What are the normal secretions of the stomach and small intestine?

Normal Secretions of the Stomach and Small Intestine

The stomach and small intestine produce essential secretions that facilitate digestion and absorption of nutrients, with the stomach primarily secreting gastric juice containing acid and enzymes while the small intestine produces alkaline secretions that neutralize stomach acid and contain digestive enzymes crucial for nutrient breakdown and absorption.

Gastric Secretions

• Daily gastric secretions amount to approximately 2.0 liters of gastric juice 1
• Gastric juice contains hydrochloric acid produced by parietal cells through a process involving intracellular formation in tubulovesicles and subsequent secretion 2
• Gastric acid creates a highly acidic environment (low pH) that initiates the digestive process, particularly protein digestion 3
• Pepsin, secreted as pepsinogen by chief cells, is activated in the acidic environment and begins protein breakdown 1
• Gastric lipase contributes to initial fat digestion, which becomes more important during pathological conditions such as pancreatitis 1
• The stomach regulates the delivery of acidic chyme into the duodenum at a rate that optimizes digestion and absorption in the small intestine 3

Small Intestinal Secretions

Duodenal Secretions

• The duodenum receives approximately 1.5 liters of pancreatico-biliary secretions daily 1
• Pancreatic secretions contain:
  • Bicarbonate to neutralize gastric acid 1
  • Digestive enzymes including amylase (for carbohydrates), lipase and colipase (for fats), and proteases such as trypsin, chymotrypsin, and carboxypeptidases (for proteins) 1
• Bile from the liver enters the duodenum and contains:
  • Bile salts essential for fat emulsification and absorption 1
  • Bilirubin and other waste products 4

Jejunal and Ileal Secretions

• Jejunal mucosa produces passive secretions to render the lumen isotonic during passage and digestion of nutrients 1
• Intestinal epithelial cells secrete water and electrolytes to maintain appropriate luminal environment 4
• Brush border enzymes are produced by enterocytes and include:
  • Disaccharidases (maltase, sucrase, lactase) for final carbohydrate digestion 1
  • Peptidases for final protein digestion 1
• Intestinal secretions contain protective mucus and antimicrobial compounds 4

Regulation of Digestive Secretions

• Secretions are regulated through three phases: cephalic, gastric, and intestinal 1
• The cephalic phase is vagally mediated and contributes up to 40% of the overall pancreatic response 1
• The intestinal phase is the most important mechanism for pancreatic secretion, triggered by the presence of nutrients in the duodenum 1
• Cholecystokinin (CCK) plays a crucial role in stimulating pancreatic enzyme secretion, acting both as a hormone and as a neuromodulator of cholinergic pathways 1
• Inhibitory mechanisms involving somatostatin, pancreatic polypeptide (PP), peptide YY (PYY), and glucagon-like peptide-1 (GLP-1) help terminate secretory responses 1
• In the fasting state, secretions occur in concert with the migrating motor complexes (MMC), with minimal secretion during phase I and moderate outputs during phase II 1

Coordination of Secretions with Motility

• Gastrointestinal motility and secretions are tightly coordinated to optimize digestion and absorption 1
• After a meal, motility patterns change from an interdigestive state to a fed pattern, which coordinates with increased secretory activity 1
• This coordination ensures maximal nutrient assimilation in the proximal small intestine and economizes digestive secretions 1
• The coupling of motor and secretory events maintains intraluminal homeostasis, which is particularly important for fat digestion 1
• Disruption of this coordination, as seen in enteric neuropathies, can lead to symptoms such as abdominal pain after eating 1

Clinical Significance

• Inadequate digestive secretions can lead to malabsorption of nutrients 4
• Pancreatic lipase is particularly vulnerable to acid denaturation in the duodenum and destruction by pancreatic proteases during small intestinal transit, making lipid digestion susceptible to pathologic conditions 1
• Impaired migrating motor complexes can lead to bacterial overgrowth due to inadequate clearance of intestinal contents 1
• Understanding normal secretory functions is essential for managing conditions such as short bowel syndrome, where adaptation of remaining bowel segments is crucial 1, 5