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Intestinal Absorption Details

Intestinal Absorption Details

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Intestinal Absorption Details
Summary
  • Uptake of nutrients and water from the lumen of the digestive tract into the bloodstream and lymphatic vessels.
  • Small intestine (specifically the duodenum) is the major site of nutrient absorption.
  • Transport occurs on both the apical and basolateral surfaces of the intestinal epithelium for absorption into the bloodstream or lymphatic system.
– Each nutrient utilizes a specific transport mechanism across either surface.
Small Intestine
Monosaccharides -
  • Glucose and Galactose
– Apical: secondary active transport with Na+ – Basolateral: facilitated diffusion
  • Fructose
– Apical: facilitated diffusion – Basolateral: facilitated diffusion
Proteins and their digested products
  • Free Amino Acids
– Apical: secondary active transport with Na+ – Basolateral: facilitated diffusion
  • Di- and Tripeptides
– Apical: secondary active transport with hydrogen exchange. – Basolateral: facilitated diffusion
  • Intact proteins
– Apical: endocytosis – Basolateral: exocytosis
Fats
  • Short-chain fatty acids:
– Apical and basolateral: simple diffusion
  • Long-chain fatty acids and glycerol:
– Apical: simple diffusion
  • Long-chain fatty acids and glycerol form chylomicron in small intestine epithelial cell.
– Basolateral: facilitated diffusion
Vitamins
– Absorbed with fats: dissolved in fat droplets, micelles, and chylomicrons.
  • Water-soluble vitamins:
– Simple or facilitated diffusion
  • Vitamin B-12: Must be bound to intrinsic factor (IF) for B-12 absorption in the ileum
– Apical: endocytosis – Basolateral: exocytosis
Note: Vitamin B-12 Must be bound to intrinsic factor (IF) for its absorption in the ileum: Intrinsic Factor is secreted in the duodenum where binds with vitamin-B12.
Small and Large Intestine
Water and electrolytes: Remember that water absorption is most relevant to the function of the large intestine for efficient feces concentration prior to elimination.
Three general steps involving water and electrolyte absorption.
  • Step 1
– Active sodium absorption across the basolateral surface allows facilitated diffusion across the apical surface and creates a net positive charge in the cell
  • Step 2
– Facilitated chloride diffusion across apical membrane down the electrical gradient.
  • Step 3
– Water is absorbed via osmosis.
Large Intestine (mainly absorbs water (and also bacterial products))
  • Fiber
– Converted to short chain fatty acids and absorbed via simple diffusion.
  • Vitamins B and K
– Dissolved in water for absorption.
Clinical Correlation: Roux-en-Y Procedure
  • Commonly used as a weight-loss (bariatric) surgery: it reduces the stomach size and amount of nutrient absorption.
– Creates a small gastric pouch that comprises only the fundus and bypasses rest of stomach (the body and antrum) and the duodenum. – Jejunum is attached to the gastric fundus. – Duodenum attached further down along the jejunum; this allows digestive juices from the stomach and pancreas as well as bile from the gallbladder release into the intestine. – "Roux Limb" = portion of the jejunum between two attachments.
  • Following surgery, food passes: esophagus ? gastric pouch ? Roux Limb ? through Roux Limb ? remaining jejunum.
– Because the duodenum is bypassed, these patients lack the absorptive and secretive properties of the duodenum. – Patients supplement diets with: iron, vitamin B12, and other vitamins and mineral supplements (no longer absorbed) – Also: no longer secrete intrinsic factor from bypassed duodenum ? cannot bind to vitamin B-12 in bypassed duodenum ? unable to absorb vitamin B12 in ileum.
  • Physicians prescribe vitamin B-12 supplements (nasal spray, sublingual drops, or tablets) ? ensures B-12 directly absorbed into circulation.

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