physiology notes

• Absorption is the movement of the products of digestion from the lumen of GIT into the blood or lymph
• Structural arrangements in the small intestine that maximize absorption include:

  • Circular folds
  • Villi and crypts
  • Microvilli
• Circular folds in the duodenal mucosa increase the surface area of the small intestine by a factor of 3
• Villi in the small intestine increase the absorptive surface area by a factor of 10
• Microvilli on enterocytes increase the absorptive surface area by a factor of 20
• Three structural features increase the absorptive area of the small intestine by as much as 600-fold
• Absorption of nutrients is maximal in the duodenum and declines towards the ileum
• Two pathways for absorption of nutrients in the small intestine:

  • Paracellular pathway
  • Transcellular pathway
• Passive transport of ions and water occurs along the gradient and does not require energy
• Active transport of ions and water occurs against the gradient and requires energy (ATP)
• Primary active transport in the small intestine involves:

  • Transport of Na+ from enterocyte to interstitial fluid
  • Absorption of proteins via transcytosis in infants
  • Exocytosis of chylomicrons from enterocyte to the lymphatic capillary
• Secondary active transport in the small intestine involves:

  • Co-transport with Na+ for absorption of glucose, galactose, amino acids, and water-soluble vitamins
  • Co-transport with H+ for absorption of short peptides
  • Co-transport with HCO3- for absorption of Cl-
• Absorption of monosaccharides in the small intestine:

  • Fructose passes through the luminal membrane via facilitated diffusion using the GLUT-5 transporter
  • Glucose and galactose pass through the luminal membrane via secondary active transport coupled to the transport of Na+ using the sodium-glucose transporter (SGLT)
  • Monosaccharides move through the basolateral membrane via facilitated diffusion using the GLUT-2 transporter
• Absorption of proteins in the small intestine occurs in two forms:

  • Amino acids
  • Short peptide molecules (dipeptides, tripeptides)
• Absorption of amino acids, dipeptides, and tripeptides in the small intestine involves:

  • Absorption at the luminal membrane of enterocyte via secondary active transport
  • Amino acids are coupled to Na+ ions for absorption
  • Dipeptides and tripeptides are coupled to H+ ions
  • Short peptides are hydrolyzed by intracellular peptidases to single amino acids before crossing the basolateral membrane via facilitated diffusion
• Transcytosis of proteins in infants allows the transfer of antibodies from mother to child during breastfeeding
• Absorption of fat in the small intestine occurs in four steps:

  • Carriage of fatty acids and glycerol by the micelle
  • Crossing the luminal membrane of enterocyte via simple diffusion
  • Resynthesis of fatty acids and glycerol to triglycerides (chylomicrons)
  • Exocytosis of chylomicrons across the basolateral membrane of enterocyte into the lymphatic capillary
• Chylomicrons are released to the lymphatic capillary via exocytosis (primary active transport)
• Fatty acids and glycerol inside the intestinal cell are recombined to form triglycerides
• Triglycerides aggregate into globules and become coated with proteins (lipoproteins)
• Protein-coated lipid globules are called chylomicrons
• Chylomicrons leave the enterocyte via exocytosis
• Chylomicrons enter the lymphatic capillaries (lacteals) due to their large size and the larger pores in the lymphatic capillaries
• Fat-soluble vitamins (A, D, E, K) are processed like dietary lipids
• In the lumen of the small intestine, fat-soluble vitamins are incorporated into micelles and transported to the luminal membrane of enterocytes
• Fat-soluble vitamins cross the luminal membrane of enterocytes via simple diffusion
• Inside the enterocyte, fat-soluble vitamins are incorporated into chylomicrons, which cross the basolateral membrane via exocytosis and enter the lymph
• Water-soluble vitamins include B1, B2, B6, B12, vitamin C, biotin, folic acid, nicotinic acid, and pantothenic acid
• Vitamin B6 is absorbed via simple diffusion
• Vitamin B12 is absorbed via facilitated diffusion with the help of intrinsic factor secreted in the stomach
• Other water-soluble vitamins are absorbed via secondary active transport involving Na+-dependent transporters
• Absorption of calcium is largest in the duodenum and progressively declines across the rest of the small intestine
• Calcium absorption in the enterocyte involves both transcellular and paracellular pathways in the duodenum, while in the jejunum and ileum, it involves the paracellular pathway only
• Transcellular absorption of calcium involves facilitated diffusion at the luminal membrane, binding to calbindin, transport to the basolateral membrane, and absorption into the blood via secondary active transport coupled with Na+ or H+
• Iron is absorbed in the duodenum only through the transcellular pathway
• A portion of ingested iron is absorbed by enterocytes from heme and ferrous iron (Fe2+)
• In the enterocyte, absorbed iron is divided into two pools, one leaving the cell via facilitated diffusion involving ferroportin, and the other becoming bound to ferritin and stored within the cell
• Water absorption in the small intestine occurs via osmosis depending on the absorption of Na+
• Na+, K+, Cl-, and HCO3- are mostly absorbed in the jejunum (Na+, K+, Cl-) and ileum (HCO3-)
• Na+ absorption occurs via two pathways: simple diffusion and transcellular pathway involving Na+-glucose, Na+-amino acid, and Na+-H+ co-transporters