OVERVIEW OF GI SYSTEM Flashcards
1
Q
What germ layer is the GI system derived from?
A
The endoderm
2
Q
Whats the basic structure throughout the length of the GIT?
A
Mucosal layer
Submucosa
Muscular layer
Serosal/adventia layer
3
Q
What does the mucosa consist of?
A
Epithelium
Laminae propria
Muscularis mucosa (very thin layer of smooth muscle)
4
Q
What is the submucosa?
A
A thick layer of loose connective tissue surrounding the mucosa
It also contains glandular pits, blood vessels, lymphatic vessels and nerves
5
Q
Whats the structure of the smooth muscle layer of the GIT?
A
Arranged in an inner circular layer and an outer longitudinal layer with the myenteric plexus between the 2
6
Q
Whats the structure of serosal layer?
A
Double membrane layer made up of epithelium. Visceral layer stuck to organs and parietal layer closest to body cavity
Serous fluid in between layers
7
Q
What part of the GIT does the serosa surround?
A
It wraps around intraperitoneal organs e.g. liver, stomach, spleen, first part of duodenum, ileum and jejunem, transverse colon and sigmoidal colon.
8
Q
Whats the difference between serosa and adventitia?
A
Serosa is epithelia and surrounds intraperitoneal organs
Adventitia is loose connective tissue and it wraps organs aoutside the abdominal cavity and retro-peritoneal organs and binds them to the walls of the abdominal cavity.
9
Q
What part of the GIT does adventitia surround?
A
RETROPERITONEAL ORGANS
Pancreas (except tail)
Ureters
Second and third part of duodenum
Caecum
Ascending and descending colon
Adrenal glands
Aorta and IVC
Kidneys
Oesophagus
Rectum
10
Q
Where is the myenteric plexus found?
A
Between the layers of smooth muscle in the muscularis propria
11
Q
Whats the myenteric plexus also known as?
A
Auerbachs plexus
12
Q
Whats the function of the myenteric plexus?
A
provides motor innervation to both layers of the muscular layer of the gut, having both parasympathetic and sympathetic input from the vagus nerve
Function is for peristaltic movement of the bowels
13
Q
Where is meissners plexus found?
A
Within the submucosa
14
Q
What is meissners plexus also known as?
A
The submucosal plexus
15
Q
Whats the function of the meissners plexus?
A
regulates the configuration of the luminal surface
controls glandular secretions
alters electrolyte and water transport
regulates local blood flow.
16
Q
What is peristalsis?
A
a radially symmetrical contraction and relaxation of muscles that propagate in a wave down a tube, in an anterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, which is preceded by a simultaneous contraction of the longitudinal muscle and relaxation of the circular muscle in the lining of the gut
17
Q
What are the functions of the GI tract?
A
Digestion of food
Motility
Storage of food waste and waste materials
Excretion of waste products
Exocrine secretions
Endocrine secretions
Paracrine secretuons
Defence
18
Q
What makes up the foregut?
A
Pharynx
Oedeophagyus
Stomach
Proximal half of duodenum
And gives rise to the liver, gallbladder and pancreas
19
Q
What make sup the midgut?
A
Distal half of duodenum
Jejunum
Ileum
Caecum
Ascending coon
Proximal 2/3rds transverse colon
20
Q
What makes up the hindgut?
A
Distal 1/3rd of the transverse colon
Descending colon
Sigmoid colon
Proximal 2/3rds of anorectal canal
21
Q
Whats the blood supply to the foregut?
A
Coeliac artery
22
Q
Whats the blood supply to the midgut?
A
Superior mesenteric artery
23
Q
Whats the blood supply to the hindgut?
A
Inferior mesenteric artery
24
Q
Where does referred pain from the foregut go?
A
Epigastrium
25
Where does referred pain from the midgut go?
Umbilical region
26
Where does referred pain from the hindgut go?
Hypogastric region
27
What can cause right hypochondriac pain?
Liver diseases e.g. hepatitis, abscess, alcohol, meds, etc
Biliary diseases e.g. cholecystitis, cholangitis, gallstones, choledocholithiasis
RLL pneumonia
Subdiaphragmatic abscess
28
What can cause left hypochondriac pain?
Pancreatitis
Splenic infarct
Splenic rupture
Splenic abscess
Gastritis, gastric ulcer, GERD
29
What can cause epigastric pain?
Gastritis
GERD
Gastric perforation
Gastroparesis
Pancreatitis
Peptic ulcer disease
Oesophagitis
Hiatal hernia
Carcinoma
Gastroenteritis
30
What can cause lumbar pain?
Kidney stones
Pyelonephritis
Colitis
31
What can cause umbilical pain?
Gastroenteritis
Mesenteric ischaemia
Ruptured AAA
Umbilical hernia
Appendicitis
Duodenal ulcer
IBD
Diverticulitis transverse colon
Coeliac disease
Colon cancer
IBS
Small bowel obstruction
32
What can cause right iliac pain?
Appendicitis
Colitis
Ovarian torsion
Ovarian cyst
Ectopic
Turbo-ovarian abscess
PID
Diverticulitis (caecal or meckels)
Mesenteric adenitis
Intestinal neoplasm
Ureteric colic
Torted testes
Inguinal hernia
33
What can cause left iliac pain?
Colitis
Ovarian torsion
Ovarian cyst
Ectopic pregnancy
Turbo-ovarian abscess
PID
Diverticulitis
Intestinal neoplasm
Ureteric colic
Torted testes
Inguinal hernia
34
What can cause pain in the Hypogastric region?
Cystitis
PID
STI
Pregnancy
Ectopic
Appendicitis
35
Where do the liver and pancreas develop from?
Endoderm all diverticula that bud off the duodenum in weeks 4-6
36
Where do the muscles of mastication, mylohyoid and anterior belly of digastric muscles develop from?
First pharyngeal arch (mandibular) - so innervated by trigeminal nerve
37
When does the primitive gut tube form?
During week 3
38
Where does the primitive gut tube come from?
The primitive gut is formed when a portion of the yolk sac becomes incorporated into the embryo, which occurs due to the cephalocaudal and lateral folding of the embryo. The portions that remain outside the embryo are the yolk sac and the allantois. The primitive gut forms a blind-ended tube on both the cephalic and caudal ends of the embryo, forming the foregut and the hindgut, respectively. The middle part forms the midgut, but remains temporarily connected to the yolk sac via the vitelline duct (yolk stalk).
39
What are the retroperitoneal structures?
adrenal glands
kidneys
ureter
aorta
inferior vena cava
anal canal
the duodenum, except for the proximal first segment, which is intraperitoneal
ascending and descending portions of the colon
pancreas, except for the tail, which is intraperitoneal
40
How does the midgut initially communicate with the yolk sac?
Via the Vitelline duct
41
Where does the pharynx develop from?
The 4th and 6th pharyngeal arches
42
How do the lungs and oesophageal form simply?
The foregut gives rise to oesophagus. There lung bud which sprouts as an outpouching from the anterior wall of the foregut. During week 4 the tracheoesophageal septum separates the lung bud from the foregut. Anterior compartment develops into trachea and lungs and posterior compartment into oesophagus.
43
Outline how the stomach develops?
Begins as a small dilation of the foregut
The ventral mesogastrium attaches the ventral border to the anterior wall whilst the dorsal mesogastrium attaches the dorsal border to the posterior body wall
The dorsal border grows faster forming the greater curvature whilst the ventral border grows slower, becoming the lesser curvature
Stomach undergoes 90 degrees clockwise rotation along its length, pulling the dorsal and ventral mesogastria with it. This moves the greater curvature to the right side of the body and lesser curvature to the left
The stomach rotates once more on the frontal plane after mesogastrium and bursas have formed. This repositions the superior end of the stomach, forms the pylorus and turns the duodenum into a C-shaped loop
44
What forms the lesser omentum?
The ventral mesogastrium
45
What is the lesser omentum?
double layer of peritoneum that extends from the liver to the lesser curvature of the stomach, and to the first part of the duodenum.
46
The lesser omentum is usually divided into two connecting parts. What are they?
the hepatogastric ligament, and the hepatoduodenal ligament
47
What forms the omental bursa?
As the dorsal mesogastrium grows and bends as the stomach rotates, a cavity forms between the stomach and the posterior body wall - forming the omental bursa
48
What forms the greater omentum?
As the omental bursa grows and fills with peritoneal fluid, it develops 2 projections; upper and lower recess
The sheets of the dorsal mesogastrium that form the lower recess fuse to form the greater omentum
49
What does the liver bud (hepatic diverticulum) give rise to?
Liver
Gallbladder
Biliary duct system
50
When do hepatocytes start producing bile?
Week 12
51
How do the liver and gallbladder develop?
The liver bud divides into 2 parts;
The larger, superior portion becomes the liver whilst the smaller, inferior portion becomes the gallbladder
52
How does the pancreas form?
2 pancreatic buds from the last portion of the foregut fuse to form an entire organ
The dorsal bud forms the tails body and part of the head
The ventral part forms most of the head
53
When does the pancreas begin producing insulin?
Week 10
54
Outline the development of the midgut?
During rapid gut tube growth, primary intestinal loop herniates through the Vitelli nerve duct and develops inside the umbilical cord. The superior mesenteric artery grows between the loop’s 2 limbs. The loop rotates 90 degrees counterclockwise around the axis of SMA which moves the cranial limb to the right side of the artery. This cranial limb becomes convoluted which marks the future jejunal and ileal anses. The caudal limb develops a small dilation which will eventually become the caecum and appendix
In week 10 the loop rotates final 180 degrees and moves into the abdominal cavity. The formerly caudal limb now frames the developing small intestine loops becoming the ascending colon and right 2/3rds of the transverse colon
55
Where is the anal canal’s lower portion derived from?
Primitive anus - proctodeum
56
What is the proctodeum?
A pit of ectoderm that forms below the cloacal membrane
57
When does the urorectal septum form?
In week 4
58
What is the urorectal septum?
It separates the cloaca into anterior urogenital sinus and posterior anal canal
59
When does the anal membrane rupture, forming a continuous anal canal?
End of week 7
This opens the anal canal into the embryos tail-region
60
What are the functions of saliva?
Aid in digestion, softening and deglutition of food
Keeps mucosal surfaces moist and lubricated to protect against abrasion
Control oral bacteria by secreting lysozyme
Secretes calcium and phosphate for tooth formation and maintenance
Secretes amylase to begin digestion of starches
61
What are the 2 types of acinar cells in salivary glands and what are their function?
Serious acinar sells secrete proteinaceous and enzymatic components of saliva
Mucous acinar cells secrete watery mucus
62
What are the 3 major salivary glands and what type of gland are they?
Parotid - serous gland
Submandibular - serous and mucous gland
Sublingual - large mucous gland
63
Whats the parasympathetic innervation of each of the 3 major salivary glands?
Parotid - glossopharyngeal
Submandibular - facial nerve
Sublingual - facial nerve
64
What are our 2 sets of teeth?
Deciduous teeth - 20 teeth that appear by the age of 3
Permenant teeth that occur after the age of 6 and consist of 32 teeth
65
How is the pharynx divided?
Nasopharyngeal, oropharynx and laryngopharynx
66
Whats the mucosa of the oropharynx and laryngopharynx?
Stratified squamous epithelium interdispersed with mucous glands
67
Outline the structure of the oesophagus?
Upper 1/3rd is skeletal muscle and lower 1/3rd is smooth muscle with the middle 1/3rd being a mixture
Muscular layers are arranged in an outer longitudinal layer and inner circular layer
68
Outline how the lower oesophageal sphincter opens as food enters the oesophagus?
As the peristaltic wave carries a bolus of food to the stomach, release of NO causes relaxation of LES and allows food to enter the stomach
69
Whats the difference between parietal and visceral peritoneum?
Parietal peritoneum – an outer layer which adheres to the anterior and posterior abdominal walls. Visceral peritoneum – an inner layer which lines the abdominal organs. It's made when parietal peritoneum reflects from the abdominal wall to the viscera.
70
What are mesenteries?
Double layer of visceral peritoneum that reflects from the inner wall of the abdomen and envelopes portions of the abdominal viscera
It suspends parts of the bowel and conveying vessels, lymphatics and nerves
71
Where is the greater omentum?
Hanging from the greater curvature of the stomach and folding back on itself to attach to the transverse colon
72
Where is the lesser omentum?
Extending from the lesser curvature of the stomach and proximal duodenum to the liver via hepatoduodenal and hepatogastric ligaments
73
How does the lesser sac (omental bursa) communicate with the greater sac?
Via the epiploic foremen of Winslow
74
What is the ‘greater sac’?
The entire rest of the peritoneal cavity
75
What is the omental bursa/lesser sac?
A cul-de-sac that forms posterior to the stomach and anterior to the retroperitoneal pancreas as a result of the twisting of the stomach in embryonic life
76
What 4 cells are found in the base of gastric pits in the stomach?
Mucous neck cells
Chief cells
Parietal cells
Enteroendocrine cells
77
Whats the function of mucus neck cells?
Secrete mucus to protect the stomach lining
78
Whats the function of chief cells?
Secrete pepsiongen which is converted to pepsin once it contacts the gastric juice - aids digestion of proteins
79
Whats the function of parietal cells?
Secrete HCL and intrinsic factor which allows vit B12 to be absorbed
80
Whats the function of enteroendocrine cells?
Secrete a host of hormones or hormone-like syvsrabces thar help regulate digestion
81
What drains the midgut structure?
Hepatic portal system
82
What are the 4 parts of the duodenum?
Superior
Descending
Inferior
Ascending
83
What happens at the superior part of the duodenum?
It’s an attachment site for the hepatoduodenal ligament of lesser omentum
84
What happens at the descending part of the duodenum?
Where bile and pancreatic ducts empty
85
What happens at the inferior part of the duodenum?
Crosses IVC and aorta and is crossed anteriorly by superior mesenteric vessels
86
What happens at the ascending part of the duodenum?
Portion is tethered by suspension ligament at the duodenojejunal flexure
87
How is the jejunum different from the ileum?
It has a large diameter
It has thicker walls
It has greater vascularity
Less fat in the mesenteric
Fewer lymph nodules
Large and taller plicae circulares
88
How is the surface area of jejunum and ileum increased?
Via plicae circularis, villi and microvilli
89
What epithelium lines the bowel?
Simple columnar
90
What immune defences can be found in the small intestine?
Peters patches (aggregated lymphatic nodules)
91
What are Crypts of Lieberkuhn (intestinal glands)?
tubular glands formed from the mucosa of the small intestine in between the bases of the villi. The cells of these glands secrete intestinal juice.
92
How does the number of crypts of Lieberkuhn and peyers patches change as you move towards the distal ileum?
They increase
93
Whats the function of the duodenum?
The duodenum is largely responsible for the breakdown of food in the small intestine, using enzymes.
The duodenum also regulates the rate of emptying of the stomach via hormonal pathways. Secretin and cholecystokinin are released from cells in the duodenal epithelium in response to acidic and fatty stimuli present there when the pylorus opens and emits gastric chyme into the duodenum for further digestion. These cause the liver and gall bladder to release bile, and the pancreas to release bicarbonate and digestive enzymes such as trypsin, lipase and amylase into the duodenum as they are needed.
94
Whats the function of the jejunum?
The lining of the jejunum is specialized for the absorption by enterocytes of small nutrient particles which have been previously digested by enzymes in the duodenum.
Once absorbed, nutrients (with the exception of fat, which goes to the lymph) pass from the enterocytes into the enterohepatic circulation and enter the liver via the hepatic portal vein, where the blood is processed
95
Whats the function of the ileum?
The main function of the ileum is to absorb vitamin B12, bile salts, and whatever products of digestion were not absorbed by the jejunum.
The DNES (diffuse neuroendocrine system) cells of the ileum secrete various hormones (gastrin, secretin, CCK) into the blood.
Cells in the lining of the ileum secrete the protease and carbohydrase enzymes responsible for the final stages of protein and carbohydrate digestion into the lumen of the intestine
The villi contain large numbers of capillaries that take the amino acids and glucose produced by digestion to the hepatic portal vein and the liver.
Lacteals are small lymph vessels, and are present in villi. They absorb fatty acid and glycerol, the products of fat digestion.
96
What are brunners glands and where are they found?
They secrete an alkaline fluid containing mucin, which protects the mucosa from the acidic stomach contents entering the duodenum.
located in the submucosa of the duodenum
97
Whats the function of the large intestine?
Reabsorb water and electrolytes from faeces
To store faeces until eliminated from body
98
Whats the difference in walls in small and large intestine?
Just that large intestine mucosa does not have villi or circular folds
99
What cells in the large intestine are responsible for secreting mucus? Why is this important?
Goblet cells - lubricate the bowel lumen and facilitates passage of faces
100
The mucosa in the large intestine has partial folds. What are these called?
Plica semilunaris
101
How is the outer longitudinal smooth muscle layer of the large intestine organised?
Into 3 thickened bands known as taeniae coli that run from the caecum to the rectum and help propel the faces along the length of the bowel
102
What are haustrae and where are they found?
Contraction of the muscle layers in the walls of the large testing produces sacculations called haustrae
103
What are appendices epiploicae?
adipose structures protruding from the serosal surface of the colon
104
Describe the 2 anal sphincters?
Internal anal sphincter is smooth muscle
External anal sphincter is skeletal muscle
105
What are the 4 lobes of the liver?
Right lobe
Left lobe
Quadrate lobe (between gallbladder and round ligament)
Caudate lobe (between IVC, ligamentum venosum and porta hepatis)
106
Whats the blood supply to the liver?
The right and left side each recieve a major branch of the hepatic artery (25%) and hepatic portal vein (75%)
107
Whats the venous drainage of the liver?
Hepatic veins and biliary drainage
108
What is the round ligament of the liver?
Aka ligamentum teres
a ligament that forms part of the free edge of the falciform ligament of the liver. It connects the liver to the umbilicus. It is the remnant of the left umbilical vein.
109
Whats the falciform ligament of the liver?
A peritoneal reflection off anterior abdominal wall with round ligament in its margin
Divides the liver into right and left side
a remnant of the ventral mesentery
110
What is the ligamentum venosum?
a fibrous remnant which travels superiorly from the porta hepatis of the liver to the inferior vena cava. It is often obliterated in adults.
In the fetus, it is patent and known as the ductus venosus which shunts blood returning from the placenta in the umbilical vein to the inferior vena cava thus bypassing the liver.
111
What are the coronary ligaments?
parts of the peritoneal reflections that hold the liver to the inferior surface of the diaphragm.
112
What are triangular ligaments?
asymmetrical bilateral structures that help to hold the liver in place. There are two in total, right and left, both of which are continuations of the coronary ligament.
113
What is the bare area?
An area of liver pressed against the diaphragm that lacks visceral peritoneum
114
What is the porta hepatis?
The site at which vessels, ducts, lymphatics and nerves enter/leave the liver
115
Whats the function of the gallbladder?
Stores and concentrates bile which is then secreted by the hepatocytes in the liver
116
How much bile does the liver produce a day?
900ml
117
Whats the storage capacity of the gallbladder for bile?
30-50ml
118
What is the laminate propria?
The layer of connective tissue that sits immediately beneath the epihtiulm
It contains numerous peyers patches
It possesses a rich vascular and lymphatic network that is important for the absorption of nutrients
119
What are the smooth muscle pacemaker cells of the GIT?
The interstitial cells of Cajal
120
Where are the interstitial cells of Cajal found?
Within the muscularis external, between the inner and outer layers
121
Whats the function of interstitial cells of cajal?
generate electrical slow waves in gastrointestinal (GI) smooth muscles.
122
What does the enteric nervous system consist of?
Submucosal and myenteric plexuses
123
How does the parasympathetic nervous system affect motility and secretions? Which nerve
Vagus nerve
Secretion of ACh which binds muscarinic receptors
= increases motility and promotes secretions
124
How does the sympathetic nervous system affect motility and secretions? Which nerve
Splanchnic nerves
Release of catecholamines = decrease motility and inhibit secretion
125
What are the constituents of saliva?
Water 99.5%
Amylase
Mucus
Lysozyme
Lactoferrin
126
Where do the sublingual glands exit into the mouth?
Small ducts in floor of the mouth
127
Where do the submandibular glands exit into the mouth?
Passes into Wharton’s duct that exits into the oral cavity either side of the frenulum on the underside of the tongue
128
Where do the parotid glands exit into the mouth?
Via stensen’s duct at the 2nd superior molar
129
What mechanisms help to prevent entry of the bolus into passageways other than the oesophagus?
Tongue prevents re-entry into mouth
Soft palate prevents entry into nasal passages
Closure of rima glottis and movement of epiglottis over the closed glottis
Pharyngeal muscles contract to move the bolus to the top of the oesophagus
130
Outline the structure of the stomach?
Cardia is where oesophagus meets stomach
Fungus is the uppermost part
Body is the main portion of the stomach
Muscular antrum is the lower section of the stomach
Pylorus forms the distal aspect and it ends at the pyloric sphincter
131
What are the 3 main types of gastric glands?
Cardiac glands
Oxyntic glands
Pyloric glands
132
Where are cardiac glands found and what is their function?
At lower oesophageal sphincter
Contains mucous cells which produce alkaline mucous to protect epithelium of stomach and act as a lubricant
133
Where are oxyntic glands found and what is their function?
Found in body and fungus
Primarily contain important exocrine cells including mucous cells, chief cells, parietal cells, mucous neck cells, stem cells, neuroendocrine cells
134
Where are pyloric glands found and what is their function?
Found in antrum and pylorus
Mucous cells are the predominant cell type and produce an alkaline mucous that protects the stomach epithelium and acts as a lubricant
135
What are the 2 important endocrine cells in the pyloric gland?
G cells and D cells
136
Whats the function of G cells?
Involved in the secretion of gastrin which stimulates parietal cells to secrete HCL
137
Whats the function of D cells?
Secrete somatostatin which has an inhibitory action on gastric secretions
138
Whats the cephalic stage of digestion?
Higher cortical functions, senses, chewing and swallowing drive the increase in gastric secretions
This leads to fatal stimulation which increases ACh release
ACh acts directly on parietal and chief cells = secrete HCL and pepsinogen respectively
ACh can also act indirectly via endocrine cells (G cells and enterochromaffin-like cells)
139
Whats the gastric stage of digestion?
Food entering the stomach leads to stimulation of the enteric nervous system and endocrine cells
= increased secretion of pepsinogen and HCL
140
Whats the function of HCL?
Denatures proteins
Kills microorganisms
Activates pepsinogen
141
What does it mean when its said that pepsin is able to autocatalyse itself?
Pepsin can activate pepsinogen = significant increase in the availability of active enzyme
142
Whats the intestinal stage of digestion?
Gastric emptying of chyme into duodenum= Activation of D cells = release of somatostatin = decreased gastric secretions
Gastric emptying = loss of chyme from stomach = decreased gastric secretions
143
Whats the hepatobiliary system?
this consists of the liver, gallbladder and bile ducts.
144
Where does the common bile duct enter the duodenum?
Ampulla of vater
145
What does the exocrine pancreas produce?
Amylase
Lipase
Proteolytic enzymes e.g. trypsin
Alkaline pancreatic juice
146
When does the pancreas first begin to develop?
5th week of gestation
147
Outline the embryology behind the development of the pancreas?
developing from an outgrowth of the foregut. It develops as two separate buds (ventral and dorsal) which eventually fuse. The ventral bud forms the uncinate process and head of the pancreas whilst the dorsal bud forms the neck, body and tail of the pancreas. The duct associated with the ventral bud forms the main pancreatic duct (of Wirsung) whilst the duct associated with the dorsal bud forms the accessory duct (of Santorini).
148
What causes anatomical variations in the pancreas?
Abnormal fusion between ventral and dorsal buds
149
What is an annular pancreas?
incomplete rotation of the ventral bud leads to its surrounding of the 2nd part of the duodenum.
150
What is a pancreas divisum?
caused by failure of fusion of the ventral and dorsal buds. It is common, affecting around 4-15% of individuals
151
What is the major duodenal papilla?
The major duodenal papilla (papilla of Vater) is the point where the dilated junction of the pancreatic duct and the bile duct (ampulla of Vater) enter the duodenum.
152
What is the minor duodenal papilla?
Where the accessory duct which drains part of the pancreatic head exits into the duodenum
153
What cells are found in the endocrine pancreaS?
Alpha cells - glucagon secretion
Beta cells - insulin secretion
Delta cells - somatostatin secretion
154
What proportion of the pancreas is exocrine?
98-99%
155
How is the exocrine pancreas organised?
into secretory sacs termed acini. The ductal cells, which line the pancreatic ducts, are important in the secretion of a bicarbonate-rich alkaline solution. Acidic drain to ducts which connect to form the main pancreatic duct which empties through the ampullae of vater
156
Whats the function of pancreatic amylase?
Digestion of starch (polysaccharides to the disaccharide maltose)
157
Whats the function of pancreatic lipase?
Breaks down triglycerides into monoglycerides and FFA
Reliant on bile for emulsification of fats
158
What proteolytic enzymes does the pancreas rrleasE?
Trypsinogen
Chymotrypsinogen
Procarboxypeptidase
159
What are proteolytic enzymes secreted in an inactive state?
To prevent proteolytic damage to the pancreas
160
What enzyme is required to activate the proteolytic enzymes?
Enterokinase (luminal membrane enzyme)
161
Outline how the proteolytic enzymes are activates?
Enterokinase cleaves trypsinogen into trypsin rendering it active.
Trypsin then cleaves chymotrypsinogen into chymotrypsin and procarboxypeptidase into carboxypeptidase.
162
Whats the function of the biliary system?
to transfer bile, produced by hepatocytes, into the duodenum to help with the emulsification of fats.
163
What is the sphincter of oddi?
The muscular valve surrounding the ampulla of vater = prevents bile outflow into duodenum during times of fasting and prevents reflux of duodenal content into ducts
164
What are the important components of bile?
bile salts, bile pigments, lecithin (phospholipid), aqueous solution, cholesterol and inorganic salts.
165
Whats the function of bile salts?
To help break up the large triglyceride globules into smaller ones which increases the surface area through which pancreatic lipase may act
166
What enzyme is required before pancreatic lipase can digest fat globules?
Colipase
167
What is colipase?
a protein co-enzyme required for optimal enzyme activity of pancreatic lipase. It is secreted by the pancreas in an inactive form, procolipase, which is activated in the intestinal lumen by trypsin.
It displaces bile salts and then binds to lipase and anchors it to the surface of the fat globules
168
Whats a micelle?
The structure which digested lipids are formed into. They have a lipid-rich core surrounded by a hydrophilic shell of bile salts and lecithin
169
What enzymes conjugates the bilirubin in the liver?
Glucuronyltransferase
170
What converts bilirubin into urobilinogeb?
Bacteria
171
Whats the major pathway for excretion of bilirubin?
involves urobilinogen being converted by bacteria into stercobilin and urobilin. This remains within the intestines and is excreted within the faeces. These breakdown products give faeces its characteristic brown colour.
172
Whats the minor pathway for excretion of bilirubin?
involves some urobilinogen being reabsorbed into the circulation and then excreted by the kidneys. Excessive excretion of urobilinogen can lead to darkening of the urine.
173
When is secretin released?
In response to acid chyme entering the duodenum
The s cells secrete secretin
174
Whats the function of secretin?
Stimulates pancreatic ductal cells to secrete an alkaline fluid which is rich in bicarbonate ions
It also stimulates the bile duct epithelium to secrete an alkaline rich fluid
This neutralises the acidic chyme that enters the duodenum
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What stimulates the release of cholecystokinin?
Entry of fat and protein products in duodenum = stimulates I cells = secrete CCK
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Whats the function of CCK?
Stimulates pancreatic acinar cells to secrete digestive enzymes into the pancreatic duct
Also initiates contraction of the gallbladder = release of bile
Also stimulates relaxation of sphincter of Oddi = bile can flow into duodenum
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Outline how the pancreatic enzymes are released?
All 3 are packaged into zymogen granules and are released together by exocytosis on stimulation of I cells
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Whats a lacteal?
The central lymphatic duct found within each virus - important in absorption and distribution of fats
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What cells are found within the crypts of lieberkuhn?
Stem cells
Enterocytes
Paneth cells
Endocrine cells
Goblet cells
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Whats the function of the stem cells found within the crypts of lieberkuhn?
Impprtant in regeneration of the epithelial lining. They can differentiate into enterocytes and goblet cells and migrate towards the top of the villus over 3-6 days before undergoing apoptosis
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Whats the function of the paneth cells found within the crypts of lieberkuhn?
Important functions in host defence and secretion of antimicrobial peptides
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X Whats the function of the goblet cells found within the crypts of lieberkuhn?
To secrete mucus
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Whats the function of the enterocyte cells found within the crypts of lieberkuhn?
predominantly responsible for the production of intestinal fluids in the small intestine
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What are the 4 main functions of the small intestine?
Secretion
Digestion
Absorption
Motility
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How are carbohydrates broken down in the small intestine?
by disaccharidases such as maltase, sucrase and lactase. These enzymes break disaccharides into single monosaccharide units. Sucrase converts sucrose into glucose and fructose, maltase converts maltose into two glucose molecules and lactase converts lactose into galactose and glucose.
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How are proteins broken down in the small intestine?
Enterokinase converts trypsinogen into the active enzyme trypsin. Trypsin is then able to activate further trypsinogen and chymotrypsinogen (to chymotrypsin). Trypsin, chymotrypsin along with elastase convert proteins to polypeptides. Carboxypeptidases (also released by the pancreas) can cleave individual amino acids from small polypeptides. Digestion also occurs on the luminal surface by peptidases on the luminal membrane of enterocytes. They produce small peptides (e.g. di-, tri-peptides) and single amino acids.
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How are lipids broken down in the small intestine?
Bile, released by the hepatobiliary system, emulsifies fat. Pancreatic lipase breaks down triglycerides to free fatty acids and monoglycerides. Micelles are formed with an outer lining of bile salts and products of fat digestion internally, that can be absorbed by enterocytes.
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What are the 2 important mechanisms of motility in the small intestine?
Segmentation and migrating motility complex
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What is segmentation?
the process of ring-like contractions of the small intestines that help to mix and propel intestinal content. These contractions occur due to oscillating circular muscle contraction. They are controlled by the basic electrical rhythm (BER). The BER refers to oscillations in membrane potential that bring the muscle closer and further from threshold potential.
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What is the migrating motility complex?
Peristaltic waves are generated, initially in the stomach, and cause a wave of depolarisation throughout the digestive tract, which moves intestinal content forward. Over 100-150 minutes the remaining intestinal content is swept towards the colon. The cycle can then be repeated once finished.
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When does migrating motility complex occur?
During times of fasting when segmentation stops and MMC takes over
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What regulates migrating motility complex?
The hormone motilin
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How are amino acids absorbed in the small intestine?
Sodium/potassium ATPase pumps on the basolateral membrane generate an electrochemical gradient. This allows the movement of sodium down its concentration gradient via a symporter on the luminal membrane.
The symporter is coupled with the movement of amino acid molecules thereby facilitating absorption. This process is secondary active transport. Once inside the cell, amino acids can be absorbed into the portal circulation by a variety of different passive carriers on the basolateral membrane.
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How are peptide fragments absorbed in the small intestine?
Sodium/potassium ATPase pumps on the basolateral membrane generate an electrochemical gradient. Sodium then moves into the enterocyte by an antiporter, which shuttles hydrogen ions out of the cell on the luminal membrane.
The movement of hydrogen ions out of the cell generates a second electrochemical gradient. Hydrogen then moves into the cell via a symporter on the luminal membrane. This symporter is coupled with the movement of peptide fragments. This process is tertiary active transport.
Once inside the enterocyte, these peptide fragments are broken into amino acids by intracellular peptidases before absorption into the portal circulation.
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How is glucose and galactose absorbed in the small intestine?
odium/potassium ATPases on the basolateral membrane generate an electrochemical gradient. This allows sodium to moves down its electrochemical gradient into the enterocyte through the sodium-glucose transporter. The movement of sodium into the cell is coupled with the movement of glucose/galactose. This process is secondary active transport.
Glucose/galactose are then passively absorbed by facilitated diffusion across the GLUT-2 transporter on the basolateral membrane.
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How is fructose absorbed in the small intestine?
Fructose is transported into the enterocyte by facilitated diffusion through the GLUT-5 transporter on the luminal membrane. Once inside the cell, fructose moves into the portal circulation via the GLUT-2 transporter on the basolateral membrane. This process is passive diffusion.
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How are lipids absorbed from the small intestine?
Micelles carry products of fat digestion and lipid-soluble components (e.g. cholesterol, fat-soluble vitamins) to the surface of enterocytes.
Free fatty acids and monoglycerides at the centre of the micelle freely diffuse across the luminal membrane into enterocytes. Inside the enterocyte, fatty acids and monoglycerides reform to make triglycerides. Triglycerides are then coated with a layer of lipoprotein created by the endoplasmic reticulum. This forms the smallest lipoprotein; a chylomicron.
Chylomicrons are then excreted into the central lacteal of the villi and transported through the lymphatic system. At the junction of the lymphatic duct with the subclavian vein, chylomicrons join the systemic circulation and are taken to muscle and adipose tissue where free fatty acids can be liberated. The remnant chylomicron is then taken to the liver and broken down. This transport of lipids from the intestines to the liver is called the exogenous pathway.
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How is iron absorbed in the small intestine?
Ferrous iron and haem are absorbed across the luminal membrane in different transporters:
Ferrous iron: transported by divalent metal transporter 1 (DMT).
Haem: transported by haem carrier protein 1.
Once inside the cell, iron can be converted into a cytosolic store of ferritin. Iron that is needed is absorbed across a basolateral transporter called ferroportin. Iron absorption across ferroportin can be regulated by the molecule hepcidin. This molecule is secreted by the liver when iron levels are high.
Binding of hepcidin to ferroportin leads to the lysosomal destruction of the receptor preventing iron absorption. Ferroportin can also be found in the reticuloendothelial system on the plasma membrane of macrophages. Once iron exits enterocytes it is transported within the blood by the molecule transferrin.
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What happens when we ingest to much iron?
Iron that is not required by the body remains in enterocytes as ferritin. From here it is either utilised or lost in the faeces during epithelial regeneration.
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How is calcium absorbed in the small intestine?
Calcium is initially absorbed down its concentration gradient across the TRPV6 receptor on the luminal surface of enterocytes. It is then transported across the cell to the basolateral membrane by the molecule calbindin.
Calcium can then be absorbed into the bloodstream at the basolateral membrane by two different mechanisms:
Calcium ATPase pump: primary active transport.
Sodium/calcium antiporter: secondary active transport.
Vitamin D ( a fat-soluble vitamin), once activated, enhances every stage of this process to aid with calcium absorption from the small intestines. It binds to nuclear vitamin D receptors that induce gene transcription. The gene transcription products are molecules important in calcium absorption (e.g. calbindin, TRPV6).
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Outline the muscle type seen in the oesophagus?
In the upper 1/3rd both the outer longitudinal layer and inner circular mucle layers are striated
In the lower 2/3rds both layers are composed of smooth muscle
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Outline the epithelium seen in the oesophagus?
Lined by stratified squamous epithelium which extends dismally to the squamocolumnar junction where the oesophagus joints the stomach - recognised by the Z line just above the most proximal gastric folds
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What is the Z-line in the oesophagus?
The squamocolumnar junction where the squamous mucosa of the oesophagus and the columnar mucosa of the stomach meet
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What are the 2 types of peristaltic contractions in the oesophagus?
Primary peristalsis
Secondary peristalsis
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What is primary peristalsis
This occurs when the bolus enters the oesophagus during swallowing.
Initiated and controlled by the swallowing centre and coordinated by the myenteric plexus
It forces the bolus down the oesophagus and into the stomach
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What is secondary peristalsis?
If the bolus gets stuck or moves slower than the primary peristaltic wave (as can happen when it is poorly lubricated), then stretch receptors in the esophageal lining are stimulated and a local reflex response causes a secondary peristaltic wave around the bolus, forcing it further down the esophagus, and these secondary waves continue indefinitely until the bolus enters the stomach
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Where is the swallowing centre?
In the medulla oblongata
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Outline the mechanism of the peristaltic reflex?
Food bolus -> stretch of the gut smooth muscle -> serotonin secreted -> sensory neurones activated
Sensory neurones activate neurons of the myenteric plexus, which then proceed to split into two cholinergic pathways: a retrograde and an anterograde.
Activated neurons of the retrograde pathway release substance P andACh to contract the smooth muscle behind the bolus.
The activated neurons of the anterograde pathway instead release NO and VIP to relax the smooth muscle caudal to the bolus.
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What are the 3 muscular layers of the stomach?
Outer longitudinal
Inner circular
Innermost oblique
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Where in the stomach are most parietal ands chief cells found?
The mucosa of the upper 2/3rds of the stomach
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Where are D cells and G cells found in the stomach most predominantly?
Antrum
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Where in the stomach are mucus secreting cells found?
Throughout the stomach (note mucus is made of glycoproteins called mucins)
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How is HCL released from parietal cells?
H+ are formed from the dissociation of carbonic acid. H+ is transported into the stomach lumen via the H+/K+ ATPase ion pump
The HCO3- ion is transported out of the cell into the blood in exchange for CL-. CL- is then transported into the stomach lumen via a chloride channel
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How is gastric acid production increased?
Vagus nerve releases ACh upon activation by seeing/smelling/chewing food
G cells secrete gastrin activated by vagus nerve, gastrin-relayed-peptide and peptides in stomach. Gastrin binds to CCK receptors on parietal cells which elevates calcium levels and increases vesicular fusion
Enterochromaffin like cells secrete histamine which bind H2 receptors on parietal cells upon activation by presence of gastrin and ACh
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How is gastric acid production decreased?
Accumulation of acid in stomach leads to lower pH = D cells release somatostatin = inhibits secretion of gastrin
As food passes into the duodenum the enterogastric reflex is triggered due to distension of duodenum = enteric nervous system sends signals too stomach and medulla oblongata = reducing fatal stimulation of stomach
Presence of chyme in duodenum also stimulates entero-endocrine cells to release CCK and secretin = inhibit gastric acid secretion
GIP and VIP also decrease acid production
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How does gastrin increase HCL secretion?
Gastrin binds CCK receptors on basal membrane of parietal cells and increase expression of K/H ATPase
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Outline how the number of H/K ATPase channels can change?
At rest the number of pumps is minimal
Upon stimulation the tubovesicles fuse with the cell membrane, increasing insertion of H/K+ ATPase into the membrane = allows increased movement of H+ into stomach = increased acid production
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What nervous system controls the functioning of the small bowel?
Enteric nervous system
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Where are iron and folate absorbed?
Duodenum and proximal jejunum
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Where is vit B12 and bile salts absorbed?
Terminal ileum
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Whats the main function of CCK?
Gallbladder contraction and sphincter of oddi relaxation
Trophic effects on duodenum and pancreas
Minor role in pancreas secretion
Role in satiety - acting on CNS
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Whats are the main functions of gastrin?
Stimulate acid secretion
Trophic to mucosa
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Whats are the main functions of secretin?
Stimulation of pancreatic bicarbonate secretion
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Whats are the main functions of vasoactive intestinal peptide?
Intestinal secretion of water and electrolytes
Neurotransmitter
Splanchnic vasodilation
Stimulates insulin release
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Whats are the main functions of glucose-dependant insulinotropic peptide?
Incretin effect - causes insulin release by islets in beta cells
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Whats are the main functions of glucagon like peptide 1?
Stimulates insulin synthesis
Incretin
Trophic to islet cells
Inhibits glucagon secretion and gastric emptying
Stimulates growth of enterocytes
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Whats are the main functions of motilin?
Increases gastric emptying and small bowel contraction
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Whats are the main functions of ghrelin?
Stimulates appetite
Increases gastric emptying
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Whats are the main functions of obestatin?
Opposes ghrelin
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Whats are the main functions of oxyntomodulin?
Inhibits appetite
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Whats are the main functions of somatostatin?
Inhibits secretion and action of most hormones
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Whats are the main functions of substance P?
Enhances gastric acid secretion
Smooth muscle contraction
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Whats are the main functions of neurotensin?
Affects gut motility
Increases jejunal and ileal fluid secretion
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Outline digestion of carbohydrates?
Salivary amylase catalyses polysaccharide hydrolysis in the mouth
Pancreatic amylase catalyses polysaccharide hydrolysis in the upper intestine
Breakdown products are maltose, maltriose, sucrose and lactose. These are further hydrolyses by specific oligo- and disaccharidases on microvillous membrane to form glucose, galactose and fructose
These monosaccharides are then transported across enterocytes via a SGLT1 transporter (unless fructose then GLUT5) and into blood via GLUT-2
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Outline digestion of protein?
Proteolytic enzymes are secreted by the pancreas as pro-enzymes and transformed to active forms in the lumen. These enzymes break down protein into oligopeptides. Some di- and tripeptides are absorbed intact while the remainder and broken down into free amino acids by peptidases on microvillous membranes of enterocytes.
Amino acids enter enterocytes via a Na+ dependant co-transported system
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Outline the process of formation of micelles?
Bile containing the amphipathic detergents, bile acids and phospholipids enters the duodenum following gall bladder contraction. These substances act to solubilize fat and promote hydrolysis of triglycerides in the duodenum by pancreatic lipase to yield fatty acids and monoglycerides.
Bile acids, phospholipids and the products of fat digestion cluster together with their hydrophilic ends on the outside to form aggregations called mixed micelles. Trapped in the centre of the micelles are the hydrophobic monoglycerides, fatty acids and cholesterol.
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Outline how micelles enter enterocytes and how they then enter the blood stream?
At the cell membrane, the lipid contents of the micelles are absorbed, while the bile salts remain in the lumen. Inside the cell, the monoglycerides and fatty acids are re-esterified to triglycerides. The triglycerides and other fat-soluble molecules (e.g. cholesterol, phospholipids) are then incorporated into chylomicrons to be transported into the lymph.
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What is the ileal brake?
The ileum secreted peptide YY which delays gastric emptying. This can be triggered by any unabsorped lipids that reach the ileum
This allows more time for absorption of lipids in the small intestine
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What are some physical defences of the GIT?
The mucus layer
Continuous shedding of surface epithelial cells
Physical movement of the luminal contents
Colonisation resistance - indigenous microbiota can outcompete pathogens for survival in the gut
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What are the innate chemical defences in the GIT?
Enzymes - lysozyme and phospholipids A2 secreted by paneth cells at the base of crypts
Antimicrobial peptides secreted from enterocytes and paneth cells e.g. defensins
Trefoil peptides secreted by goblet cells - stabilise mucus
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What innate immunoglobulin defence mechanisms does the GIT have?
Hum oral defence - IgA mediates mucosal immunity and is secreted from immunocytes in the lamina propria as dimers joined by a J-chain (polymeric IgA)
B cell sensitisation
Cellular defence - intestinal T lymphocytes in GALT, lamina propria and surface epithelium
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What is the gut microbiome?
comprises all intestinal microorganisms residing along with the gastrointestinal (GI) tract which include commensal, symbiotic, and pathogenic microorganisms
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How does the gut microbiome change as you move further down the colon?
Small numbers in stomach and small intestine which very large numbers in the colon.
As you move further towards the colon, anaerobes increase.
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Whats the function of the appendix?
The function of the appendix is unknown. One theory is that the appendix acts as a storehouse for good bacteria, “rebooting” the digestive system after diarrheal illnesses
245
What are the 2 main anal sphincters and what are they innervated by?
Internal anal sphincter - parasympathetic fibres which relax involuntarily
External anal sphincter - skeletal muscle controlled by somatic nerve supply from inferior anal branch of pudendal nerve (S2-S4) which allows conscious control of defecation
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Outline the physiology of defecatuon?
When the rectum is distended the rectosphincteric reflex is initiated and relaxes the internal sphincter. If defecation is not desired, voluntary contraction of the external sphincter can delay it.
If defecation is appropriate, then a series of reflexes take place that lead to: Relaxation of the external sphincter, contraction of abdominal wall muscles and relaxation of pelvic wall muscles.
Peristaltic waves then facilitate the movement of faeces through the anal canal. Defecation can also be assisted by taking a deep breath and attempting to expel the air against a closed glottis, this is known as the Valsalva maneuver
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How long does food stay in the stomach for?
6-8 hours
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What is the American Society of Anaesthesiologists classification?
A simple categorization of a patient's physiological status that can help predict operative risk
249
What is ASA 1 in the American Society of Anaesthesiologist classification?
A normal healthy patient
E,g, non smoker, no/minimal alcohol use, healthy
250
What is ASA 2 in the American Society of Anaesthesiologist classification?
Patient with mild systemic disease e.g. current smoker, social alcohol drinker, pregnancy, obesity, well controlled diabetes, hypertension, mild lung disease
251
What is ASA 3 in the American Society of Anaesthesiologist classification?
A pt with severe systemic disease e.g. 1 or more moderate-severe disease (for example poorly controlled diabetes, COPD, morbid obesity, active hepatitis, alcohol deendance, ESRD, MI)
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What is ASA 4 in the American Society of Anaesthesiologist classification?
A pt with severe systemic disease that is a constant threat to life e.g. recent MI, cerebrovascular accident, sepsis, DIC, ESRD not undergoing regular dialysis
253
What is ASA 5 in the American Society of Anaesthesiologist classification?
A moribund pt who is not expected to survive without the operation
E.g. ruptured aneurysm, massive trauma, intracranial bleed with mass effect, ischaemic bowel in the face of significant cardiac pathology, multiple organ dysfunction
254
What is ASA 6 in the American Society of Anaesthesiologist classification?
A declared brain-dead patient whose organs are being removed for donor purposes
255
What are examples of operations in which chance of diffusion is nearly definite and therefore you need to cross match 4-6 units?
Total gastrectomy
oophorectomy
oesophagectomy
Elective AAA repair
cystectomy
hepatectomy
