9.2.7 The Small Intestine

• Small intestine


  Divided into 3 parts: Duodenum, jejunum, ileum

  
• Duodenum


  • Where digestive enzymes are added from the pancreas and where bile enters from the gallbladder
• Ileum


  • Where absorption of food and water occurs
• Most water is absorbed in the small intestine (and the rest is absorbed in the colon)
• Trypsin


  A protease that breaks down proteins to amino acids
• Lipase


  Breaks down lipids to fatty acids and glycerol
• Maltase


  A carbohydrase that breaks down maltose into glucose
• Maltase is found on the membranes of the epithelium lining the small intestine
• Absorption in the ileum


  • Small, soluble molecules can move across the small intestine walls into the bloodstream

  
• Starch digestion


  1. Starch is a complex carbohydrate, it is large and insoluble

  2. Enzymes break down the large insoluble molecule (maltose) into small, soluble molecules, glucose

  3. The small soluble molecules have been absorbed into the bloodstream
• Adaptations of the ileum for absorption


  • Large surface area – due to being long and folded
  • Thin (1 cell thick) – digested food doesn't have to travel far to reach the blood
  • Good blood supply – to maintain the concentration gradient for diffusion between the ileum and bloodstream
  • Villi and microvilli – finger like projections that further increase surface area

  
• Features of the villi that aid absorption

  • Good blood supply – a villus has a large network of capillaries. Once blood becomes high in digested food products it is transported away and replaced with blood that is low in digested food. This maintains the concentration gradient necessary for diffusion between the ileum and bloodstream
  • Lacteal – a tube that absorbs the products of fat digestion (fatty acids and glycerol). The lacteals connect with lymph vessels
  • Single layer of surface epithelium cells – this reduces the diffusion distance that digested food products have to travel in order to enter the bloodstream
  • Permeable – digested food can pass through easily