GI Flashcards
The wall of the GI tract is formed of 4 layers:
•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
Oesophagus
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
Stomach
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
Gastric mucosa
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)
Functions of the Stomach
• Secretes intrinsic factor: allows b12
absorption
• Some absorption: water, alcohol, some drugs
• Endocrine: ghrelin and gastrin secretion
Duodenum
Biliary tract enters
GI tract here
Jejunum and Ileum
Ileum ends at the
ileo-caecal valve in
the RIF
Small intestinal mucosa
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
Colon/large intestine
Caecum • Colon – Ascending – Transverse – Descending – Sigmoid • Rectum • Anal canal
Rectum
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)
Wall of the colon
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
Appendix
May have a role in
gut microflora
Peritoneum
– Viseral – lines the organs, is their serosa – Parietal – lines the walls of the abdo cavity
Gallbladder
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
Pancreas
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)
Parenchymal cells are supported by fine reticular fibres
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. `
Types of liver vessels
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)
Hepatic lobular blood flow is:
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.
LIVER ACINUS
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.
Inside the liver lobule: Liver sinusoids
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)
The sinusoidal wall provides for:
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
Kupffer cells:
phagocytosis and
immunologically active cells
Hepatocytes
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
Hepatocyte/hepatic cell
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.
Bile pathways
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.
LIVER LYMPHATIC SYSTEM
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.
liver Metabolism
Glucose metabolism
• Bilirubin metabolism
Routine Liver Function Tests (LFTs)
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
jaundice
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
Which organ and what cells produce acid and
what are their functions?
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)
What enzymes are involved in digesting food
and where are they produced?
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]
B12
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
Folate
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
Iron
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
Gastric Acid Plays An Important Role
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
