Changes

* '''pH at site of absorption'''

* '''pH at site of absorption'''

Many drugs are either a '''weak base''' or a '''weak acid''' and so will exist in both an unionised and ionised form in the same solution. The ratio of the two forms is dependent upon the pH of the location of the drug. It must be remembered that ionised molecules don't pass easily through lipid membranes. Unionised molecules will diffuse easily as long as they are lipid soluble.

Many drugs are either a '''weak base''' or a '''weak acid''' and so will exist in both an non-ionized and ionized form in the same solution. The ratio of the two forms is dependent upon the pH of the location of the drug. It must be remembered that ionized molecules do not pass easily through lipid membranes. Unionized molecules will diffuse readily as long as they are lipid soluble.

The ionisation reaction of a weak acid is

The ionization reaction of a weak acid is

  AH =^'''Ka''' A^- + H⁺

  AH =^'''Ka''' A^- + H⁺

  pK_a = pH + log₁₀ ([AH]/[A^-])

  pK_a = pH + log₁₀ ([AH]/[A^-])

The ionisation reaction of a weak base is

The ionization reaction of a weak base is

  BH⁺ =^'''Ka''' B + H⁺

  BH⁺ =^'''Ka''' B + H⁺

  pK_a = pH + log₁₀ ([BH⁺]/[B])

  pK_a = pH + log₁₀ ([BH⁺]/[B])

The dissociaton constant allows one to measure the strength of an acid or base and to determine the charge on a molecule in any givem pH. Thus the extent of ionisation of a drug and so it's extent of absorbtion, depneds upon its '''pK_a''' and the pH within the body compartment.

The dissociaton constant allows one to measure the strength of an acid or base and to determine the charge on a molecule in any givem pH. Thus, the extent of ionization of a drug (extent of absorption), depneds upon its '''pK_a''' and the pH within the body compartment.

Due to the above drugs can become '''ion trapped''' in certain body compartments. If it is assumed that the ionised fraction is unable to cross cell membranes and the unionised fraction is; then a weak acid will become greatly concentrated in an environment with a high pH. This is because it will donate it's spare protons to the basic elements in the high pH environment and then will be unable to cross out of the environment as the drug is now ionised; it has been '''trapped'''.

Due to the above, drugs can become '''ion trapped''' in certain body compartments. If it is assumed that the ionized fraction is unable to cross cell membranes and the non-ionized fraction is, then a weak acid will become greatly concentrated in an environment with a high pH. This is because it will donate it's spare protons to the basic elements in the high pH environment and subsequently be unable to cross out of the environment as the drug is now ionized. The drug has now been '''trapped'''.

  '''Insert Diagram Here'''

  '''Insert Diagram Here'''

'''In summary a weakly acidic drug will become ion trapped in an environment with a high pH and a weakly basic drug will become ion trapped in an environment with a low pH.'''

'''In summary, a weakly acidic drug will become ion trapped in an environment with a high pH and a weakly basic drug will become ion trapped in an environment with a low pH.'''

* '''The Area of the absorbing surface'''

* '''The Area of the absorbing surface'''

The larger the surface area for absorption the greater the absorption.

The larger the surface area for absorption the greater the absorption.