env of stomach
• Most carb digestion occurs in SI by action of pancreatic a-amylase - more alkaline env - pH 8
carbohydrate absorption
diagram
liver - hepatic portal supply
diagram
transport from portal vein
- Glucose, galactose and fructose directly transported to liver
- Fructose and galactose converted to glucose derivatives and have same fate as glucose (liver glycogen/maintain blood glucose)
- Glucose enters organs by fac transport, skeletal muscle and adipose tissue regulated by insulin (released from pancreas)
blood glucose response to ingestion 50g glucose/starch
- Polysaccharide has to be broken down first
- Slower appearance with glucose
- Glycaemic index - area under curve - ingesting carb
protein digestion
- Begins in stomach (pepsin - protease) - acid env - responsible for around 15%
- Pepsin activity halted by alkaline pancreatic secretions
- Majority protein (50% food, 25% gut secretions, 25% epithelial cells) digestion occurs in SI by action pancreatic enzymes - activated in duodenum
- Enzymes activated outside pancreas (to prevent digestion of pancreatic proteins) by trypsin
classes of protein digestive enzymes
o Endopeptidases - cleave interior peptide bonds
o Exopeptidases - cleave external peptide bonds
protein digestion/absorption
- Di- and tripeptides further broken down into AAs by intracellular peptidases of enterocyte (epithelial cell)
- Absorption of AAs occurs by diffusion and sodium dependent transport
blood AA response to ingestion 20g AAs/milk protein
- No digestion with AAs
- More steps in protein digestion
- More sustained release
liver functions
o Bile production
o Met - glucose and glycogen storage and production
o Detoxification of blood
liver - digestion
• Gallbladder contracts following meal to eject into duodenum
o Stimulated by secretin and CCK
o 0.5-1.5/day
o 90% bile salts reabsorbed and recycled - continuously
digestion of fat
- Lipids emulsified by bile
- Lipases break down triglycerides into FAs and monoglycerides
- FAs and monoglycerides packaged into micelles absorbed by microvilli
- FAs and monoglycerides converted back into triglycerides - aggregate with cholesterol, proteins and phospholipids to form chylomicrons
- Chylomicrons move into lymph cap, which transports them to rest of body
blood lipid response to 70g triglyceride
- Less rapid
- Longer digestion and absorption process
fat soluble vitamin absorption in SI
o Some, but not all, require digestion absorbed from micelle mostly in SI
o Enter chylomicrons and lymph system
o Stored in tissues
water soluble vitamin absorption in SI
o Diffusion (high concs) - pass epithelial cells o AT (low concs) o Not stored
water
• Approx. 8l absorbed each day, 6l secreted in GI tract
• 90% absorbed in SI, 10% LI (colon has greater absorption capacity if necessary)
• Water moves rapidly across intestine in response to osmotic grads ensuring luminal contents isosmotic with interstitial fluid
o Hypotonic solution increases water absorption
the serosa is located where?
outer cover of the small intestine
how much do the buccal glands contribute to saliva production?
5%
what does alpha amylase break down?
amylose (1, 4) bonds
what are the major hormones involved in digestion?
secretin
gastrin
cholecystokinin
in what order do the stages of swallowing occur in?
Buccal phase
Pharyngeal stage
Oesophageal Phase
which digestive phase of the stomach is pepsinogen released?
cephalic
how long does it take food to reach the stomach?
a matter of s
how long does a bolus usually stay in the stomach?
2-4h
in the duodenum and jejunum, triacylglycerol digestion requires?
bile salts
lipases
bicarbonate
how much saliva can we produce?
up to 6ml/min-1
intrinsic enteric NS
o Myogenic mechanisms (inherent to smooth muscle) responsive to stretch - created within muscle - creates reflex where it contracts
o pH
o products of digestion such as FAs
o osmolality - more hypertonic chyme has greater effect on contraction
bile
o Bile salts (digestion and absorption of fats) and HCl- - alkaline solution
o Continuously produced
o Between meals stored in gall bladder and concentrated
o Aids secretion bile pigments, cholesterol, steroids, heavy metals, drugs
secretions in gastric juice
o Ions, mucus (protection)
o Pepsins, from chief cells (protein digestion), lipases
o Intrinsic factor from parietal cells (vit B12 absorption)
o HCl (activate pepsin, breakdown of tissue, kill bacteria
