absorption

• Products of chemical digestion and other substances (vitamins, minerals, water) are absorbed through internal surface of small intestine into the blood
• a large surface area is needed for efficient absorption, achieved through :

  • Small intestine is very long
  • Mucosa has folds that extend into interior
  • Mucosa has projections (villi) extending from folds
  • Cells covering villi have microscopic projections from their internal surfaces (microvilli)
• Villus is suited to its function of nutrient absorption because :

  • 1mm long, but longer in jejunum than duodenum and ileum 
  • Covered by single layer of cells
  • Inside is lacteal surrounded by blood capillaries 
  • Continually moved by muscles, constantly bringing villi into contact with different parts of intestinal content which is also constantly changing
• Absorption occurs through simple diffusion or active transport, either going along or against the concentration gradient
• Simple sugars, amino acids, water, water-soluble vitamins are absorbed into blood capillaries through walls of the villi. they are carried by hepatic portal vein to liver where they are either removed for further processing or remain in blood to be carried to other body cells
• Fatty acids, glycerol recombine in cells of villi to form fats, fat-soluble vitamins enter lacteals where they are transported in the lymph system and eventually emptied into the blood through veins in the upper chest
• Amino acids are absorbed by active transport into the blood capillaries.
• Simple sugars such as glucose are absorbed by active transport. They pass through the cells on the outside of the villi and into the blood capillaries.
• Fatty acids and glycerol are absorbed by simple diffusion.
• Water and water-soluble vitamins are absorbed into the blood capillaries by diffusion.