Digestive System 2 (WK7)

What are exocrine glands

Specialised epithelia that produce and secrete various substances

What is the acinus and its morphology

• Secretory portion of the exocrine gland

• Cluster of epithelial cells

• Pyramidal with nuclei situated basally

• Secretory products accumulate apically

• Morphology = alveolar, tubular, tubuloalveolar

What is the duct and its morphology

• Conduit for secreted products

• Epithelium lined > progresses from cuboidal to columnar

• Morphology = simple and compound (branching)

What are the 3 types of secretions

• Serous = watery with enzymes 

• Mucous = viscid, glycoprotein mixture (mucus)

• Mucoserous = combination of both

What are the 3 secretory release mechanisms

• Merocrine = secretory products are membrane-bound and are released via exocytosis

• Holocrine = accumulation of secretory products within cell that is simultaneously undergoing apoptosis

• Apocrine = secretory product accumulate at cell’s apical end and “pinches” off. 

What are the major salivary glands and their histological features

• Parotid (by ear) = entirely serous

• Submandibular (above mandible) = mixed but predominately serous

• Sublingual (below tongue) = mixed but mostly mucous

What are the functions of the liver

• Located in upper right quadrant of abdomen

• Produces bile

• Emulsify lipids

• Metabolises nutrients and detoxifies poisons in blood

• Converts excess blood glucose to glycogen

Explain the features (surfaces and lobes) of the liver

• Surfaces = diaphragmatic (anterior and superior) and visceral (posterior)

• Lobes:

  • Left and right on diaphragmatic

  • Caudate and quadrate on visceral

What is the porta haptis?

• Entry/exit site for portal vein, hepatic artery and common hepatic duct, lymph nodes and autonomic nerves 

• Portal vein = nutrient-rich but deoxygenated blood

• Hepatic artery = brings oxygen-rich blood

• Common hepatic blood = union of left and right hepatic ducts

Explain histology of the liver

• Hexagonal lobules made of:

  • Hepatocytes > cuboidal cells arranged in cords

  • Sinusoids > vascular spaces with discontinuous (fenestrated) epithelium

  • Central vein > delivers blood from the sinusoids to the hepatic vein, then IVC

• Portal areas in stroma:

  • Hepatic arteriole > transports blood from hepatic artery towards the lobule

  • Portal venule > transports blood from portal vein towards the lobule

  • Bile ductule > transports bile away from the lobule 

Explain blood/ bile flow

• Blood = centripetally (toward centre)

  • Mixes within sinusoids and Kupffer cells

  • Central vein > hepatic vein > IVC > heart 

• Bile = centrifugally (out from centre)

  • Produced by hepatocytes

  • Drains via canaliculi surrounding hepatocytes toward bile ductule

Identify the gallbladder and its histological features

• Muscular sac that rests in fossa on visceral surface of liver

• Permits bi-directional flow of bile (stores, absorbs, expels)

• Histology:

  • Mucosa = simple columnar epithelium

  • Muscularis = thin, randomly orientated

  • Serosa = loose connective tissue lined with epithelium

Identify the pancreas and its histological features

• Mixed exocrine and endocrine gland that sits posterior to stomach (retroperitoneal)

• Produces enzymes and hormones (insulin and glucagon)

• Histology:

  • Secretory unit = pancreatic acinus

  • Ducts = intercalated, interlobular and interlobar ducts within connective tissue

Describe formation of the billary tree and its functions

• System of ducts conveying secretions from the liver, gallbladder, pancreas to duodenum

• Includes (top to bottom):

  • Left and right hepatic ducts

  • Common hepatic duct

  • Cystic duct

  • Bile duct

  • Main pancreatic duct

  • Major duodenal papilla

  • Hepatopancreatic ampulla

What is the arterial supply of the digestive system?

• Coeliac trunk = foregut

• Superior mesenteric artery = midgut

• Inferior mesenteric artery = hindgut

Explain coeliac trunk and what it supplies

• Arises from abdominal aorta at T12

• Supplies foregut organs > stomach, part of duodenum, liver, gallbladder, pancreas, spleen

Explain superior mesenteric artery and what it supplies

• Arises from abdominal aorta at LV1

• Branches travel within the mesentery

• Supplies midgut organs > distal part of duodenum, jejunum and ilium, appendix, ascending colon, transverse colon

Explain inferior mesenteric artery and what it supplies

• Arises from the abdominal aorta at LV3

• Supplies hindgut organs > transverse, descending and sigmoid colon, rectum and anal canal

What is the portal system

• Two capillary bed system

• First capillary bed within submucosa of GIT

• Second capillary bed within liver parenchyma

What is the portal vein

• Union of splenic vein and superior mesenteric vein

• Directs nutrient-rich blood from the gut towards the liver

• Enters porta hepatis and divides into portal venules

• After processing, blood leaves the liver via hepatic veins that drain into IVC

What are the anatomical regions of gastric secretions

• Fundus

• Corpus

• Antrum

What are the functional regions of gastric secretions

• Oxyntic > fundus and corpus (supper 80%)

  • Location of acid-secreting cells > parietal cells

• Pyloric > antrum (lower 20%)

  • Location of gastrin-secreting cells

Explain what each types of cells secrete

• Surface mucous and mucous neck cells = mucous

• Parietal cells (oxyntic) = acid (H+) intrinsic factor

• Chief cells = pepsinogen

• Endocrine/ paracrine cells = hormones gastrin, histamine, somatostatin

What is the role of a parietal cell

• Epithelial cells that help break down protein (HCI)

  • Intrinsic factor facilitates absorption of vitamin B12

• Unstimulated cells = tubulovesicles containing H+/K+ ATPase proton pumps

  • Upon stimulation > vesicles fuse with canalicular membrane

  • Proton pumps on surface = HCI secretion

What are the 3 hormones that stimulate acid secretion

• Acetylcholine

• Gastrin

• Histamine

• Stimulate > increase Ca+ levels > vesicular fusion > HCI

What are G cells and ECL cells

• G cells = located primarily in antrum of stomach

  • Secrete gastrin into systemic circulation > delivery to parietal and ECL cells

• ECL cells = located close to parietal cells

  • Secrete histamine which acts on parietal cells

What are D cells

• Neuroendocrine cells in the antrum of the stomach

• Produce somatostatin in response to low gastric pH

• Inhibits gastric acid secretion

  • Directly and indirectly

Which 3 hormones inhibit gastric acid production

• Somatostatin (D cells) > inhibits G, ECL and parietal cells

• Secretin (S cells) > inhibits G and pareital cells

• Cholecystokinin (I cells) > inhibits gastric emptying

What are the 3 phases of digestion

1. Cephalic

2. Gastric

3. Intestinal

What are the components and functions of pancreatic juice

• Components = watery alkaline secretion high in HCO3

  • Acts to neutralise acidic stomach contents as it enters small intestine

• Functions = 2 key ones

  • Production of enzymes to breakdown food (exocrine)

  • Production of hormones to aid digestion (endocrine)

Explain where bile is produced, stored and secreted

• Contains bile salts, bile pigments and other substances in an alkaline electrolyte solution

• Secreted by the liver (hepatic bile)

• Stored and concentrated in gallbladder until needed in duodenum