GI physiology Flashcards
1
Q
Name the three salivary glands and state their locations.
A
Submandibular - under mandible
Sublingual - under tongue
Parotid - on side of face
2
Q
What is the composition of saliva and what do each of the components function as?
A
Water - acts as solvent, to avoid water flooding into mouth, softens and moistens food
Mucins - combine with water to form mucus, lubricating function
alpha-amylase - breaks down polysaccharides into maltose and glucose
Electrolytes - affect tonicity and pH, e.g. bicarbonate keeps pH high to avoid damage to the teeth
Lysozyme - bactericidal (cleaves polysaccharide component of bacterial cell walls)
3
Q
What is the nervous innervation of the salivary glands?
A
ANS - parasympathetic and sympathetic both stimulator.
Parasympathetic - watery, high volume saliva, stimulated by glossopharyngeal and facial nerves.
Sympathetic - small volume, viscous. High amylase content stimulated by beta2-adrenoceptors. alpha1-adrenoceptors.
Reflex control - presence of food in mouth stimulates chemoreceptors/pressure receptors to trigger saliva production.
4
Q
What is the histology of salivary glands?
A
Alveolar feeding into ducts. 3 types of alveoli: - mucus - secrete mucus - serious - secretes enzymes, e.g. amylase - mixed - serous and mucus
5
Q
What are the lower and upper oesophageal sphincters? What are their functions?
A
Thickened rings of smooth muscle (contract/relax to control open/closed state).
UOS above level of pharynx, prevents food regard
LOS before cardia of stomach (peristalsis of oesophagus in coordination with LOS relaxation). LOS should only be open when food/liquid passing through.
6
Q
Where are the three constrictions of the oesophagus?
A
- cervical - 15cm from incisors
- thoracic - crossed by arch of aorta and left main bronchus (22.5cm and 27.5cm from IT respectively)
- diaphragmatic - passing oesophageal hiatus (40cm from IT)
7
Q
At what vertebral level does the oesophagus start and end?
A
Lower level of Cricoid cartilage (C6) to T11-12 (where it enters the stomach)
8
Q
How is the oesophagus peritonised?
A
Retroperitoneal.
9
Q
What is the function of the oesophagus?
A
Carry found via contraction of the circular muscle (peristalsis) to the stomach.
10
Q
Describe the process of swallowing to food reaching the stomach.
A
Bolus pushed to back of mouth by tongue.
Presence of bolus initiates reflex contractions of pharyngeal muscles (coordinated by swallowing centre (medulla). UOS relaxes and epiglottis covers larynx opening. UOS contracts (once food through).
Propulsion of bolus to stomach (10s) by peristalsis.
LOS relaxes and bolus enters stomach.
Vagal reflexes cause relaxation of thin, elastic smooth muscle of gastric fundus and body.
11
Q
Why doesn’t the stomach rip with the pressure of food entering it?
A
Stomach arranged in rugae (pleats), which unfold and allow for increased space for food, without an increase in pressure of the stomach. When food leaves the stomach again, the stomach can fold back into rugae.
12
Q
Why do we chew?
A
Prolong taste, defence against respiratory failure.
13
Q
What is chewing controlled by?
A
Voluntary by somatic nerves (skeletal muscles of jaw/mouth). Chewing can also be involuntary.
Reflex - contraction of jaw muscles creates pressure of food against gums/hard palate - sensed by mechanoreceptors that inhibit jaw muscles causing reduced pressure and another contraction.
14
Q
What is the nervous supply to the oesophagus?
A
Vagus and sympathetic trunk.
15
Q
How does the oesophagus form embryologically?
A
From cranial part of primitive gut tube as a laryngo-tracheal diverticulum from the ventral wall. A tracheo-oesophageal septum divides the forgot into the trachea and oesophagus.
16
Q
What are the four tunics of the oesophagus?
A
Mucosa - non-keratinised stratified squamous epithelium.
Submucosa - ducts from glands responsible for lubrication.
Musculares externa - upper 1/3rd skeletal, lower 2/3rd smooth muscle.
Adventitia - connective tissue.
17
Q
What are the 5 parts of the stomach?
A
Cardia Fundus Body Pyloric antrum Pylorus
18
Q
What is the fundus mostly filled with? What is its function?
A
Gas
Storage (stretchy and thin)
19
Q
What causes hiccups?
A
If stomach too full can push on diaphragm and irritate it and cause hiccups.
20
Q
What is the function of the body of the stomach?
A
Mucus, HCl and intrinsic factor production.
21
Q
What is the function of the antrum of the stomach?
A
Mixing and grinding. Gastric production (enters circulation).
22
Q
What is the function of the pylorus?
A
Controls discharge of chyme into the duodenum.
23
Q
What is the function of the rugae?
A
Pleats that allows for expansion of stomach without an increase in pressure.
24
Q
What pH is the stomach lumen at?
A
2.
25
What are the functions of the stomach?
Temporary storage of food (prevents mass digestion and osmotic pull)
Initiation of digestion
Control of delivery to small intestine
Sterilization (HCl)
Intrinsic factor production (produced by parietal cells)
26
Where is the intrinsic factor B12 complex absorbed?
Ileum.
27
Peristaltic waves move from where to where?
Body to antrum.
28
Which part of the stomach has the most powerful contractions?
Antrum (thick muscle), body only has thin muscle (just where secretions move out), contraction of the pyloric sphincter creates rotation of blood.
29
Which muscle is responsible for the squeezing type motion that breaks down food?
Oblique muscle layer.
30
What cells are responsible for creating the peristaltic rhythm of the stomach?
Pacemaker cells (in longitudinal muscle where UOS is).
31
What is slo wave contraction and BER in the stomach?
Slow waves caused by spontaneous depolarisation to create basic electrical rhythm (BER) - fundamental property of longitudinal muscle fibres.
Slow wave depol is sub threshold so requires further depolarisation to induce APs leading to contraction.
32
What determines the stretch of contraction of the stomach?
Number of APs fired.
33
What things control gastric motility?
Gastrin - increases contraction
Food in stomach --> distension -- vagus/ENS reflexes --> ACh --> increased contraction
Fat/AAs/hypertonicity in duodenum --> inhibition of motility.
34
What cells release gastrin?
G cells in the antrum of the stomach.
35
What junctions are parietal cells connected by?
Tight junctions.
36
Describe the cellular process by which parietal cells pump HCl into the stomach lumen.
CO2 diffuses into cell, combines with water --> carbonic acid (enzyme carbonic anhydrase catalyses this reaction). Carbonic acid splits into hydrogen ion and bicarbonate.
Hydrogen pumped out (actively by KH pump in exchange for K).
At basolateral membrane, bicarbonate transported out in exchange for Cl (=transient alkalinity of blood).
Cl exits apical membrane --> osmotic potential --> water follows via paracellular pathway.
Leads to pH 2 of stomach lumen.
37
On the basolateral surface of the parietal cell what receptors for what substances can be found?
Gastrin, histamine, prostaglandins and acetylcholine which all influence the KH pump.
38
What happens when gastrin binds to its receptor on the parietal cell?
It increases intracellular Ca, which activates its protein kinase A which activates KH pump --> decreased pH.
39
What happens when histamine binds to its receptor on the parietal cell?
Histamine binds unique stomach receptor (H2) - it is G coupled receptor which activates adenyl cyclase which converts ATP to cAMP. cAMP activates protein kinase A --> stimulates KH pump --> decreased pH.
40
What happens when ACh binds to its receptor on the parietal cell?
Acts on M3 muscarinic receptor --> increase in intracellular Ca --> activates its protein kinase C --> stimulates KH pump --> decreases pH.
41
What happens when prostaglandin binds to its receptor on the parietal cell?
Receptor coupled to inhibitory G protein which inhibits adenyl cyclase --> prevents ATP being made into cAMP and reduces KH activity --> increased pH.
42
What sort of chemicals are histamine and gastrin?
Gastric is a hormone and histamine is a paracrine substance.
43
What is involved in the cephalic phase?
Sight, smell, taste of food triggers vagus to produce ACh and triggers G cells to produce gastrin. Gastrin/ACh act on ECL (enterochromaffin like) cells to produce histamine.
44
What is involved in the gastric phase?
Food arrives in stomach causing distension of stomach --> vagal/enteric reflexes --> release of ACh, peptides in lumen stimulate G cell cells --> gastrin. ACh/gastrin act on ECL cells --> histamine --> increased parietal cell activity.
45
What are the mechanisms inhibiting gastric secretion?
Cephalic phase - stopped eating so vagal activity reduced.
Gastric phase - decreased pH means gastrin is reduced (neg feedback).
Intestinal phase - acid in duodenum triggers release of secretin and enterogastric reflex which both decrease in gastrin secretion, fat in duodenum causes GIP.
46
What is GIP?
Gastric inhibitory polypeptide - decreases gastrin secretion and parietal HCl secretion.
47
Why must you increase the pH of the chyme entering the duodenum?
Pancreatic enzymes denatured at pH 2 and only function above pH 5.
48
What are enterogastrones?
Hormones released from gland cells in the duodenal mucosa (secretin, CCK, GIP) in response to acid, hypertonic solutions, fatty acids or monoglycerides in duodenum. They prevent further build up in duodenum.
49
What are the two strategies enterogastrones use to prevent further build up of acid in the duodenum?
Reducing gastric emptying (inhibiting motility of wall/contract pyloric sphincter) and inhibit gastric acid secretion.
50
What cells secret pepsinogen?
Chief cells.
51
What does zymogen storage prevent?
Autodigestion.
52
What is the function of pepsinogen?
Pepsin liberated at low pH --> its back on pepsinogen and further cleaves itself (positive feedback).
53
What inactivates pepsins?
Neutral pH.
54
What kinds of cells are present in gastric glands and what do each secrete?
Mucous neck cells - mucus (also produced by surface epithelium)
Chief cells - pepsinogen
Parietal cells - HCl, intrinsic factor
55
What is the function of the mucus in the stomach?
Cytoprotective - prevents mucosal surface from mechanical injury, neutral pH (HCO3) protects against acid corrosion and pepsin digestion. Creates gel layer that prevents bicarbonate diffusing away.
56
Describe the embryological development of the stomach.
Wk 4 - gut tube dilates to produce stomach, differential growth leads to lesser and greater curvature.
Stomach undergoes 90 degree rotation around longitudinal axis and then anterior-posterior rotation.
57
What does the fact the stomach undergoes an anterior-posterior rotation mean in terms of its nervous supply?
R side of primitive stomach supplied by R vagus and L by L, so when stomach rotates, R vagus supplies posterior surface and L supplies anterior surface.
58
What are the four tunics of the stomach wall?
Serosa - connective tissue
Muscularis externa - longitudinal muscle, circular muscle, oblique muscle
Submucosa and mucosa (rugae)
Muscularis mucosa contraction helps expel contents of gastric glands.
59
What are the two major and minor lobes of the liver?
Major - right and left lobe
| Minor - caudate and quadrate lobes
60
What does the falciform ligament of the liver separate?
Right and left lobe and attaches liver to anterior wall.
61
What does the coronary ligament attach the liver to?
Underside of the diaphragm on bare area.
62
What is the porta hepatis?
Fissure of the liver where the hepatic artery, portal veins and common hepatic duct, lymphatic and nerves enter/leave the liver.
63
What is the bare area of the liver?
Non-peritoneal area of R lobe where coronary ligament attaches to the liver.
64
How do secretions from the liver enter the duodenum?
Enters right and left hepatic ducts, which combine to form common hepatic duct, which combined with cystic duct of gallbladder to form the common bile duct which joints the pancreatic duct to enter the duodenal lumen via the major duodenal papilla.
65
What ribs is the liver lying deep to?
7-11.
66
What covers the liver?
Visceral peritoneum and connective tissue capsule.
67
What is the alimentary role of the liver?
Production of bile.
68
What are the components of bile?
Bile acids, lecithin (emulsification agent), cholesterol, bile pigments, toxic metals (modified by liver), bicarbonate.
69
What colour are hepatocytes?
Brown.
70
What are the functions of the hepatocytes?
Bile salt production, storage of excess nutrients, interconversion of nutrients and storage of important vitamins, Cu, Fe and removal of toxic substances (modification).
71
Where do bile pigments come from?
Breakdown of Hb from old/damage RBCs.
Bilirubin main pigment.
72
What does bilirubin change the colour of?
Makes bile yellow
Bacterial modification of bilirubin in the gut causes the brown colour of faeces and reabsorption from gut into blood stream and then excretion in urine makes urine yellow.
73
What are bile acids synthesised from?
Cholesterol.
74
What is the issue with bile acids?
They are very insoluble so need conjugation with glycine or taurine before secretion.
75
How are secreted bile salts recycled?
Via enterohepatic circulation (taken back up at end of ileum and transported back to liver by portal vein).
76
What controls the delivery of bile into the duodenum?
Sphincter of Oddi.
77
Describe the embryological development of the liver.
Liver primordium in 3rd wk (overgrowth of endoderm at distal foregut).
Liver bud (hepatic diverticulum) grows into septum transversum (mesodermal plate bw pericardial cavity and yolk sac).
Connection between liver bud and foregut narrows to form bile duct. Ventral outgrowth from bile duct forms gallbladder and cystic duct.
Diverticulum develops into core of mesochyme between heart and forgot.
Septum transversum gives rise to vessels of liver.
78
What is the blood supply to the liver?
Hepatic arteries (coeliac trunk branches) and hepatic portal vein.
The coeliac artery gives rise to common hepatic artery which gives rise to R and L hepatic arteries (supply R and L lobe of liver) and proper hepatic artery - supplies liver, gallbladder, part of stomach.
79
Blood from the hepatic arteries and portal veins travels through what in the liver?
Sinusoids (discontinuous capillaries).
80
What is the parasympathetic and sympathetic supply to the liver?
PS - vagus
| S - coeliac plexus fibres
81
What is the lymphatic supply to the liver?
Lacteals found in centre of villi (absorb fat) and drain into mesenteric lymph nodes, then superior mesenteric or ileocolic lymph nodes before reaching cisterna chyli.
82
What creates the distinct septa of the liver.
Connective tissue capsule of portal hepatic extends inside liver and splits it into distinct septa (support). Vessels, ducts and nerves follow the septa and divide the liver into hexagonal lobules (at each corner there is a portal triad).
83
What is a portal triad?
Portal vein, hepatic duct and hepatic artery.
84
What is found at the centre of each lobule of the liver?
Central vein (these anatomise to form hepatic vein).
85
Where do the hepatic veins drain into?
IVC.
86
What structures radiate out from central veins?
Hepatic cords.
87
What lies in between each cord of hepatocytes?
Bile canaliculi (cleft like lumen).
88
What do bile canaliculi come to form?
Hepatic ducts.
89
What are the different anatomical parts of the gallbladder?
Fundus, neck and body.
90
Where is the gallbladder found?
Inferior surface of liver.
91
What is the function of the gallbladder?
Storage and concentration of bile (15-20x, more concentrated if not had fatty meal in a while) - absorbs Na and water follows.
92
How much bile can the gallbladder store?
Up to 50ml.
93
What is the function of the sphincter of Oddi?
Controls release of bile and pancreatic juice into duodenum. When contracted forces bile back up into gallbladder.
94
What causes the release of CCK?
Fat/amino acids in duodenum.
95
What is the function of CCK?
Relaxation of sphincter of odd and contraction of gallbladder muscularis to discharge bile into duodenum.
Also causes decrease in gastric emptying and increase in pancreatic enzyme secretion to increase digestion.
96
What is the function of secretin?
Decreases gastric acid secretion, gastric emptying and increases duodenal bile and pancreatic HCO3 secretion --> neutralisation.
97
What is secretin release in response to?
Acid in the duodenum.
98
How does the gallbladder dehydrate bile?
NaK ATPase on baso-lateral membrane of gallbladder cells creates constant inward driving force for sodium to enter through sodium channels, which pulls water with it --> dehydration.
99
What does the gallbladder form from embryologically?
Ventral outgrowth from primordial bile duct.
100
What is the blood supply to the gallbladder?
Cystic a. (from hepatic artery properly) and drainage via cystic vein (drains into portal vein).
101
Cystic arteries lie in which structure?
Triangle of Calot - formed by inferior surface of liver, common hepatic duct and cystic duct.
102
What is the parasympathetic and sympathetic supply to the gallbladder?
S (and sensory) - coeliac plexus
| PS - vagus (leads to increased contraction of gallbladder)
103
Where does lymph from gallbladder drain into?
Cystic lymph nodes --> coeliac lymph node.
104
What are the 3 tunics of the gallbladder wall?
Mucosa (rugae)
Muscularis (contraction)
Serosa (connective tissue)
105
What are the different parts of the pancreas?
Head, tail, body.
106
What vessel forms behind the neck of the pancreas?
Portal vein.
107
What is the function of the pancreas?
Secretion of bicarbonate by duct cells.
| Secretion of digestive enzymes by zymogen acing cells.
108
What is the optimum pH for zymogens?
7.
109
Why are enzymes release in zymogen form?
To prevent auto digestion.
110
Describe the pathway that enzymes produced in the acinar cells move out into the main pancreatic duct.
Acinar cells in lobules connected by intercalated ducts which feed into intralobular ducts which come together into interlobular ducts that join together to form the main pancreatic duct.
111
If the route via hepatopancreatic ampulla is blocked how else can digestive enzymes from the pancreas be released into the duodenum?
Via an accessory pancreatic duct.
112
Where are enzymes stored in the pancreas?
Apical end of acini.
113
What mechanisms lead to release of digestive enzymes from the pancreas?
Bicarb secretion stimulated by secretin (in response to acid in duodenum).
Zymogen secretion in response to CCK (n response fatty acids/amino acids in duodenum).
Vagus/local reflexes (triggered by arrival of nutrients in duodenum).
114
Where is enterokinase found?
Bound to brush border of duodenal enterocytes.
115
What is the function of enterokinase?
Converts trypsinogen to trypsin. Trypsinogen then converts all other zymogens to active form.
116
What are the different pancreatic enzymes?
Proteases - cleave peptide bonds
Nucleases - hydrolyse RNA/DNA
Elastases - collagen/connective tissue
Phospholipases - phospholipid breakdown into FAs
Lipases - triglyceride breakdown into FAs and glycerol
alpha-amylase - starch breakdown into glucose and maltose
117
Describe the embryology of the pancreas?
2 buds form from endodermal lining of duodenum in wk 5.
Dorsal pancreatic duct lies in dorsal mesentery and ventral pancreatic duct lies close to bile duct.
As duodenum rotates, they lie closer to one another and eventually fuse.
118
What part of the liver did the ventral bud form?
only uncinate processes and inferior part of head of pancreas (dorsal forms rest).
119
What formed the main pancreatic duct?
Ventral duct plus distal part of dorsal duct (proximal part may form accessory duct).
120
Which ribs are related to the spleen?
9-11
121
What is the function of the spleen?
Graveyard of RBCs.
122
What is the lifetime of a RBC?
120 days.
123
What are two causes of splenomegaly?
Anaemia and typhoid fever.
124
What must you be careful of in splenectomy?
Damaging the islets of Langerhans in tail of pancreas.
125
What does the spleen develop from at the end of wk 4?
Mesenchymal condensation in dorsal mesogastrium.
In wk 5 - differentiates to form spleen. Up to wk14 has haemoatopoietic function, in wks15-17 acquired lobular shape and colonised by T lymphocytes. During wks 23 B cell precursors arrive - spleen starts lymphoid function.
126
What are the midgut boundaries?
2nd part of duodenum - distal 3rd of transverse colon.
127
What is the blood supply to the midgut?
Superior mesenteric artery.
128
What are the brain branches of the superior mesenteric artery?
Jejunal and ileal arteries (others incl. middle and right colic and ileocolic).
129
How is the small intestine peritonised?
Intraperitoneal. Connected to the posterior wall via greater mesentery of small intestine.
130
What are the main functions of the duodenum?
Gastric acid neutralisation, bulk of digestion, iron absorption.
131
What are the main functions of the jejunum?
95% nutrient absorption.
132
What are the main functions of the ileum?
NaCl/water reabsorption (chyme dehydration) - takes in 1-1.5L/day.
133
What are the enterocytes?
Function intestinal epihtleum.
134
What are the function of the villous cells?
Absorb NaCl, monosaccharides, amino acids, peptides, fats, vitamins, minerals and water.
135
What are crypt cells?
Secrete chloride ions and water.
136
What transporter absorbs peptides?
PepT.
137
What needs to occur to ingested foodstuffs before it can be broken down?
Needs to be broken down into monomeric units. Which will then be rebuilt up to prevent osmotic drag.
138
What are monosaccharides?
Hexose sugars, galactose, glucose and fructose.
139
What are the main dietary disaccharides?
Lactose (glucose and galactose), sucrose (glucose and fructose), maltose (glucose and glucose).
140
What enzymes breakdown the main dietary disaccharides?
Lactase, maltase, sucrase.
141
What are the main dietary disaccharides?
Starch (plant storage of glucose), cellulose and glycogen.
142
What is the structure of starch?
Amylose (glucose in straight chains) and amylopectin (glucose in very branched chains) linked by alpha (1-->4) bonds which can be broken down my amylase (salivary, pancreatic).
143
What is the structure of cellulose?
Unbranched, linear chains of glucose monomers linked by Beta(1->4) bonds. Require bacteria with cellulase to digest.
144
What is the structure of glycogen?
Glucose monomers linked by alpha(1-->4) bonds.
145
When does the body start to produce glycogen?
When BG >5mmoles/L.
146
What sort of glycosidic bonds can be broken down by alpha-amylase?
Alpha(1-->4) bonds but NOT beta(1-->4) bonds.
147
What are the two ways that molecules can move into the blood from the gut tube?
Transcellular pathway
| Paracellular pathway
148
What is the transcellular pathway?
Through apical membrane of enterocytes and back out basolateral membrane - can be done by non-polar molecules or transporter proteins (if charged).
149
What is the paracellular pathway?
Down the side of the epithelial cells.
150
How is glucose transported into the blood from the gut tube?
Sodium-glucose linked transporter 1 - Na and glucose bind and transports into cell. When cell glucose level >5mmol/L - creates a concentration gradient to push it into the blood.
Inward driving force for sodium created by Na-K-ATPase.
GLUT2 transporter transports it past basloateral surface into the blood.
151
How is fructose transported into the blood?
SGLT1 does not recognise fructose so GLUT5 is transporter for fructose and it is not linked to Na (no gradient for fructose in blood), and GLUT2 just transported it out into the blood.
152
What passes through the paracellular pathway?
Water. When SGLT1 or SAAT1 transporters glucose and amino acids into blood, pulls water with it (glucose more so).
153
What are small proteins (of about 3-10 aa's in length) known as?
Peptides.
154
What enzymes are responsible for protein digestion?
Proteases/peptidases (hydrolysed peptide bonds).
155
What is the difference between an endopeptidase and an exopeptidase?
Endopeptidases breakdown the centre of proteins, exopeptidase only act on terminal amino acids (if on amino end = amino peptidase, if on carboxyl end = carboyxpeptidases).
156
How are amino acids transported from the gut tube into the blood?
Sodium coupled amino acid transporter (SAAT1) which works the same as SGLT1.
157
What does PepT1 transport? How does it do this?
Di and tripeptides. Coupled up with hydrogen ions (proton motive force)
158
What is the acid microclimate and how is it created?
Layer of mucus that creates a diffusion barrier (prevents H getting away). NHE3 pumps out hydrogen ions in exchange for Na ions (NHE3 essential for life).
159
In what form is most ingested fat?
Triacylglycerol.
160
What enzymes is involved in gat digestion?
All fat digestion by pancreatic lipase (= monoglyceride and 2 FAs).
161
What is the issue with pancreatic lipase? How is this combated?
It is water soluble so can only work at surface of droplet (v slow, would = steatorrhoea).
Mechanical disruption of large lipid droplets by muscularis externa contraction and emulsification of small droplets produced by bile salts and phospholipids (to prevent them reforming into large droplets).
162
What are triacylglycerides composed of?
3 stearic acids and glycerol.
163
How is the absorption of fats further enhanced (aside from mechanical disruption/emulsification by bile salts/phospholipids)?
Formation of micelles (extracellular fat taxis) that move fat to brush border and release fats (FAs and monoglycerides) here, where they diffuse across plasma membrane of absorbing cells.
164
What are micelles composed of?
Bile salts, monoglycerides, fatty acids and phospholipids.
165
How does the micelle release the FAs and monoglycerides at the right area?
Acid microclimate (tumbling around until they reach it).
At this point FA take on hydrogen ions and become uncharged and able to cross lipid membrane.
166
There is a dynamic equilibrium between FAs and monoglycerides in solution and FAs and monoglycerides in .....?
Micelles.
167
After entering the epithelial cells what happens to the FAs and monoglycerides?
Enter sER and are reformed into TAGs. TAGs coated with amphiphatic protein and emlusification takes place in sER. TAG droplets transported through cell in vesicles formed from sER membrane. Processed through golgi apparatus and exocystosed into ECF at serosal membrane.
168
What are chylomicrons?
Extracellular fat droplets (also contain phospholipids, cholesterol, fat soluble its).
169
Where do chylomicrons pass into?
Lacteals between endothelial cells (cannot get into capillary as too large), carried in lymphatic system and eventually enter vena cava (coupled to carrier proteins).
170
What are the fat soluble vitamins?
A, D, E and K.
171
What are the water soluble vitamins?
B group, C and folic acid.
172
What is the function of B12?
Allows RBCs to mature properly and form their biconcave shape to allow them to transport oxygen efficiently.
173
How much of ingested iron is absorbed into the blood?
10%
174
How is iron transported across the brush border?
Via DMT1 (divalent metal transporter 1) into duodenal enterocytes. Only takes up Fe2+ (Fe3+ must be converted to Fe2+ first).
175
What is ferritin?
12 iron ions in a protein complex used as intracellular iron store.
176
What happens to unbound iron in the enterocytes?
Transported across serosal membrane via specific transporter and into blood. Here, it binds to transferrin which is taken up by the liver and made into Hb etc.
177
What is ferritin expression dependent on?
Body's needs.
| E.g. - anaemia - decreased ferritin and more Fe released into blood and opposite for hyperaemia.
178
How is absorbed iron excreted?
Cells in small intestine undergo maturation/divison cycle (start in erupts and migrate up and differentiate on villi), after 5 days shed off and so ferritin is release in faeces.
179
What nutrients does the gut regulate?
ONLY iron.
180
What does the small intestine passively secrete and why does it do this?
1.5L/day - due to active Cl secretion into lumen. This comes rom the epithelial cells lining the crypts of Lieberkuhn.
181
Why is water secretion into the small intestine important?
Maintain liquid state of contents to aid nutrient presentation to absorbing surface, promoting mixing with digestive enzymes and dilute/wash away hazardous substances.
182
What is the water reabsorbed by?
Villi.
183
How is Cl transported from the blood to the gut lumen?
Via basolateral Na-K-Cl co transporter (2Cl for 1K and 1Na) - driven by Na-K-ATPase and leaky K channels. Pushing Cl into negative cell creates uncomfortable environment so cAMP activated CFTR channel allows it into the lumen which pulls water in.
(cAMP comes from adenylate cyclase on basolateral membrane, this acts on protein kinase A which activates CFTR).
184
What are the vasa recta?
Straight arteries coming from the arterial arcades in the mesentery of the jejunum and ileum.
185
What are the arterial arcades?
Anastomoses of jejunal and ileal arteries (branches of SMA).
186
When does segmentation of intestine occur?
During meal.
187
What is segmentation?
Contraction (few s) moves chyme up and down to areas of relaxation, relaxed areas contract and push chyme all around. Allows mixing with digestive enzymes.
Occurs at leisurely pace.
188
What initiates segmentation?
Depolarisation generated by pacemaker cells in longitudinal muscle.
189
What is the BER?
Basic electrical rhythm - produces oscillations in membrane potential to reach threshold hold and fire APs to initiate contraction.
AP frequency determined by BER (no change with hormonal influence).
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Why is there net movement of chyme down the intestines?
BER proximally faster than distal regions, creating net downward migration.
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What is the parasympathetic supply to the intestines?
Vagus - stimulation = increased contraction.
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What does sympathetic supply to the intestines cause?
Decreased contraction.
193
Does ANS affect BER?
No.
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What is the gastroileal reflex?
Gastric emptying leads to increased segmentation in ileum leading to opening of ileocaecal valve, so chyme enters large intestine leading to distension of the colon which causes relaxation of ileocaecal valve (prevents reflux back into small intestine).
195
When does peristalsis of small intestine occur?
After finished a meal.
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Where does peristalsis start?
In antrum.
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How does peristalsis occur?
As migrating motility complexes (as one ends another starts). Contraction behind bolus and relaxation in front moves downwards to clear out.
NB - if food arrives MMC terminates and segmentation begins again.
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What is the role of the migrating motility complexes?
Move undigested food to large bowel
| Limit bacterial colonisation
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What hormones is involved in the initiation of an MMC?
Motilin.
200
What is the law of the intestine?
If intestinal smooth muscle is distended (by chyme) then:
muscle on oral side contracts, muscle on anal side relaxes, bolus moved to area of relaxation towards colon, mediation by neurone in myenteric plexus and syncytium connected by gap junctions in small intestine.
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How does neutralisation of acid in duodenum take place?
Production of bicarbonate from submucosal Brunner glands.
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What does acid in the duodenum trigger?
Vagal and ENS reflexes to stimulate Brunner's glands.
| Secretin release from S cells which stimulates bicarbonate release from pancreas and liver (self-limiting neg feedback).
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What part of the gut does the small intestine develop from embryologically?
1st and 2nd part of duodenum from foregut, rest of gut from midgut.
204
What causes the duodenum to become C shaped?
Rotation of the stomach.
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Describe the embryological development of the jejunum and ileum.
Rapid enlongation of gut tube and associated mesentery (primary intestinal loop)
Wk6 - 90 degree anticlockwise rotation around axis of superior mesenteric a. and physiological herniation into extraembryonic cavity in umbilical grow (allows for rapid growth)
Wk10 - retraction of herniated loops (regression of other organs), as this occurs further 180 degree rotation leads to transverse colon lying in front of duodenum.
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What is a Merckle's diverticulum?
A little protrusion from gut from vitelline duct (embryonic yolk sac) present in small % of population.
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What is the drainage of the small intestine?
Superior mesenteric vein (joins splenic vein to form hepatic portal vein).
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What is the nervous supply to the duodenum?
Sympathetic - intestinal plexuses
| Parasympathetic - vagus
209
What ist he nervous supply to the ileum and jejunum?
Sympathetic - T5- spinal nerves (greater, lesser splanchnic nerves)
Parasympathetic - vagal trunks (synapse on postganglionic cell in mesenteric and submucosal plexus)
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What are the four tunics composing the small intestine wall?
Mucosa, submucosa, muscularis externa, serosa.
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What is the surface of the small intestine composed of?
Plicae circularis (foldings).
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What cells make mucous?
Goblet cells.
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What is the purpose of villi?
Increase SA.
214
What are on villi?
Microvilli.
215
______ cells are separated by ______ ________ and create a base-lateral and apical membrane of the villi.
Columnar.
Tight junctions.
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What are the handout boundaries?
Distal 3rd transverse colon - anus.
217
What is the blood supply to the hindgut?
Inferior mesenteric artery.
218
What are the main branches of the inferior mesenteric artery?
Left colic, sigmoid arteries, superior rectal artery (terminal branch of inferior mesenteric artery).
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What path would food take following the large intestine?
Comes from ileum --> ileocaecal valve --> caecum --> ascending colon --> hepatic flexure --> transverse colon --> splenic flexure --> descending colon --> sigmoid colon --> rectum --> anus.
220
What are the differences between the small and large intestine?
Large intestine has teniae (3 thickened bands of smooth muscle), contractions of teniae coli make haustra, and omental appendices (small peritoneal sacs of fat).
221
What is the function of the large bowel?
Activate transport of sodium from lumen to blood leading to osmotic pull for water to renter blood and dehydration of chyme leading to solid stool.
Bacterial colonisation in large bowel leads to bacterial fermentation of undigested carbs (= vit K production and straight chain FA production).
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What are enterotoxigenic bacteria?
Bacteria e.g. vibrio cholera, E. coli which produce protein enterotoxins which turn on intestinal Cl secretion via elevating intracellular secondary messengers (cAMP, cGMP, Ca) leading to increased water secretion.
= profuse watery diarrhoea.
223
What is the composition of flatus?
N, CO2, H2, methane, hydrogen sulphide.
224
How is the normal anus kept closed?
Internal and external anal sphincters.
225
What happens to the anus following a meal?
Wave of intestine contraction from colon --> resctum. Distension of rectal wall produced by mass movement of faecal material into rectum --> mechanoreceptors --> defection reflex --> urge to defaecate.
226
What nerves is the defection reflex under?
Parasympathetic control via pelvic splanchnic nerves.
227
What does the defaecation reflex lead to?
Contraction of rectum, relaxation of the mineral and contraction of external anal sphincters, increased peristaltic activity in colon --> increased pressure on external anal sphincter (only relaxes under voluntary control --> expulsion).
228
What is delay of defaecation by?
Descending pathways.
229
Define diarrhoea.
Too frequent passage of faeces that are too liquid.
230
What can cause diarrhoea?
Pathogenic bacteria, viruses, protozoans, toxins, nervousness, food.
231
How do you treat diarrhoea?
Oral dehydration therapy, sodium and glucose solution.
232
What symptoms are associated with constipation?
Headaches, nausea, loss of appetite, abdominal distension.
233
What is the venous drainage of the large intestine?
Superior and inferior mesenteric veins.
234
What lymph nodes drain the large intestine?
Superior mesenteric lymph nodes.
235
What is the histology of the large intestine like?
Simple columnar epithelium. Large, straight crypts with large no. of goblet cells for lubrication of movement of faeces.
236
What is the appendix?
Blind intestinal diverticulum with masses of lymphatic tissue that enters the caecum.
237
What is the most common position for the tail of the appendix?
Retrocaecal.
238
What is the rectum?
Straight muscular tube between end of sigmoid colon and anal canal.
239
Where is the recto-sigmoid junction?
Anterior to S3 vertebrae.
240
What is the mucosa layer of the rectum like?
Simple columnar epithelium.
241
What ist he muscularis externa of the rectum like?
Thicker than other regions of GIT.
242
Describe the embryological development of the anal canal?
Terminal part of hindgut joins posterior part of cloaca (primitive naal canal).
Allantois enters into anterior part of cloaca (primitive urogenital sinus).
Cloaca is an endoderm lined cavity with surface ectoderm at its ventral boundary.
Allantois is ventral diverticula of cloaca.
Cloacal membrane separates cloacal and anal pit (ectoderm).
Mesoderm (urorectal septum) separates allantois and hingut.
Septum merge to cover yolk sac and to surround allantois.
Wk 7 - cloacal membrane ruptures = opening for hangout. Ectoderm of anal canal proliferates, closing caudal end.
Wk9 - anal anal reopens.
243
What cells line the mucosa of the anal canal?
Simple columnar
244
What muscle lined the internal anal sphincter?
Smooth muscle (involuntary).
245
What muscle lines the external anal sphincter?
Skeletal muscle (voluntary).
246
What lines the epithelium of the anal canal?
Stratified squamous.
247
What are the three layers of the mucosa?
```
Epithelium
Lamina propria (loose connective tissue, glands, blood/lymph vessels, gives background support to epithelium)
Muscularis mucosae (thin, insignificant layer of smooth muscle)
```
248
In which layer of the GIT wall are nerves found?
Submucosa
249
What are the muscle layers in the muscularis externa?
Circumferential muscle (contraction --> motility) and longitudinal muscle (shortens length).
250
Which two plexuses form the ENS?
Submucosal and myenteric plexuses
251
Where are the two ENS plexuses found?
Myenteric (Auerback's) found between circular and longitudinal muscle.
Submucosal lies in the submucosa.
252
What is the parasympathetic innervation to the abdominal organs?
Vagus and S2-4 splanchnic nerves.
253
What is the sympathetic innervation to the abdominal organs?
Abdominopelvic splanchnic nerves (greater T5-9, lesser T10-11, least T12)
Prevertebral sympathetic ganglia
Abdominal aortic plexus
254
What drains the stomach?
Gastric veins.
255
What drains the pancreas?
Splenic vein.
256
What are the three main groups of lymph nodes draining the alimentary organs?
```
Preaortic (coeliac, superior and inferior mesenteric)
Lateral aortic (drains organs supplied by lateral aortic branches)
Retro-aortic (drains posterior abdominal wall)
```
257
What does ectoderm go on to form?
Epidermis, CNS, PNS, retina of eye.
258
What does the endoderm go on to form?
Respiratory tract, GIT, glands, liver, pancreas.
259
What does the mesoderm go on to form?
Most of CVS, connective tissues, muscles, blood cells, bone marrow, vessels.
260
What is the primordial gut derived from?
Endoderm lining the yolk sac (wk4).
261
What closes the primordial gut at the caudal and cranial end?
Cranial - oropharyngeal membrane
| Caudal - cloacal membrane
262
How is the omental bursa formed?
Stomach attached to dorsal and ventral wall by mesentery (mesogastrium). When it rotates, and its disproportionate growth alters position of mesogastrium, pulls the dorsal mesentery to the left creating the lesser sac.
263
What else does the endoderm of the hindgut form?
Lining of bladder and urethra.
264
As you go down the gut tube are the microbes increasingly aerobic or anaerobic?
Conditions increasingly anaerobic therefore occupied by anaerobic organisms.
265
Where is the longest transit time in the gut tube?
Large intestine.
266
Carbohydrate metabolism by microbes produces what products?
Acetate, propionate, butyrate and gases (CO2, H2, CH4).
267
Protein metabolism by microbes produces what products?
Isobutyrate and isovalarate.
268
What are the functions of the gut flora?
Competition and barrier function (against toxic bacteria).
269
What functions do we have against gut bacteria?
pH inhibition, mucus layer.
270
What are the consequences of antibiotic use?
Clears out commensal bacteria, so when it grows back their is a block of diversity and resistant populations thrive (CDAD is an example).
271
What are prebiotics?
Food for commensal gut bacteria.
272
What are probiotcs?
Gut bacteria.
273
What are synbiotics?
Probiotic + prebiotic.
