Rat Small Intestine Physiology: Nutrient Digestion, Absorption, Hormone Secretion, and Immunity

The rat small intestine comprises duodenum, jejunum, and ileum, performing functions analogous to humans with quantitative differences.

1. Digestion & Absorption

• Duodenum: Chyme mixes with pancreatic enzymes (amylase, lipase, proteases) and bile; major site for iron, calcium, folate absorption.
• Jejunum: Primary site for carbohydrate, protein, fat, water-soluble vitamin absorption. Villi tall, dense; microvilli express SGLT1, PEPT1, CD36.
• Ileum: Bile acid reabsorption (ASBT), vitamin B12-intrinsic factor complex, residual nutrients.

2. Nutrient Transport

• Capillaries in villi → portal vein → liver (first pass).
• Lymphatic lacteals → chylomicrons → thoracic duct → systemic circulation.
• Active transport (Na⁺-coupled), facilitated diffusion, paracellular.

3. Hormone Secretion

• Gastrin (G cells, antrum/duodenum): stimulates gastric acid.
• CCK (I cells, duodenum/jejunum): gallbladder contraction, pancreatic secretion, satiety.
• Secretin (S cells): pancreatic bicarbonate.
• GLP-1, GIP, PYY (L/K cells): incretin effect, appetite regulation.

4. Barrier & Immunity

• Tight junctions, mucus layer, antimicrobial peptides (defensins).
• GALT: Peyer’s patches (ileum), isolated lymphoid follicles, intraepithelial lymphocytes.
• Oral tolerance induction.

5. Fluid/Electrolyte Balance

• Absorbs ~90% of intestinal water; Na⁺/H⁺ exchanger (NHE3), Cl⁻/HCO₃⁻ exchanger.
• Diarrhea = failed absorption or active secretion (e.g., cholera toxin → CFTR).

Rat models are foundational for translational GI research.