Gastroenterology

Question 1

Outline pancreatic embryology

Answer 1

Abdominal accessory organs arise as foregut outgrowths. The proximal duodenum rotates clockwise. On the dorsal side of the pancreas can be found the dorsal pancreatic bud and duct. On the ventral side can be round the ventral pancreatic bud and duct, the liver buds (develop into the liver). The joint between the liver and the foregut eventually narrows and extends to become the bike duct, which gives rise to its own outgrowth, which then becomes the gallbladder and the cystic duct.

Question 2

What happens by week 11 of pancreatic embryology?

Answer 2

By week 11, rotation is essentially complete and the organs are in their final locations. The ventral pancreatic bud and duct rotate clockwise and have then fuse with the dorsal pancreatic bud and duct, forming the main pancreatic duct, which emerges at the major papilla, where it joins the distal common bile duct. The dorsal duct emerges at the minor papilla and is known as an accessory pancreatic duct. This structure often degenerates and is absent in many adults.

Question 3

Where is the pancreas located?

Answer 3

The pancreas is retroperitoneal organ, meaning that it doesn’t exist within the abdomen, but behind the posterior peritoneum of the abdomen. Anterior to the pancreas are the transverse colon and the stomach

Question 4

Outline the anatomy of the pancreas

Answer 4

The pancreas is composed of 4 regions:

1) The head - fits into the duodenum, which is ‘C’-shaped
2) The neck - contain the superior mesenteric vein, which comes behind the pancreas to join the splenic vein and become the portal vein.
3) The body
4) The tail

Question 5

Outline the anatomy of the duodenum

Answer 5

The duodenum is described as being D1, D2, D3 and D4.

Question 6

What is located behind the pancreas

Answer 6

The coeliac axis and the superior mesenteric artery are both located posterior to the pancreas. But the superior mesenteric vein, the splenic vein, the portal vein, the IVC, the left renal vein and the right renal vein are all also located behind the pancreas.

Question 7

How is the pancreas imaged?

Answer 7

1) Computed tomography (CT) scan - this is the most useful way of imaging the pancreas.
2) Magnetic Resonance CholangioPancreatography (MRCP)
3) Angiography - used for interventional purposes, particularly patients that are bleeding

Question 8

Why do patients get recurrent episodes of pancreatitis?

Answer 8

In some, the main pancreatic duct is not fused with the ventral duct, this goes by the name pancreas divisum. Many patients with this get recurrent episodes of pancreatitis because it’s normally the ventral duct that has a large enough calibre to cope with flow of pancreatic juice, but in these patients, a large flow has to go through the minor duct instead.

Question 9

What’s the difference between endocrine and exocrine excretion?

Answer 9

1) Endocrine secretion describes secretion into the blood stream to have effect on distant target organ (Autocrine/Paracrine) and involves ductless glands.
2) Exocrine secretion describes secretion into a duct to have direct local effect.

Question 10

What are the main endocrine secretion of the pancreas?

Answer 10

1) Insulin: anabolic hormone
• Promotes glucose transport into cells & storage as glycogen
• Decreases blood glucose
• Promotes protein synthesis & lipogenesis
2) Glucagon: Increases gluconeogenesis & glycogenolysis (increases blood glucose)
3) Somatostatin: “Endocrine cyanide”

Question 11

Outline pancreatic endocrine vs exocrine secretion

Answer 11

1) Endocrine - 2% of the pancreatic gland
•Islets of Langerhans
•Secrete hormones into blood - Insulin & Glucagon (also Somatostatin and Pancreatic Polypeptide)
•Regulation of blood glucose, metabolism & growth effects - (Endocrine course)
2) Exocrine - 98% of the pancreatic gland
•Secretes pancreatic juice into duodenum via main pancreatic duct (MPD)/sphincter of Oddi/ampulla
•Digestive function

Question 12

Outline pancreatic cell differentiation

Answer 12

1) Acini
•They are attached to ducts
•Acini are grape-like clusters of secretory units
•Acinar cells secrete pro-enzymes into ducts
2) Islets
•Derived from the branching duct system
•Lose contact with ducts – become islets
•Differentiate into α- and β-cells secreting into blood
•More islets in the tail of the pancreas than in the head

Question 13

Outline the microanatomy of the pancreas

Answer 13

Pancreatic acini are found around ducts and consist of pancreatic acinar cells. Between the acinar cells are intercellular cananiculi, which drain into pancreatic ducts. These ducts get bigger and bigger, and come to an intercalated duct, which becomes an intralobular duct which then join with the main pancreatic ducts.

Question 14

What are centroacinar cells?

Answer 14

These are specific cells between the acinus and between the duct.

Question 15

Outline the anatomy of islets

Answer 15

The islets have no connection with the ducts, but have numerous capillaries within them. The fact that the islets are highly vascular, ensure that all endocrine cells have close access to a site for secretion. The islets are surrounded by acini.

Question 16

Outline the endocrine composition of islets

Answer 16

1) α-cells form about 15-20% of islet tissue and secrete glucagon
2) β-cells form about 60-70% of islet tissue and secrete insulin
3) δ-cells form about 5-10% of islet tissue and secrete somatostatin

Question 17

Outline the exocrine composition of acini

Answer 17

1) Secretory acinar cells
•Large with apical secretion granules
2) Duct cells
•Small & pale

Question 18

What is pancreatic juice?

Answer 18

Pancreatic juice is made of 2 components:

1) Produced by acinar cells - these are low volume, are viscous and are enzyme-rich
2) Produced by duct and centroacinar cells - these are high volume, are watery and are HCO3- rich

Question 19

Outline bicarbonate secretion to produce pancreatic juice

Answer 19

1) Produced by duct & centroacinar cells
2) Pancreatic Juice is very high in bicarbonates
• ~120 mM (mmol/L) of bicarbonate in pancreatic juice, only ~25 mM in plasma
•pH 7.5-8.0
3) The purpose of the bicarbonate is to neutralise acid chyme from stomach
• This prevents damage to duodenal mucosa which cannot resist low pH
• Raises pH to optimum range for pancreatic enzymes to work
4) Washes low volume enzyme secretion out of pancreas into duodenum

Question 20

How does duodenal pH effect pancreatic bicarbonate secretion rate?

Answer 20

1) When Duodenal pH < 5, this causes a linear increase in pancreatic HCO3- secretion
2) Duodenal pH <3, there is not much more increase in HCO3- secretion

Question 21

Why does pancreatic bicarbonate secretion stop when pH is still acid?

Answer 21

Bile also contains HCO3- and helps neutralise acid chyme. As well as this, Brunners glands secrete alkaline fluid.

Question 22

Outline the mechanism of pancreatic bicarbonate secretion

Answer 22

1) CO2 enters the pancreatic duct cells are reacts with water to separate H+ and HCO3-. This step is catalysed by carbonic anhydrase. Simultaneously, Na+ moves down gradient via paracellular (“tight”) junctions and water (H2O ) follows.
2) Cl-/HCO3- exchange at lumen (via the anion exchanger [AE]). Simultaneous Na+/H+ exchange at basolateral membrane into bloodstream (sodium-hydrogen exchanger (antiporter) type 1 [NHE-1]). These exchanges driven by electrochemical gradients (ie high extracellular Na+ compared to intracellular and high Cl- in lumen compared to intracellular).
3) The Na+ gradient into the cell from blood is maintained by Na+/K+ exchange pump, which uses ATP - Primary active transport.
4) K+ returns to blood via K+-channel. Cl- returns to lumen via Cl-channel (cystic fibrosis transmembrane conductance regulator [CFTR]).

Question 23

Where else is bicarbonate produced in the body?

Answer 23

The exact reaction between carbon dioxide and water to produce H+ and HCO3- in the pancreas, is same reaction in gastric parietal cells:

1) In the stomach, H+ is secreted into the gastric juice and HCO3- enters the blood. Hence, gastric venous blood is alkaline.
2) In the pancreas, HCO3- is secreted into pancreatic juice and H+ enters the blood. Hence, pancreatic venous blood is acidic.

Question 24

Which enzymes do acinar cells secrete?

Answer 24

Acinar cells secrete enzymes for the digestion of:

1) Fat (lipases)
2) Protein (proteases)
3) Carbohydrates (amylase)

Question 25

Where the acinar cell enzymes synthesised and stored?

Answer 25

The enzymes are synthesised and stored in zymogen granules found within acinar cells. Zymogens are stores of pro-enzymes, essentially inactive forms of subsequent enzymes.

Question 26

What protective mechanisms prevent acinar cell enzymes from auto-digestion?

Answer 26

1) Proteases are released as inactive pro-enzymes
• Protects acini & ducts from auto-digestion
2) Pancreas also contains a trypsin inhibitor to prevent trypsin activation
3) Enzymes only activated in the duodenum, where they start to digest food
4) Blockage (e.g. gallstone) of the main pancreatic duct (MPD) may overload protection → auto-digestion (= acute pancreatitis)

Question 27

What is the function of the duodenal mucosa?

Answer 27

The duodenal mucosa secretes an enzyme - Enterokinase (a type of enteropeptidase produced in the brush border). Enterokinase then converts trypsinogen into its active form, trypsin.

Question 28

What is the function of trypsin?

Answer 28

Trypsin converts all other proteolytic & some lipolytic enzymes.

Question 29

How is lipase secreted?

Answer 29

Lipasea are secreted in their active form but require colipase (also secreted as precursor) which binds them to fat globules, preventing them from causing any damage. Lipases require the presence of bile salts for effective action.

Question 30

What may alter pancreatic enzyme function?

Answer 30

1) Pancreatic secretions adapt to diet (e.g. ↑ protein, ↓ carbs leads to ↑ proportion of proteases & ↓ proportion of amylases).
2) Pancreatic enzymes (+ bile) essential for normal digestion of a meal:
• Lack of these can lead malnutrition even if dietary input is OK (unlike salivary or gastric enzymes).

Question 31

What are the side effects of the anti-obesity drug, Orlislat, which inhibits pancreatic lipases?

Answer 31

Increased faecal fat, which occurs when pancreatic lipase secretion decreases. Much like cystic fibrosis and chronic pancreatitis, Orlistat decreases intestinal fat absorption, leading to fatty stool or steatorrhea.

Question 32

How is pancreatic juice secretion controlled?

Answer 32

1) Cephalic phase
•Reflex response to sight/smell/taste of food
•Enzyme-rich component only.
•Low volume - “mobilises” enzymes
2) Gastric phase
•Stimulation of pancreatic secretion originating from food arriving in the stomach
•Same mechanisms involved as for cephalic phase
3) Intestinal phase (= 70-80% of pancreatic secretion)
•Hormonally mediated when gastric chyme enters duodenum.
•BOTH components of pancreatic juice stimulated
•enzymes & HCO3- juice flows into duodenum

Question 33

How is pancreatic juice controlled?

Answer 33

The two components of pancreatic juice are separately controlled.
1) Pancreatic juice enzyme secretion controlled in acini by:
> Vagus nerve –
• Cholinergic (acetylcholine)
• Nerve vagal stimulation of enzyme secretion (& communicates information from gut to brain)
> Cholecystokinin (CCK) (Ca2+/PLC)
2) Pancreatic juice bicarbonate secretion is controlled in duct & centroacinar cells by:
• Secretin (cAMP)

Question 34

What are the mechanisms for controlling cholecystokinin (CCK)?

Answer 34

There are numerous mechanisms responsible for controlling release of CCK from duodenal I cells:

1) Stimulatory effects - fatty acids and amino acids within the lumen of the duodenum stimulate I cells in the duodenum.
2) Inhibitory effects - trypsin
3) CCK-releasing peptide (CCK-RP) has stimulatory effusive
4) Gastrin-releasing peptide (GRP) has stimulators effects

Question 35

How is bicarbonate secretion controlled in acini and ducts?

Answer 35

1) Acinar fluid is isotonic
•Resembles plasma in its concs of Na+, K+, Cl− & HCO3−.
2) Secretion of acinar fluid & proteins it contains is stimulated primarily by CCK
3) Secretin stimulates secretion of H2O & HCO3- from cells lining extralobular ducts.
4) Secretin-stimulated secretion is richer in HCO3− cf acinar secretion because of Cl−/HCO3− exchange

Question 36

How is HCO3- secretion controlled in ducts?

Answer 36

Classic negative-feedback loop (ie decreasing luminal pH in the duodenum stimulates S cells to release Secretin which stimulates pancreatic duct at HCO3- secretion, which increases pH and inhibits the decrease in luminal pH in the duodenum).

Question 37

How do stimulants effect the rate of bicarbonate secretion?

Answer 37

1) CCK alone has no effect on bicarbonate secretion, but the maximum effect is reached when stimulated by both CCK and secretin
2) The Vagus nerve has similar effect to CCK, secretin has the most effect individually
3) Secretin has NO EFFECT on enzyme secretion from the acute

Question 38

How does pH change when food is eaten?

Answer 38

1) Food mixed, digested in stomach, pH 2
2) Chyme squirted into duodenum
3) H+ ions in duodenum cause the secretion of secretin which causes increased pancreatic juice. This, added with bile and Brunner’s gland secretions increase pH to neutral/alkaline.
4) Peptides & fat in duodenum cause a sharp increase in CCK & vagal nerve stimulation. This in turn stimulates pancreatic enzyme release, which peaks by 30 mins and continues until the stomach is empty.
5) CCK potentiates effects of secretin on aqueous component. This is necessary as most of duodenum is not at a low pH.

Question 39

What is the function of the small bowel?

Answer 39

To absorb nutrients, salt and water.

Question 40

Outline the structure of the small bowel

Answer 40

The small bowl is approximately 6m long and 3.5cm in diameter. It is formed of 3 regions:
1) Duodenum - 25 cm long
2) Jejunum - 2.5 m long
3) Ileum - 3.75 m long
There is no sudden transition between them and they all have same basic histological organisation.

Question 41

What is the function of the mesentery?

Answer 41

Mesentery has several functions:
1) Suspends small & large bowel from posterior abdominal wall
•anchoring them in place
•whilst still allowing some movement
2) Provides a conduit for blood vessels, nerves & lymphatic vessels.

Question 42

Outline the structure and function of the superior mesenteric artery

Answer 42

The artery comes out of the inferior border of the pancreas. It supplies all the jejunum and all the ileum, via jejunal and ileal arteries.

1) The ileocolic artery supplies the terminal ileum, the ascending colon, and the caecum.
2) The right colic artery sillies the ascending colon.
3) The middle colic artery supplies the hepatic flexure, the transverse colon and the splenic flexure.

Question 43

Outline the digestive epithelium of the small bowel

Answer 43

The outer covering of the small bowel is the serosa. Underneath is the longitudinal muscle, followed by the circular muscle (important for motility).

Question 44

What are the plicae cicularis?

Answer 44

Plicae circularis are found on the smooth muscle and contain villi. The apex of the villus in turn has numerous microvilli. The villi are surrounded by cells called enterocytes. At the centre of the villi can be found arteries, veins and lymphatic vessels. From the apex downwards, between villi, is a crypt.

Question 45

What are villi?

Answer 45

1) They only occur in the small intestine
2) They are motile
3) They have a rich blood supply and lymph drainage for absorption of digested nutrients
4) They have good innervation from the submucosal plexus
5) They have simple epithelium, which is 1 cell thick and dominated by enterocytes (columnar absorptive cells)

Question 46

What cell types are found within the small bowel?

Answer 46

1) Villi (mucosa) lined with a simple columnar epithelium consisting of:
•primarily enterocytes (absorptive cells)
•scattered goblet cells
•scattered enteroendocrine cells
2) Crypts of Lieberkühn - epithelium includes:
•Paneth cells
•Stem cells

Question 47

What are enterocytes?

Answer 47

These are the most abundant cells in small bowel:

1) They are tall columnar cells with microvilli & a basal nucleus.
2) They are specialised for absorption & transport of substances.
3) They have a short lifespan of 1-6 days.

Question 48

How is the surface area of the small bowel increased?

Answer 48

The cylindrical internal surface area of small bowel is only 0.4m2. The plicae circularis, folds, villi and microvilli increase the surface area to ~200m2 (the size of a tennis court). This is an increase of the absorption and digestive surface by at least a 500 fold.

Question 49

What are microvilli?

Answer 49

1) Microvilli, which are ~0.5-1.5 micrometres high, make up the “brush border”
2) There are several thousand microvilli per cell/enterocyte
3) The surface of microvilli is covered with glycocalyx

Question 50

What is glycocalyx?

Answer 50

1) Glycocalyx is a rich carbohydrate layer on the apical membrane
2) It serves as protection from digestional lumen, yet allows for absorption
3) It traps a layer of water and mucous known as “unstirred layer”
•This regulates the rate of absorption from intestinal lumen

Question 51

What are goblet cells?

Answer 51

1) These are the 2nd most abundant epithelial cell type
2) They are mucous containing granules which accumulate at the apical end of cell, causing their ‘goblet’ shape.
3) Mucous is a large glycoprotein that facilitates the passage of material through the small bowel
4) The abundance of goblet cells increases distally along the length of the bowel
• There are virtually none in the duodenum
• There are lots in the colon (solid faeces requires lubrication for passage)

Question 52

What are enteroendocrine cells?

Answer 52

1) These are columnar epithelial cells which are scattered among enterocytes.
2) They are most often found in the lower part of crypts.
3) They are hormone secreting (e.g. to influence gut motility)
4) In older text books they are referred to as chromaffin cells (affinity for chromium/silver salts).

Question 53

What are paneth cells?

Answer 53

1) These are found only in the bases of crypts
2) They contain large, acidophilic granules
3) The granules contain:
•antibacterial enzyme lysozyme (protects stem cells)
•Glycoproteins & zinc (essential trace metal for a large number of enzymes)
4) They also engulf some bacteria & protozoa
5) They may have a role in regulating intestinal flora

Question 54

What are stem cells?

Answer 54

1) They are undifferentiated cells which remain capable of cell division to replace cells which die
2) Epithelial stem cells are essential in the GI tract to continually replenish the surface epithelium
3) They continually divide by mitosis
4) They migrate up to tip of villus, escalator-like, replacing older cells that die by apoptosis
•The dead cells are digested and reabsorbed
5) They can differentiate into various cell types (pluripotent)

Question 55

Why do enterocytes and goblet cells of the small bowel have short life spans

Answer 55

1) Enterocytes and goblet cells have short life spans, of about 36 hours, as enterocytes are the first line of defense against GI pathogens & may be directly affected by toxic substances in the diet.
2) Having shorter life spans means that the effects of agents which interfere with cell function, metabolic rate etc will be diminished.
3) As well as this, any lesions will be short-lived.
4) If the escalator-like transit of enterocytes is interrupted through impaired production of new cells (e.g. radiation) severe intestinal dysfunction will occur.

Question 56

Outline the structure and function of the duodenum

Answer 56

The duodenum can be distinguished by presence of Brunner’s glands, which are submucosal coiled tubular mucous glands which secrete alkaline fluid. They open into the base of the crypts and neutralise the acidic chyme from stomach, protecting the proximal small bowel, and help to optimise pH for action of pancreatic digestive enzymes.

Question 57

Compare the structure and function of the jejunum and ileum

Answer 57

1) The duodenum leads on to the jejunum.
2) The jejunum is wider, thicker walled and redder than the ileum.
3) The jejunum is thicker because the plicae circularis are larger, more numerous and more closely set.
4) The jejunal mesentery, attached to the posterior abdominal wall, is above and to the left of the aorta. The ileal mesentery is attached below and to the right of the aorta.
5) The jejunal mesenteric branches have long infrequent terminal vessels to the intestine, with only 1-2 arcades. The ileal mesenteric branches have numerous short terminal, with 3-4 arcades.
6) Peyer’s patches, which are aggregations of lymphoid tissue, only occur in the lower ileum and the anterior mesenteric border.

Question 58

Outline the functions of small bowel motility

Answer 58

1) Segmentation: mix ingested food with digestive secretions & enzymes
2) Migrating motor complex: facilitate contact between contents of intestine & the intestinal mucosa
3) Peristalsis: propel intestinal contents along the alimentary tract

Question 59

How does small bowel motility allow segmentation (mixing) of the contents of the lumen?

Answer 59

1) This occurs by stationary contraction of circular muscles at intervals.
2) More frequent contractions in duodenum compared to the ileum
3) Allow pancreatic enzymes & bile to mix with chyme
4) Although chyme moves in both directions, net effect is movement towards the colon

Question 60

How does small bowel mobility allow peristalsis (propelling)?

Answer 60

1) Involves sequential contraction of adjacent rings of smooth muscle
2) Propels chyme towards colon
3) Most waves of peristalsis only travel about 10cm
4) Segmentation & peristalsis result in chyme being segmented, mixed & propelled towards the colon

Question 61

How does small bowel mobility allow the migrating motor complex?

Answer 61

1) Cycles of smooth muscle contractions sweeping through gut
2) Begin in stomach → small intestine → colon → next wave starts in duodenum
3) Prevents migration of colonic bacteria into ileum

Question 62

Outline the digestion and absorption mechanisms in the duodenum

Answer 62

1) Digestion in the small bowel occurs in an alkaline environment.
2) Pancreatic digestive enzymes & bile enter the duodenum from the main pancreatic duct (MPD) and the common bile duct (CBD).
3) The duodenal epithelium also produces its own digestive enzymes
4) Digestion occurs in lumen & in contact with the membrane

Question 63

Outline the digestion of carbohydrates

Answer 63

1) Carbohydrates contain ~50% of ingested calories in the western diet.
2) Digestion begins in mouth by salivary alpha- amylase, which is destroyed in the stomach due to its acid pH.
3) Most of digestion of carbohydrates occurs in small intestine.

Question 64

Outline the types of carbohydrates

Answer 64

1) Simple carbohydrates
•monosaccharides -  glucose & fructose
•disaccharides -  sucrose & maltose
2) Complex carbohydrates
•starch, cellulose, pectins
•→ sugars bonded together to form a chain

Question 65

Outline the function of pancreatic alpha-amylase

Answer 65

1) It is secreted into the duodenum in response to a meal
2) It continues the digestion of starch & glycogen in the small bowel (started by salivary amylase)
3) It requires Cl- for optimum activity & neutral/slightly alkaline pH
4) It acts mainly in the lumen (some also adsorbs to brush border)
5) Digestion of amylase products & simple carbohydrates occurs at the brush border

Question 66

Which enzymes digest different carbohydrates, and where?

Answer 66

1) In the lumen, alpha-amylase digests starch down to maltose, maltotriose and alpha-dextrins.
2) At the brush border, different oligosaccharides are digested:
> Lactase splits lactose
> Glucoamylase (maltase) removes glucose monomers for transport
> Sucrase-isomaltase is formed of 2 enzymes: the sucrase moiety splits sucrose as well as maltose and maltotriose, the isomaltase moiety splits alpha-doctrine as well as maltose and maltotriose.

Question 67

How are monosaccharides absorbed?

Answer 67

1) Absorption of glucose & galactose is by secondary active transport
•Carrier protein is SGLT-1 on the apical membrane
2) Absorption of fructose is by facilitated diffusion
•Carrier protein is GLUT-5 on the apical membrane
3) GLUT-2 facilitates exit at basolateral membrane
4) The human small intestine can absorb 10kg of simple sugars/day

Question 68

How are proteins digested?

Answer 68

1) Protein digestion begins in lumen of stomach by pepsin
•Pepsin is then inactivated in the alkaline duodenum
2) 5 pancreatic proteases are secreted as precursors into the lumen of the small bowel (e.g. trypsinogen)
3) Trypsinogen is activated into trypsin by enterokinase, an enzyme located on duodenal brush border
4) Trypsin then activates other proteases, which hydrolyse proteins into single amino acids (AA) & oligopeptides (AA)n

Question 69

How are digested proteins absorbed?

Answer 69

1) A variety of peptidases at brush borders of enterocytes progressively hydrolyse oligopeptides [(AA)n] into amino acids (AAs)
2) Enterocytes directly absorb some of the small oligopeptides [(AA)n] via action of H+/oligopeptide cotransporter PepT1
3) These small oligopeptides are digested to amino acids (AAs) by peptidases (e.g. tripeptidase and dipeptidase) in the cytoplasm of enterocytes

Question 70

How are lipids digested in the small bowel?

Answer 70

Lipids are poorly soluble in water, so complicated to digest. Hence, there is a 4 stage process of digestion in the small bowel:
1) Secretion of bile salts & pancreatic lipases
2) Emulsification - increases surface area for digestion
3) Enzymatic hydrolysis of ester linkages to produce fatty acids and monoglycerides
> Colipase complexes with lipase – prevents bile salts displacing lipase from the fat droplet
4) The solubilisation of lipolytic products in bile salt micelles

Question 71

How are lipids absorbed?

Answer 71

Unlike amino acids and simple sugars, lipids are transformed as they’re absorbed via enterocytes:
1) Fatty acids (FAs) & monoglycerides (MG) leave micelles and enter enterocytes
2) FAs & MG are resynthesized into tri-glycerides (TGS) by 2x pathways:
> Monoglyceride acylation (major pathway )
> Phosphatidic acid pathway (minor pathway )
3) Chylomicrons are then formed - lipoprotein particles synthesised as an emulsion (80-90% TGs, 8-9% phospholipids, 2% cholesterol, 2% protein, trace carbohydrate) in Golgi apparatus
4) Chylomicrons are then secreted across the basement membrane by the active process of exocytosis
5) Chylomicrons enter a lacteal/lymph capillary, the lymphatic system within the villi then transports them away from bowel

Question 72

What is the ileocaecal valve?

Answer 72

The ileum is separated from the colon by the ileocaecal valve. Relaxation and contraction of this valve controls the passage of material into colon, and prevents back flow of bacteria into the ileum.