Ileum

• 5 features of gas exchange in the ileum:

  • Large surface area - many villi which have a wall made up of microvilli epithelial cells
  • permeable - microvilli epithelial cells are permeable to soluble molecules e.g. glucose and amino acids
  • moist - molecules e.g. glucose can dissolve and diffuse into the blood
  • thin for a short diffusion pathway - walls of villi are 1 cell thick
  • maintaining a concentration gradient - large network of capillaries and lacteals present in the villi
• Wall of the ileum:

  A) capillaries

  B) epithelial cells with microvilli

  C) lacteal

  D) venule

  E) muscularis mucosa

  F) arteriole

  G) goblet cells

  
• The lacteal joins to the thoracic duct and eventually the subclavian vein
• Muscularis mucosa - contract and relax to move the villi to increase the contact between villi and soluble food
• 3 ways in which the cells lining the villi ae adapted for efficient absorption of molecules:

  • folded membrane increases surface area for absorption
  • many mitochondria produce ATP for active transport (of Na+/K+)
  • Co-transport proteins to absorb glucose and amino acids
• Absorption in the ileum of lipids

  1. fatty acids and glycerol diffuse through the phospholipid bilayer into the epithelial cell from the lumen
  2. In the smooth endoplasmic reticulum they are combined into triglycerides (+ chylomicrons)
  3. Triglycerides and chylomicrons exit the basal membrane of the epithelial cells by exocytosis and enter the lacteal
  4. lacteal forms part of the lymphatic system and joins the bloodstream at the thoracic duct / subclavian vein junction
• Absorption of carbohydrates and proteins:

  1. Na+ are actively transported out of the epithelial cell
  2. This means there is a lower concentration of Na+ in the cells than the ileum lumen
  3. Na+ diffuse into the epithelial cells by facilitated diffusion from the lumen and co-transport a glucose with them
  4. there is a higher concentration of glucose in the epithelial cells compared to the capillary
  5. glucose diffuses out of the basal membrane of the epithelial cell and into the capillary, to the vena cava then the heart then the body