nutrients in si

• Absorption of nutrients, water, and electrolytes in the small intestine

  • Structural and physiological basis for the absorption of nutrients
  • Mechanisms that govern absorption of carbohydrates, proteins, and fat
  • Mechanisms that govern absorption of vitamins
  • Mechanisms that govern absorption of calcium and iron
  • Mechanisms that govern absorption of water and electrolytes
• 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 allow to maximize the amount of absorption

  • Circular folds
  • Villi and crypts
  • Microvilli

  
• Circular folds in the wall of the small intestine increase its surface area by a factor of 3

  
• Villi and crypts increase the absorptive surface area by a factor of 10
• Microvilli increase the absorptive surface area by a factor of 20
• Three structural features that increase the absorptive area of the small intestine by as much as 600-fold

  
• Absorptive cell


  Present in the villi of the small intestine

  
• Molecules suitable for absorption

  Small enough to pass through the mucosal lining of the small intestine

  
• Molecules suitable for absorption

  • Monosaccharides (glucose, fructose, galactose)
  • Fatty acids and glycerol
  • Amino acids and short oligomers (dipeptides and tripeptides)
• Absorption of all nutrients is maximal in duodenum, and declines towards ileum
• Villi are the longest in the duodenum, and are the shortest in the ileum
• Paracellular pathway


  Absorption of ions and water
• Transcellular pathway


  Absorption of products of digestion of nutrients

  
• Passive transport


  Along the gradient, no energy required
• Active transport


  Against the gradient, requires energy (ATP)
• Simple diffusion


  1. Transcellular pathway: fatty acids and glycerol; fat-soluble vitamins; vitamin B6

  2. Paracellular pathway: water and ions
• Facilitated diffusion


  1. Channel-mediated (Ca2+)

  2. Carrier-mediated (Na+; iron; fructose; vitamin B12)
• Primary active transport


  1. Na+-K+ pump at the basolateral membrane of enterocyte

  2. Transport of Na+ from enterocyte to interstitial fluid

  3. Absorption of proteins via transcytosis in infants

  4. Exocytosis of chylomicrons from the enterocyte to the lymphatic capillary

  
• Secondary active transport


  1. Co-transport with Na+ (absorption of glucose, galactose, amino acids, and water-soluble vitamins)

  2. Co-transport with H+ (absorption of short peptides)

  3. Co-transport with HCO3- (absorption of Cl-)
• Absorption of monosaccharides


  1. Fructose via facilitated diffusion (GLUT-5 transporter)

  2. Glucose and galactose via secondary active transport coupled to Na+ (SGLT transporter)

  3. From cytosol to capillaries via facilitated diffusion (GLUT-2 transporter)
• Absorption of amino acids and short peptides

  1. Amino acids via secondary active transport coupled to Na+

  2. Dipeptides and tripeptides via secondary active transport coupled to H+

  3. Short peptides hydrolysed to amino acids in cytosol, then cross basolateral membrane via facilitated diffusion
• In infants, proteins can be transported across the enterocytes via transcytosis (primary active transport)
• Absorption of fat


  1. Micelle carries fatty acids and glycerol to absorptive surface

  2. Fatty acids and glycerol cross luminal membrane by simple diffusion

  3. In enterocyte, fatty acids and glycerol are resynthesized to triglycerides (chylomicrons)

  4. Chylomicrons cross basolateral membrane by exocytosis (primary active transport) and enter lymphatic capillary
• Chylomicrons


  Protein-coated lipid globules formed in enterocyte from resynthesized triglycerides
• Step 1


  Micelles transport lipids (triglycerides, fatty acids, and fat-soluble vitamins) through the watery layer covering the absorptive surface of the intestine

  
• Step 2


  Fatty acids and glycerol cross the luminal membrane of enterocyte via simple diffusion
• Step 3


  In the enterocyte, fatty acids and glycerol are converted to chylomicrons
• Step 4


  Chylomicrons are released to the lymphatic capillary via exocytosis (primary active transport)
• Chylomicrons


  • Once inside the intestinal cell, the fatty acids and glycerol are recombined to form triglycerides
  • Triglycerides aggregate into globules and become coated with proteins (lipoproteins)
  • The protein-coated lipid globules are called chylomicrons
  • Chylomicrons leave the enterocyte via exocytosis
  • As the chylomicrons are large complexes, they cannot enter blood capillaries, as the pores in their wall are too small. Instead, the chylomicrons enter the lymphatic capillaries (lacteals), which have much larger pores

  
• Summary of absorption of nutrients in the small intestine

  
• Absorption of vitamins


  • All vitamins are mostly absorbed in jejunum and ileum
• Absorption of fat-soluble vitamins

  • Fat-soluble vitamins (A, D, E, K) are processed just like the dietary lipids
  • In the lumen of small intestine, they are incorporated into micelles, and transported to the luminal membrane of the enterocytes
  • After dissociating from the micelle, the fat-soluble vitamins cross the luminal membrane of enterocyte via simple diffusion
  • Inside the enterocyte, fat-soluble vitamins are incorporated into the chylomicrons, which cross the basolateral membrane via exocytosis, and get into the lymph
• Absorption of water-soluble vitamins

  • Vitamin B6 is absorbed via simple diffusion
  • Vitamin B12 is absorbed via facilitated diffusion. The carriage across the membrane of the enterocyte is accomplished by the intrinsic factor produced in the stomach (parietal cells)
  • Other water-soluble vitamins are absorbed via secondary active transport that involves Na+-dependent transporters
• Absorption of Ca2+


  1. In duodenum, Ca2+ absorption involves both transcellular and paracellular pathway

  2. In jejunum and ileum, it involves paracellular pathway only

  3. At the luminal membrane of enterocyte, Ca2+ enters the cell via facilitated diffusion (channel-mediated)

  4. Upon entering the cell, Ca2+ binds to calbindin, a cytosolic protein which transports Ca2+ to the basolateral membrane

  5. At the basolateral membrane of enterocyte, Ca2+ is absorbed into the blood via the secondary active transport coupled with Na+ or H+ (Na+-Ca2+ exchanger, or H+-Ca2+ exchanger)

  6. Synthesis of calbindin as well as Ca2+ transporters in the enterocyte is stimulated by calcitriol
• Absorption of Fe2+


  1. At the apical membrane, Fe2+ and heme enter the cell via facilitated diffusion, which involves two separate transporters

  2. In the cell, an enzyme heme oxygenase releases Fe2+ from the heme

  3. In the enterocyte, the absorbed Fe2+ is divided into two pools. The first pool is leaving the cell at the basolateral membrane via facilitated diffusion that involves ferroportin. In the blood, the absorbed Fe2+ is converted to Fe3+, which then binds to transferrin, a protein which transports Fe3+ to the bone marrow (to form blood cells), or to the liver for storage. The second pool of Fe2+ in the enterocyte becomes bound to ferritin and stored within the cell
• Absorption of water


  1. Water absorption in GIT occurs via osmosis, i.e. it depends on absorption of Na+

  2. Water follows Na+ via osmotic gradient, both at the paracellular and transcellular pathways of absorption
• Absorption of Na+, K+, Cl-, and HCO3-

  1. Mostly absorbed in jejunum (Na+, K+, Cl-) and ileum (HCO3-)

  2. Na+ entry into the cell is facilitated by three types of transporters present at the luminal membrane (Na+-glucose, Na+-amino acid, and Na+-H+ co-transportters)

  3. K+ is absorbed passively via the paracellular pathway, pulled along by a bulk of water (solvent drag)

  4. Bicarbonate is absorbed in jejunum. It is not moved to enterocyte from the intestinal lumen. Instead, bicarbonate is generated in the cytosol of enterocyte through the action of carbonic anhydrase, and subsequently extruded from the enterocyte into the blood via facilitated diffusion across the basolateral membrane

  5. Chloride is mostly absorbed in ileum. As bicarbonate accumulates in enterocyte due to the action of carbonic anhydrase, this forms an outward concentration gradient for HCO3- (from enterocyte to the intestinal lumen). The energy of this gradient is used to move Cl- across the luminal membrane (secondary active transport). Cl- is then extruded from the enterocyte into the blood via facilitated diffusion that involves a Cl- transporter present at the basolateral membrane of the cell
• After this lecture you are able to: