Secretions of the intestine, liver, gallbladder and pancreas Flashcards
What is the role of the small intestine?
Governs the majority of chemical digestion and absorption of nutrients, electrolytes and water
What are the primary secretions of the intestine?
- intestinal juice (mucus/HCO3-)
- pancreatic juice ( digestive enzymes)
- bile (bile salts)
How are bile and pancreatic secretions regulated?
Key endocrine hormones secretin, cholecystokinin (CCK), glucose dependent insulinotrophic peptide * (GIP) regulate bile and pancreatic secretions
What are the secretory cells of the small intestine?
Villi: absorptive enterocytes and mucus secreting goblet cells
Intestinal glands: Enterocytes secreting isotonic fluid, entero-endocrine cells, paneth cells
In the duodenum only: Brunner’s glands secrete mucus and HCO3-
How doe stem cell renewal of epithelial cells work?
Derive from daughter cells at bottom of crypt
Start at bottom and move up during differentiation
What is the turnover of epithelial cells and what is the consequence of this?
- rapid turnover of epithelial cells every 3-6 days
- vulnerable to radiation, chemotherapy
Summarise the secretions of the small intestine
- Intestinal juice (1.2 L):
- fluid containing electrolytes and water (secretory enterocytes), lysozyme (Paneth cells), mucus (goblet cells), alkaline mucus containing fluid (submucosal duodenal Brunner’s glands)
- Key endocrine hormone secretion (by enteroendocrine cells) into vasculature
- CCK (I cells)– stimulate pancreatic and gallbladder secretion
- Secretin (S cells)–stimulate pancreatic and biliary bicarbonate secretion
- GIP (K cells) – may inhibit acid secretion/ stimulate insulin release
- Exocrine pancreatic juice (1.5 L): bicarbonate / digestive enzymes
- Bile (0.5-1 L) : bile salts for lipid emulsification (liver hepatocyte synthesis, gall bladder storage)
What does the pancreas secrete?
•Exocrine pancreas secretes pancreatic juice containing bicarbonate rich secretion (pH 8) and digestive enzymes essential for normal digestion and absorption
What is the structure of the pancreas?
•The pancreas consists of glandular epithelial clusters
What are the 2 types of glandular epithelial clusters found in the pancreas?
- 99% exocrine acinar clusters secreting pancreatic juice (water, electrolytes, sodium bicarbonate and pro-enzymes)
- 1% endocrine pancreatic islets (Islets of Langerhans) of 4 types secreting glucagon (alpha), insulin (beta), somatostatin (delta), pancreatic polypeptide (F cell)
Complete the diagram on the exocrine acinar cluster

Complete the diagram on acinar and ductular secretion

What is acinar and ductular secretions made of?
Ductular - NaHCO3, fluid
Acinar - enzymes, NaCl, fluid
How is acinar enzyme production regulated?
- Acetylcholine released via P/S vagus stimulation
- CCK – trigger is chyme containing fat and protein products
- Produces lower volume enzyme rich pancreatic juice
How is ductal bicarbonate and water secretions regulated?
- Secretin - trigger is H+ in highly acidic chyme
- Produces copious, HCO3- rich, low enzyme pancreatic juice
What are the 5 pancreatic enzymes?
- Proteolytic enzymes secreted in inactive form, convert proteins to peptides
- Amylase hydrolyses starch, glycogen and other carbohydrates other than cellulose to form di and trisaccharides
- Lipases hydrolyse fat into fatty acids and monogylcerides
- Nucleases digest RNA and DNA to nucleic acids
- Trypsin inhibitor prevents activation of trypsin to prevent pancreatic digestion
How are proteolytic enzymes activated?
- Proteolytic enzymes are produced as inactive precursors called zymogens
- Small intestinal brush border enterokinase enzyme cleaves hexapeptide to form active trypsin from trypsinogen
- Trypsin cleaves and activates other proteolytic enzymes
Why are proteolytic enzymes produced in the inactive form?
•Process prevents pancreatic autodigestion (+trypsin inhibitor)
What does secretin stimulate?
•Secretin stimulates high volume HCO3− rich pancreatic juice
Which cells are involved in duct secretion of sodium bicarbonate?
•HCO3− secretion out of cell into the duct lumen is via Cl−/HCO3− exchange at the apical cell membrane
How does duct secretion of sodium bicarbonate occur?
- Cl− is recycled out of the cell via the cystic fibrosis transmembrane conductance regulator (CFTR) Cl− channel under secretin stimulation via cAMP
- Na+ is secreted transcellularly into the duct lumen following HCO3− secretion down electrochemical gradient, water follows by osmosis
What does this diagram show?

•HCO3− secretion out of cell into the duct lumen is via Cl−/HCO3− exchange at the apical cell membrane
Complete the diagram with which ions each line represents

How does ionic composition of pancreatic juice depends on secretory rate when it is unstimulated and stimulated?
- Unstimulated
- Low secretion rate - electrolyte content is similar to that of plasma
- Stimulated
Higher secretion rate and rise in HCO3- from ductal cells inversely related to reduced concentration of Cl- in pancreatic juice

