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Small Intestine Physiology: Absorbed Nutrients & Transport Mecha

Small Intestine Physiology: Absorbed Nutrients & Transport Mecha

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Absorbed Nutrients & Transport Mechanisms
Summary
Absorption
  • Uptake of digested nutrients and water from the lumen of the digestive tract into the bloodstream and lymphatic vessels.
Small intestine (specifically the duodenum)
  • Major site of nutrient absorption.
Three folded mucosal structures maximize surface area for absorption* 1 - Plicae circulares: wavy, folds on the inner walls of small intestine ? form circular folds ? increase surface area 3-fold. 2 - Villi: finger-like projections that protrude from the plicae circulares ? surface area by 10-fold. Arterioles, venules, and lymphatic vessels pass through the villi and uptake absorbed nutrients. 3 - Microvilli (brush border): hair-like projections on columnar small intestine epithelial cells (face the lumen of the small intestine) ? increase surface are 20-fold. Together, all folded layers = 600-fold surface area increase
Nutrients are absorbed in the small intestine
  • Monosaccharides (digested carbohydrate products)
  • Amino acids, di-peptides and tri-peptides (digested protein products)
  • Intact proteins
  • Short-chain fatty acids, long-chain fatty acids, and glycerol (which are digested lipid products)
  • Vitamins
  • Water and electrolytes
Nutrients cross the apical and basolateral surfaces of the intestinal epithelium for absorption into circulation or the lymphatic system.
  • Apical surface: interfaces the intestinal lumen and epithelium.
  • Basolateral surface: opposite to the apical surface, lines the inside of the villi.
  • Capillaries and lacteals inside villi
– Most nutrients cross the basolateral surface and pass directly into circulation.
  • Fats, however, pass directly into lacteals (lymphatic system).
Key Transport Mechanisms
  • Apical Surface
– Secondary active transport: transporter moves an ion movement down its concentration gradient, which generates energy for it to move another ion (or molecule) against its concentration gradient. – Facilitated diffusion: Transporter passively moves an ion or molecule across the plasma membrane, down its concentration gradient. – Simple diffusion: in which non-charged, lipid, and hydrophobic molecules passively cross through the plasma membrane (without a transmembrane protein) down their concentration gradient. – Endocytosis: form of active, energy-requiring cellular ingestion, which transports large substances into the cell.
  • Basolateral Surface
– Facilitated diffusion. – Simple diffusion. – Exocytosis: the opposite of endocytosis – a vacuole actively fuses with the plasma membrane to release its contents into the extracellular environment.

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