Drug Excretion And Factors Affecting This Flashcards
What are the routes of excretion?
Lungs, bile, urine, sweat and breast milk.
How can routes of excretion be exploited in the treatment of drug overdose?
Some drugs pass straight through the GI tract.
For example in a drug overdose using charcoal (inert compound)
The drug is adsorbed irreversibly
A concentration gradient is maintained
The drug is continuously removed.
Charcoal is inert so it will be eliminated by the faeces.
How is urine alkalinised for aspirin overdose excretion?
Increase in pH so aspirin stays ionised. It’s not reabsorbed by the kidneys so is passed out via the urine.
How can net renal excretion be calculated?
Net excretion = glomerular filtration - reabsorption + tubular secretion
Rate of excretion vs clearance?
For many drugs the rate of excretion varies dependent upon drug concentration.
Clearance is a rate constant, it’s the volume of plasma cleared entirely of drug per unit time.
What is the normal glomerular filtration rate?
125ml/min
What is clearance affected by?
Glomerular filtration rate.
Solid sites of excretion?
Faeces, hair
Fluid site of excretion?
Urine, bile, sweat, tears and breast milk
Gas sites of excretion?
Exhaled breath
What are factors that determine filtration of drugs?
Molecular size - majority of drugs are small enough except heparin & biologics
Charge - glomerular capillary walls are -ve charged. So filtration of anions may be reduced.
Protein binding
Molecular size of filtration?
Molecules of a weight of less than 20 kDA can pass freely.
Which site of the kidney is most important for excretion?
Tubules
What is a normal urine pH range?
4.5 - 7.8
How does protein binding affect the filtration of drugs?
Only free drugs can be filtered
What can renal excretion be affected by?
Any pathology that can reduce glomerular filtration will reduce drug excretion.
What are the therapeutic benefits of manipulating urine pH for drug excretion?
This can be done in overdose and poisoning
For example salicylate overdose
What are the influences on renal excretion?
Filtrate or urine flow = faster/increased urine will reduce time and conc gradient
Diuretics = increases urine output
Disease states = acute conditions can lead to reduced filtration
What are the factors that can affect biliary/ hepatic excretion?
Disease states = acute/chronic liver disease alters efficiency of metabolic processes
Drug interactions can induce or inhibit CYP450 enzyme system
