Equilibria 4

• What is the purpose of partition coefficients?

  To describe solute distribution in liquids
• What does the partition coefficient represent?

  Ratio of concentrations in two immiscible liquids
• What is the formula for the partition coefficient?

  $P =$$\frac{C_o}{C_w}$
• What does Log P indicate about a molecule?

  Hydrophilicity or lipophilicity of the molecule

  
• What does Log P < 0 signify?

  Molecule is more hydrophilic (polar)

  
• What does Log P > 0 signify?

  Molecule is more lipophilic (non-polar)

  
• What are the two main classes of transport through gastrointestinal membranes?

  Passive transport and carrier-mediated transport
• What is passive transport in the gastrointestinal tract?

  Simple diffusion driven by concentration gradient

  
• What does the pH-partition hypothesis state?

  Gut membranes are more permeable to unionised drugs

  
• Why are unionised drugs absorbed more rapidly?

  Due to greater lipid solubility

  
• What factors influence drug absorption in the gastrointestinal tract?

  Extent of drug's unionised form at absorption site

  
• What happens during diffusion to maintain equilibrium?

  Rapid absorption occurs when drug is unionised

  
• What is the ionisation percentage of a drug in the stomach at pH 3.4?

  1% ionised and 99% unionised
• What is the ionisation percentage of a drug in the blood at pH 7.4?

  99% ionised and 1% unionised
• How are acids absorbed in relation to pH?

  Rapidly absorbed in low pH environments
• How are bases absorbed in relation to pH?

  Rapidly absorbed in high pH environments
• What factors limit absorption from the stomach?

  Mucous lining, small surface area, short residence time
• What factors favor absorption in the small intestine?

  Thin barrier, large surface area, longer residence time
• What role does blood play in drug absorption?

  It carries drug away from absorption site

  
• Why might a drug not be absorbed by passive diffusion?

  pH at membrane may differ from bulk pH

  
• What are the reasons for limited drug absorption from the stomach?

  • Mucous lining
  • Small surface area
  • Short residence time
• What are the advantages of the small intestine for drug absorption?

  • Thin barrier
  • Large surface area
  • Longer residence time
• What is the pH-partition hypothesis?

  • Gut membranes are more permeable to unionised drugs
  • Greater lipid solubility of unionised drugs
  • Absorption depends on drug's unionised form