GI

Question 1

The wall of the GI tract is formed of 4 layers:

Answer 1

•Mucosa - the lining of the GI tract. Itself comprises 3 layers, inc a thin layer of muscle
•Submucosa – connective tissue. This is where blood vessels and nerves lie
•Muscularis – Layers of smooth muscle and enteric nervous system
•Serosa – this is the visceral layer of the peritoneum
Some parts of the GI tract have modifications to these standard layers

Question 2

Oesophagus

Answer 2

Lined with stratified SQUAMOUS
EPITHELIUM (thick, robust) until last
1cms when COLUMNAR EPITHELIUM
Muscle are voluntary (striated) in upper
third; involuntary (smooth) in lower
third and mixed in the middle.
• SPHINCTERS
– Upper oesophageal sphincter – muscular.
Primarily cricopharyngeus. Stops air getting
into the gut.
– Lowe oeosphageal sphincter – comprises a
thickened muscular layer in the lower
oesophagus and cardia of the stomach
(intrinsic) and the diaphragm (extrinsic).
Prevents acid/food reflux

Question 3

Stomach

Answer 3

Divided into
cardia/fundus/body/antrum
The mucosa is folded into RUGAE
(folds) and within these are gastric
pits.
• Muscles: lie in oblique layers. Very
strong and effective

Question 4

Gastric mucosa

Answer 4

Glands: secrete mucous
which protects the mucosa
from the acid environment of
the stomach
• Chief cells: secrete enzymes
of gastric juice (pepsin)
• Parietal Cells: secrete
hydrochloric acid and
intrinsic factor (imp for b12
absorption)
• Endocrine cells: secrete grelin
(hormone which promotes
appetite) and gastrin
(digestive hormone)

Question 5

Functions of the Stomach

Answer 5

• Secretes intrinsic factor: allows b12
absorption
• Some absorption: water, alcohol, some drugs
• Endocrine: ghrelin and gastrin secretion

Question 6

Duodenum

Answer 6

Biliary tract enters

GI tract here

Question 7

Jejunum and Ileum

Answer 7

Ileum ends at the
ileo-caecal valve in
the RIF

Question 8

Small intestinal mucosa

Answer 8

The mucosa of the SI is
folded into villi
• Increased surface area
for absorption
Each vilius contains blood vessels and
lymph vessel
• Surface cells – enterocytes – have
microvilli this is known as the‘brush
border’
• Digestive enzymes found here
• Other cells:
– Mucus secreting goblet cells
– Enteroendocrine
cells
- Stem cells. Found in
deep crypts adjacent to villi

Question 9

Colon/large intestine

Answer 9

Caecum
• Colon
– Ascending
– Transverse
– Descending
– Sigmoid
• Rectum
• Anal canal

Question 10

Rectum

Answer 10

Repository for stool • Ends at the anal canal
where there is a
transition to squamous
mucosa
• Anal sphincter
– an
internal (smooth
muscle) and external
(striated muscle)

Question 11

Wall of the colon

Answer 11

Multiple mucus
secreting glands
• No villi but crypts
• Muscles are grouped
into dense strips
(taeniae coli) and
rings. These are
shorter than the
bowel and mean
pouches (haustra) are
formed

Question 12

Appendix

Answer 12

May have a role in

gut microflora

Question 13

Peritoneum

Answer 13

– Viseral – lines the
organs, is their serosa
– Parietal – lines the
walls of the abdo
cavity

Question 14

Gallbladder

Answer 14

Lies below the liver
• Internally mucosa
form rugae
• Functions:
– Stores bile
– Bile is crucial for fat
absorption
– When triggered by
gut hormone (CCK) it
empties

Question 15

Pancreas

Answer 15

Head lies within the
curve of the
duodenum
• Tail touches the
spleen
• Endocrine and
exocrine function

The Exocrine Pancreas
• Majority of the tissue
• Have an acinar arrangement like the liver
• Complex ductal collecting system that ends at the
pancreatic duct which empties into the duodenum
• Secrete pancreatic juice i.e. Digestive enzymes and
sodium bicarbonate
The Endocrine Pancreas
• Islands of endocrine cells ‘islet of langerhans’
• Several kinds of cell
• Secrete hormones systemically into capillaries
• Most important is insulin (from beta cells) and
glucagon (from alpha cells)

Question 16

Parenchymal cells are supported by fine reticular fibres

Answer 16

Portal vein bringing food-rich blood from the gut.

(b) Hepatic artery bringing arterial blood.
(c) Hepatic veins taking away processed blood into the vena cava.
(d) Lymphatics taking away some lymph.
(e) Hepatic ducts removing bile to the gallbladder and gut. `

Question 17

Types of liver vessels

Answer 17

Central vein / terminal hepatic venule - very thin wall; lies in the centre of a lobule,
with sinusoids converging towards and opening into it.
(b) Sublobular/intercalated vein - thicker wall; lies alone at the periphery of the lobule.
(c) Branch of portal vein - again at the periphery of the lobule, but accompanied by one or more small
hepatic arteries/arterioles, one or more bile ducts/ductules lined by cuboidal epithelium, and
lymphatics.
Portal vein, artery, and bile duct constitute a portal triad; the area in which they lie is a portal area.
(The lymphatics are ignored for this naming)

Question 18

Hepatic lobular blood flow is:

Answer 18

rom branches of the portal vein and hepatic artery; from the periphery towards the
centre;
(b) in the sinusoids, between the cell plates.
(c) Blood collected in central veins goes to sublobular veins, then to collecting veins, and
then hepatic veins leaving the liver.
Intralobular bile flow is from the lobule’s centre towards the peripheral bile ducts, and runs,
within any one cell plate, between the liver cells in bile canaliculi.

Question 19

LIVER ACINUS

Answer 19

Rappaport’sliver&acinus representsafunctionalunit
comprisingpartsofthreeorsolobules.Ittriestoexplain**
differencesinexposuretothebloodsupplyamong
variouspartsoflobules.*
• Suchdifferencesarereflectedinvariedfunctional*
activitiesanddegreesofsusceptibilitytotoxicagents= a*
metaboliczonation.
• Theterritoryofanacinushas,asitsaxis,onefinal
branchoftheportalvein,andissubdividedinto:*
• 1 periportal,*
• 2 intermediate,and
• 3 perivenous (closetothecentralvein)zones,withthe
initialperiportalzonebeingroughlyspheroid,and*
isolatedfromperiportalzonesofadjacentacini.

Question 20

Inside the liver lobule: Liver sinusoids

Answer 20

Sinusoids are low pressure vascular channels that receive blood from terminal branches of the hepatic artery
and portal vein at the periphery of lobules and deliver it into central veins.
l Are lined by fenestrated endothelial cells, loosely attached, and hold phagocytic Kupffer cells (larger, stellate, with a pale oval
nucleus), demonstrated by the vital intravascular injection of trypan blue or carbon particles, or latex particles for microscopy in
vivo.
2 Fenestrated lining cells are not tightly attached and rest on microvilli of underlying hepatic cells, without a basal lamina
intervening.
3 Plasma can thus pass through the sieve plate, formed by the lining cells, out into the perisinusoidal space of Disse to interact with
the hepatocytes. Some of this fluid may pass to the periphery of the lobule to be collected as lymph.
4 Disse’s ‘space’ contains ECM materials, but not a visible basal lamina.
5 Scarce, fat-storing, stellate cells of Ito lie outside the endothelial cells. They store vitamin A. They respond to a variety of insults
by making collagen and causing cirrhosis (fibrosis)

Question 21

The sinusoidal wall provides for:

Answer 21

lood cleansing, e.g., of gut
bacterial toxins;
(b) haemopoiesis in the embryo;
(c) bringing plasma into intimate
contact with the hepatic cell for its
many metabolic functions of storage,
transformations, syntheses, regulation
of plasma concentrations,
detoxifications, the production of bile,
and assisting defence by producing
acute-phase proteins

Question 22

Kupffer cells:

Answer 22

phagocytosis and

immunologically active cells

Question 23

Hepatocytes

Answer 23

Hepatocytes are the main functional cells of the liver and perform an
astonishing number of functions. 80% of the mass of the liver is
hepatocytes.
•In three dimensions, hepatocytes are arranged in plates that anastomose
with one another. The cells are polygonal in shape and their sides are in
contact either with sinusoids (sinusoidal face) or neighbouring
hepatocytes (lateral faces).
•A portion of the lateral faces of hepatocytes is modified to form bile
canaliculi. Microvilli are present abundantly on the sinusoidal face and
project sparsely into bile canaliculi.
•Hepatocyte nuclei are distinctly round, with one or two prominent
nucleoli. A majority of cells have a single nucleus, but binucleate cells
are common

Question 24

Hepatocyte/hepatic cell

Answer 24

large, spheroid nucleus (sometimes two), with membrane pores, and ribosomes on the outer
membrane;
• extensive granular ER (protein synthesis for enzymes, plasma proteins, etc.);
• smooth ER (steroid hormone and cholesterol metabolism; lipids are taken in, processed, and
secreted in a way very like the enterocyte’s; SER carries enzymes for detoxifications);
• mitochondria (oxidative and other enzymes);
• actin and other filaments, near the bile canaliculi and elsewhere.
• cell membrane projecting microvilli into the space of Disse, and held firmly to adjacent cells,
especially around the channel, the bile canaliculus, formed by the separation of two or three
cells’ membranes and equipped with a few microvilli;
• Golgi body lying near the canaliculus, as do the
• lysosomes; both appear to help form bile;
• peroxisomes with other enzymes, e.g., catalase;
• glycogen granules stored in association with smooth ER (an association seen elsewhere);
• fat droplets occurring briefly after meals;
• lipofuscin or aging pigment, as another normal inclusion; and sometimes brown
haemosiderin, with its iron, may be seen.

Question 25

Bile pathways

Answer 25

System of canaliculi (seen easily only with EM or special impregnation)
between the hepatic cells leads to Canals of Hering/cholangioles, with both
hepatocytes and bile duct cells in their walls.
Next come, in the portal areas, Bile ductules with only small, cuboidal cells,
firmly held by membrane interdigitations and junctional complexes, and
having a few luminal microvilli.
Bile ducts’ epithelium changes to columnar mucous cells and, extrahepatically,
the ducts acquire smooth muscle as well as Collagen tissue.
Cystic duct allows reflux into the gallbladder, when sphincter of Oddi at the
duodenal outlet of the common bile duct is closed.

Question 26

LIVER LYMPHATIC SYSTEM

Answer 26

Lymph is formed by filtration of plasma into the spaces of Disse as blood flows
through the sinusoids.
• Then lymph percolates between the space of Disse and portal tracts then
lymphatics are formed that run along portal vessels and biliary ducts.
• The exact anatomical correlation between lymphatics and the rest of liver
microanatomy is not entirely clear.

Question 27

liver Metabolism

Answer 27

Glucose metabolism

• Bilirubin metabolism

Question 28

Routine Liver Function Tests (LFTs)

Answer 28

Bilirubin [anion transport] = Assessing
Transport

Aminotransferases
(Alanine ALT and Aspartate AST) = Assessing Hepatocyte
damage

Gamma glutamyl transferase (GGT) = Assessing
Drug alone
(Impaired bile flow)

Alkaline phosphatase (Alk Phos)
(liver but also isoform in bone) = Assessing
(Impaired bile flow)
Cholestasis

Albumin = Assessing
Protein
synthesis

Prothrombin time ratio =
Assessing Protein
synthesis

Question 29

jaundice

Answer 29

associated with the accumulation of bilirubin in the skin most often caused by liver and gall bladder disorders
gall stones, tumours on head of pancreas, ampullary lesions all cause blockage of the bile ducts = cholestasis = jaundice

Question 30

Which organ and what cells produce acid and

what are their functions?

Answer 30

Stomach
• 2.5L of secretions a day
• Hydrochloric acid
• Oesophageal and duodenal
sphincters to prevent acid from
going either way
• Mucus barrier
• Parietal cells → HCl acid
• Neuroendocrine cells:
• Enterochromaffin cells → histamine
• G cells → gastrin
• D cells → somatostatin (inhibits)
• Other cells:
• Mucus cells → mucus (barrier)
• Chief cells → pepsinogen (digestion)

Question 31

What enzymes are involved in digesting food

and where are they produced?

Answer 31

Food digestion
• Amylase → Carbohydrates
• Mouth and pancreas
• Glucose absorbed in stomach and small intestine
(with help of insulin) [very important – read up!]
Proteases → Proteins
• Pepsinogen → pepsin (chief cells)
• Trypsinogen → trypsin (pancreas)
• Activated by enteropeptidase
• Chymotrypsinogen → chymotrypsin (pancreas)
• Activated by trypsin
• Absorption in small intestine
Lipase → Fats
• Pancreatic lipase and pancreatic lipase related
protein 2
• Bile salts emulsify fats and allow lipases to act
• Enterohepatic circulation [read up]

Question 32

B12

Answer 32

iberated from protein binding
by acid and pepsin in stomach
• Binds to R factors (cobalaminbinding proteins)
• Pancreatic proteases release this
complex in duodenum where it
is then bound to intrinsic factor
(produced by gastric parietal
cells)
• IF-B12 complex absorbed in
terminal ileum

Pernicious anaemia (antibodies to
parietal cells)
• Poor dietary intake
• Small bowel Crohn’s disease/postsurgery

Question 33

Folate

Answer 33

Animal products and leafy green
vegetables in polyglutamate
form
• Cleaved to monoglutamate form
in jejunum
• Key role in DNA synthesis and
repair
• Increased use in pregnancy
• Causes of deficiency:
• Poor dietary intake
• Small bowel diseases – coeliac,
Crohn’s, resection
• Drugs – methotrexate (inhibits
dihydrofolate reductase),
trimethoprim, phenytoin

Question 34

Iron

Answer 34

2 forms of iron
• Ferrous (Fe2+) - soluble
• Ferric (Fe3+) - insoluble
• Absorbed in duodenum by
enterocytes
• Transported around the blood by
transferrin
Heme iron:
• Complexed to heme
• Found in meats
• In ferrous (2+) form
• Well absorbed
• Non-heme iron:
• Vegetables, cereals etc.
• Typically in ferric (3+) form
• Not well absorbed

Question 35

Gastric Acid Plays An Important Role

Answer 35

Fe2+ can be absorbed at low or
high pH but patients with
achlorhydria (think PPI) did not
absorb Fe3+
• Gastric acid releases Fe3+
complexes, reduces to Fe2+ form
and promotes formation of
chelates
• Vitamin C can reduce Fe3+ to
Fe2+ promoting absorption