Nutritional physiology Flashcards
Just try to keep up
what is the upper GI tract
Mouth
Oesophagus
stomach
what is the middle GI tract
duodenum
jejunum
ileum
what is the lower GI tract
caecum
colon
rectum
what are the accessory organs of the GI tract
salivary glands
gallbladder
exocrine pancreas
what are the 4 layers of tissue lining the GI tract
- mucosa – inner, contacts foods, specialised for secretion/absorption
- submucosa – blood and nerve supply. Connective tissue. Has submucosal plexus
- muscularis – circular muscle layer and longitudinal muscle layer. Mixing and propulsion. Has myenteric plexus
- serosa – outer CT, simple squamous. Provides lubrication.
what are the 4 basic digestive processes
- motility – movement of intake of food
- secretion
- digestion – chemical breakdown
- absorption – mostly in SI
what are the 4 ways that digestive function is regulated
- autonomic smooth muscle
- intrinsic nerve plexus
- extrinsic nerves
- gastrointestinal hormones
explain how autonomic smooth muscle regulated digestive function
exhibits spontaneous rhythmic cycles of depolarisation/repolarisation
called SLOW WAVE POTENTIAL (smooth muscle only)
generated by Cajal cells – control frequency of contractions
fluctuations in membrane potential bringing membrane closer/further away from threshold
(different from action potentials all or nothing)
what is the enteric nervous system
network of submucosal and myenteric plexuses
3 major ENS neurobns:
1. Sensory
2. Moror
3. Interneurons
Major neurotransmitters:
1. Acetylcholine
2. Norepinephrine
3. Serotonin
how do GI hormones regulate digestive function
Regulate digestive function by acting on smooth muscle and glands, liver and pancreas
Secreted by endocerine GI cells
Excititary of inhibitory effect
Gastrin – stomach/antrum
Secretin: SI cells
CCK: SI cells
what are the 2 main functions of the mouth
mastication
secretion
explain the function of mastication
4 muscle pairs in mandible
Reflex from medulla
Consciously initiated/stopped
Function: mechanical breakdown, mix with saliva (solvent for taste), taste
describe secretion in the mouth
3 pairs of Salivary glands
Starch digestion, swallowing, antibacterial & oral hygiene, neutralise acids
1-2L/day
what are the 2 main functions of the oesophagus
swallowing
secretion - protective mucus)
explain swallowing in the oesophagus
Voluntary
Unstoppable
Bolus stimulates pharyngeal pressure receptors – relayed to swallowing centre in medulla
Stimulates reflex activation of muscles
Involuntary stage has 2 stages:
1. Pharyngeal – elevation of soft pallet/pharynx, upper sphincter opens and closes after bolus. Epiglottis seals as this happens. Swallowing centre inhibits breathing centre.
- Oesophageal – peristaltic smooth muscle contractions. Primary wave by swallowing centre, subsequent waves by intrinsic n. plexus.
what is the function of sphincters in digestive process
Pharyngoesophageal sphincter
Skeletal muscles prevents air form entering GI tract during breathing
Gastroesophageal sphincter
Smooth muscle
Prevents gastric reflux
list some functions of the stomach
Storage – hold bolus for chemical processes
Protein digestion
Food chyme
explain the process of HCl secretion
- CO2 diffuses into parietal cell from blood
- CO2 combines with water in enzyme catalysed reaction carbonic anhydrase. Forms carbonic acid
- Dissociates into bicarbonate ion + H+
- Bicarbonate ion into bloodstream. Antiporter in plasma membrane swaps bicarbonate ion for chloride ion
- Hydrogen-potassium pumps moves H+ into gastric gland duct and K+ in parietal cell
- Chloride ions diffuse into gastric gland duct
what cells secrete HCl in the stomach?
parietal cells
what cells secrete pepsinogen in the stomach
exocrine chief cells
Autocatalytic reaction
what is the gastric mucosal barrier
Mucus cells physically protect cell from acidity
Mucosal secretions alkaline to protect cells and neutralise acid
HCl (H+) is impermeable to mucosal cells
Tight junctions between cells
explain the pancreas’ role in digestion
Endocrine function:
Pancreatic islets – alpha/beta
Exocrine: function
* Pancreatic juice:
* Aqueous alkaline solution (duct cells)
* Digestive enzymes (acinar cells)
o Proteolytic enzymes
o Pancreatic amylase
o Pancreatic lipase
explain the pathway for bicarbonate ion production in the pancreas
- H2O and CO- combine through enzyme anhydrase carbonic acid
- Carbonic acid dissociates to H+ + bicarbonate ions
- H+ swapped for Na+ by antiporter in blood
- Bicarbonate ions transported to intercalated ducts in exchange for Cl-
- Sodium ions and H2O follow bicarbonate ions into ducts
- Ions and water move through intercalated ducts to interlobular ducts
how are pancreatic secretions controlled
Vagus nerve (parasympathetic) stimulates pancreatic juice secretion
Secretin from duodenum stimulates increased release of secretin. Secretin is carried by blood back to pancreas, pancreatic duct cells are stimulated to release bicarbonate rich pancreatic juice
Duodenum releases Cholecystokinin – stimulated juice release
what is bile composed of
bile salts + lecithin + cholesterol + bilirubin
what is the purpose of the biliary system in the liver
- Emulsify large fat molecules in GI tract – micelle formation
Detoxification
Hormone secretion
what is the purpose of the biliary system in the gallbladder
Store and concentrate bile 10x – water and electrolytes absorbed
explain the duct system flow of bile (bile pathway)
- Hepatic ducts (carry bile from liver lobes) combine to common hepatic duct
- Common hepatic duct + cystic duct combine = common bile duct
- Common bile duct + pancreatic duct = hepatopancreatic ampulla
- hepatopancreatic ampulla empties bile and pancreatic secretions into duodenum at major duodenal papilla
- accessory pancreatic duct empties pancreatic secretions into duodenum at minor duodenal papilla
explain the control of bile secretion and release
Vagal nerve (parasympathetic) contracts smooth muscle gallbladder to release bile
Secretin stimulates bile secretion (endocrine)
CCK stimulates gallbladder contraction
Bile salts stimulate bile secretion
what is enterohepatic circulation
Reabsorption of bile salts
Bile salts reabsorbed at terminal ileum
Through hepatic portal vein to liver
5% bile salts lost in faeces, 95% recycled.
list some non-digestive functions of the liver
Detoxification
Vitamin D activation
Nutrient storage – glycogen & lipids, vitamins
Metabolic processing
o interconversion of nutrients (e.g. GNG)
explain the cells involved in the liver structure
Hepatocytes – performs metabolic and secretory
Arranged in hexagon structure
Hepatic lobules – each has a triad (trio of vessels)
Kupffer cells – phagocytes of RBCs and WBCs
Sinusoidal endothelial cells – endocytosis and secretion
what are the digestive functions of the liver
Immediate metabolic processing of products from sugar/protein digestion
Fat digestion bypasses hepatic portal system to circulation – delivered by lymph system
what are the 4 major cells types in the small intestines mucosa
Absorptive (enterocytes, on microvilli. Contain digestive enzymes and absorption_
Goblet cells (produce mucous – protects)
Granular cells (defence)
Endocrine (regulatory hormones – secretin)
explain segmentation in terms of motility in the small intestine
Segmented sections of smooth muscle contraction
Greater frequency in beginning of SI
Basoelectrical rhythm initiated by SI pacemaker cells (but can be modified by local factors, nerve factors, hormonal factors) e.g.
o Local distention duodenal segmentation
o Gastrin empty ilium segmentation gastroileal reflux
o Extrinsic nerves modify strength: sympathetic/parasympathetic
what is the migrating motility complex
Peristaltic contractions removing any last debris to prep for next meal
Occurs in fasted state
3 phases
Regulated by motilin
what are the 2 phases of small intestinal digestion
luminal phase
membrane phase
explain the luminal phase of small intestine digestion
By enzymes from pancreas
Pancreatic amylase
Pancreatic proteolytic enzymes
Pancreatic lipase
Bile salts
explain the membrane phase of small intestine digestion
By brush border enzymes (remain bound)
1. disaccharidases
2. aminopeptidases
what are the modifications in the small intestine which suit it for absorption
mucosal lining – villi/microvilli
circular folds in lining
membrane bound enzymes
Rapid turnover of mucosal lining
New cells continuously produced
Migrate up, push older cells into lumen to be cleaned out
Cells adapt during migration: increase enzyme conc. and absorptive capacity
transepithelial transport: transport of substance through epithelium:
substances must pass through epithelial cell to interstitial fluid to be absorbed
what is the ileocecal juncture
barrier between small and large intestine
ileocecal valve and sphincter prevent back movement
prevents SI contamination
explain blood supply to the liver
Enters via hepatic artery (oxygenated from heart)
Enters via hepatic portal vein (venous from GI tract)
Exits via hepatic vein
how to bile and blood flow through the liver
Hepatic ducts: carry bile from liver
Hepatic veins: return processes molecules and waste to circulation via heart
Hepatic artery: delivers oxygen and blood metabolites
Hepatic portal vein: nutrients from intestines
list some of the unique characteristics of smooth muscle
Resting length is shorter than optimal
Stress relaxation (sudden stretch can increase tension, then muscle rapidly adjusts) e.g. in gallbladder to accommodate large volumes.
Slow contraction speed – crossbridges stay attached for longer. Uses less ATP
Versatile energy supply – can use anaerobic or aerobic respiration
outline the process of how calcium activates smooth muscle contraction
Excitation –>
APs –>
rise calcium (calcium voltage gated channels) –>
calcium calmodulin complex forms
–> activates myosin light chains kinase
–> phosphorylation of cross bridges in thick filament
what is the difference between phasic and tonic muscle contraction
Phasic: rapid activity in bursts
Tonic: Resting contraction e.g. in sphincters
compare multi unit vs. single unit muscle cells
Single unit: myogenic
Self regulated
Can be phasic or tonic
Multi unit:
Phasic: (controls in bursts)
neurogenic (under ANS control)
what are the main receptors present in the GI tract
Acetylcholine: PARASYMPATHETIC control. muscarinic receptors. E.g G protein receptor pathway
Adrenaline/noradrenaline: SYMPATHETIC control. alpha (1 & 2) and beta receptors
–> Alpha 1 receptors: excitatory response
–> Alpha 2 receptors: inhibitory response
