Pancreatic Juice
- The exocrine pancreas secretes about 1 L of fluid per day into the duodenum.
- This fluid consists of two components:
- An aqueous component rich in HCO3-, which neutralizes the acidic chyme from the stomach.
- An enzymatic component crucial for digesting carbohydrates, proteins, and lipids.
- Acinar cells produce the enzymatic component, including:
- Pancreatic amylase (digests carbohydrates)
- Pancreatic lipases (digest lipids)
- Pancreatic proteases, initially secreted in inactive forms and later activated in the duodenum. For instance, trypsinogen is converted to the active enzyme trypsin.
- Centroacinar and ductal cells produce the aqueous component, which is isotonic and initially contains Na+, Cl−, K+, and HCO3−.
- Ductal cells modify the initial secretion through transport processes that result in net secretion of HCO3− into
pancreatic juice and net absorption of H+, acidifying pancreatic venous blood.
- Notably, the concentrations of Cl− and HCO3− in pancreatic juice vary inversely with flow rate.
- Regulation of pancreatic secretion occurs in three phases:
- Cephalic phase: Triggered by sensory stimuli (smell, taste, conditioning), mediated by the vagus nerve, and primarily stimulates enzymatic secretion.
- Gastric phase: Initiated by stomach distention, also mediated by the vagus nerve, and mainly stimulates enzymatic secretion.
- Intestinal phase: Accounts for 80% of pancreatic secretion, stimulated by the presence of small peptides, amino acids, and fatty acids in the duodenum.
- Acinar cells are stimulated by cholecystokinin (CCK) (released by I-cells in response to the presence of peptides, amino acids, and fatty acids) and acetylcholine (ACh) from vagovagal reflexes.
- Ductal cells are primarily stimulated by secretin (released by S-cells in response to H+ in the duodenum), which promotes the secretion of the HCO3−-rich aqueous component. CCK and ACh potentiate the effects of secretin.
- Fat in the ileum signals the end of the intestinal phase and inhibits pancreatic secretion through the release of peptide YY and somatostatin.
- Neural regulation also plays a role:
- Parasympathetic activity (vagus nerve) stimulates pancreatic secretion.
- Sympathetic activity inhibits pancreatic secretion.
Pancreas is divided into exocrine pancreas (90%) and endocrine pancreas (2%).
Recap Questions:
- What are the two main components of pancreatic juice, and what are their functions?
- Which cells produce each component?
- Describe the three phases of pancreatic secretion regulation.
- What are the key hormonal and neural regulators of pancreatic secretion?