Pharmacokinetics 1 Flashcards
What is pharmacokinetics
How drugs are absorbed into the body, how they are distributed and how they are eliminated
What are the four areas to consider in pharmacokinetics
Absorption, distribution, metabolism, excretion
Where does the bulk flow of transfer take place
Through the bloodstream
How do lipid soluble drugs cross the membrane of cells
Passivley diffuse down their concentration gradient
What will result from a rapid absorption of a drug through the gut
Rapid increase in concentration in the blood and brain - may also mean its rapidly metabolised however
What is the partition coefficient
Determines lipid solubility - how quickly the drug will dissolve in oil rather than water
How do non-polar molecules interact with cells
Diffuse freely in lipids and penetrate the cell freely
What is the best mechanism to achieve fast absorption of drugs
Intravenous injection - preferential in emergency circumstances
What effects the rate of absorption in intrauscular injections
Depends on the perfusion of that muscle - greater blood supply = greater rate of absorption
What are inrathecal injections
Drugs of the CNS - administered into the cerebrospinal fluid - used in pain relief
What is a characteristic needed for oral medication
Has to be resistent to being broken down in the gut - some will be metabolised in the liver and kidneys but not a huge amount
How does particle size effect a drugs metabolism
Large particle sized drugs are more easily metabolised/ less is absorbed into the blood stream
When would rectally administered drugs be used
For small infants that can’t swallow tablets and for seriously ill patients that can’t stop vomiting.
Why is a low pH beneficial to the absorption of weak acids (Many drugs are weak acids or bases)
Shifts the dissociation equation of a weak acid to the associated side - so no longer carries a charge and is able to diffuse across the membrane
Give the eqn to show aspirin is in its active form at a pH of 3 in the stomach
pKa= pH + log10 [HA]/[A-] pKa = 3.5
Once aspirin gets into the bloodstream what changes occur
The pH becomes more neutral - so the drug dissociates more - this means it stays in the blood longer as it can’t move across the lipid membrane
How does the pH of the kidney effect absorption
pH or 8 so the drug dissociates more readily - This is good as this is where the associated is metabolised
How are overdose drug situations combated
Administration of NaHCO3 to neutralise the compartments so drug is in a more dissociated form in the blood so doesn’t act on CNS etc - metabolised at the kidney
